Vibrational Spectroscopy and Theory of Alkali Metal Adsorption and Co-Adsorption on Single-Crystal Surfaces

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surface science reports

Surface Science Reports 68 (2013) 305–389 www.elsevier.com/locate/surfrep

Vibrational spectroscopy and theory of alkali metal adsorption and co-adsorption on single-crystal surfaces

A. Politanoa,n, G. Chiarelloa,b, G. Benedekc,d, E.V. Chulkovc,e,f, P.M. Echeniquec,e

aUniversità degli Studi della Calabria, Dipartimento di Fisica, 87036 Rende (Cs), Italy bConsorzio Interuniversitario di Scienze Fisiche per la Materia (CNISM), Via della Vasca Navale, 84, 00146 Roma, Italy cDonostia International Physics Center (DIPC), P. Manuel de Lardizabal 4, 20018 San Sebastián—Donostia, Spain dUniversità Milano-Bicocca, Dipartimento di Scienza dei Materiali, 20125 Milano, Italy eDepartamento de Física de Materiales and Centro Mixto CSIC-UPV/EHU, Facultad de Ciencias Químicas, Universidad del País Vasco, Apdo. 1072, 20080 San Sebastián—Donostia, Spain fTomsk State University, Pr. Lenin 36, 634050 Tomsk, Russian Federation

Received in revised form 1 March 2013; accepted 30 June 2013

Abstract

Alkali-metal (AM) atoms adsorbed on single-crystal surfaces are a model system for understanding the properties of adsorption. AM adsorption, besides introducing new overlayer vibrational states, induces significant modifications in the surface vibrational structure of the metal substrate. Several studies of the vibrational properties of AM on metal surfaces have been carried out in last decades. Most of these investigations have been performed for low coverages of AM in order to make the lateral interaction among co-adsorbates negligible. The adsorbed phase is characterized by a stretch (S) vibrational mode, with a polarization normal to the surface, and by other two modes polarized in the surface plane, known as frustrated translation (T) modes. The frequencies and intensities of these modes depend on the coverage, thus providing a spectroscopic signature for the characterization of the adsorbed phases. The vibrational spectroscopy joined to an ab-initio theoretical analysis can provide useful information about surface charge re-distribution and the nature of the adatom–surface bond, establishing, e.g., its partial ionicity and polarization. Gaining this information implies a significant advancement in our knowledge on surface chemical bonds and on catalytic reactions occurring in AM co-adsorption with other chemical species. Hence, systematic studies of co-adsorption systems are essential for a more complete understanding of heterogeneous catalysis. The two principal experimental techniques for studying the vibrations of AM adsorbed phases are high-resolution electron energy loss spectroscopy (HREELS) and inelastic helium atom scattering (HAS), the former being better suited to the analysis of the higher part of the vibrational spectrum, while the latter exploits its better resolution in the study of slower dynamics, e.g., T modes, surface acoustic phonons and diffusive phenomena. Concerning AM co-adsorption systems, reflection–absorption infrared spectroscopy (RAIRS) has been also used (as well as HREELS) for obtaining information on the influence of AM adsorption on the vibrational properties of co-adsorbates.

Abbreviations: 2DEG, two-dimensional electron gas; 3HeSE, 3He spin echo (spectroscopy); AES, Auger electron spectroscopy; AM, alkali metal; amu, atomic mass units; ASP, acoustic surface Plasmon; CDO, charge density oscillation; DFPT, density functional perturbation theory; DFT, density functional theory; DME, dimethyl ether; e–p, electron–phonon; EAM, embedded atom method; FC, force constant model; FK, Fuchs–Kliewer; HAS, helium atom scattering (spectroscopy); HREELS, high-resolution electron energy loss spectroscopy; INS, inelastic neutron scattering; IRAS, infrared reflection absorption spectroscopy; LDOS, local density of states; LEED, low-energy electron diffraction; ML, monolayer; OP, organ pipe mode; OR, overlayer resonance; QHAS, quasi-elastic He atom scattering; QWS, quantum well state; RAIRS, reflection–absorption infrared spectroscopy; RT, room temperature; S, perpendicular stretch mode (2, 3 indicate 2nd and 3rd overtones); SERS, surface enhanced Raman scattering; SHG, second harmonic generation; SFG, sum frequency generation; STM, scanning tunneling microscopy; T, frustrated translation; TOF, time-of-flight; TPD, thermal programmed desorption; TRSHG, time resolved second-harmonic generation; XPS, x-ray photoemission spectroscopy nCorresponding author. Tel.: +39 0984 496107; fax: +39 0984 494401. E-mail address: antonio.politano@fis.unical.it (A. Politano).

In this review an extended survey is presented over:

a) the existing HREELS and HAS vibrational spectroscopic studies for AM adsorbed on single-crystal metal surfaces; b) the theoretical studies based on semi-empirical and ab-initio methods of vibrational structure of AM atoms on metal surfaces; c) the vibrational (HREELS, RAIRS, TRSHG) characterization of the co-adsorption on metal surfaces of AM atoms with reactive species.

Contents

1. Introduction ...... 308 2. A brief survey of surface vibrational spectroscopies ...... 309 2.1. HREELS ...... 310 2.1.1. Low-energy HREELS spectrometers for investigation of elementary surface excitations ...... 311 2.2. HAS ...... 311 2.3. QHAS ...... 315 2.4. 3HeSE ...... 317 2.5. RAIRS ...... 319 2.6. SERS ...... 320 2.7. SFG ...... 321 3. Basic concepts in the theory of surface excitations ...... 322 3.1. Surface phonons...... 322 3.2. Vibrational excitations of adsorbates ...... 324 3.3. Dynamics of AM on metals with the EAM ...... 325 3.3.1. AM/Al(111) ...... 325 3.3.2. Na/ Cu(111) ...... 326 3.3.3. AM/Pt(111)...... 327 3.4. Dynamics of AM on metal surfaces with ab-initio methods (DFPT) ...... 330 4. Vibrational spectroscopy of adsorbed AM atoms ...... 333 4.1. Single-adatom properties ...... 333 4.1.1. Mass dependence...... 333 4.1.2. Dependence on the adsorption site ...... 333 4.1.3. Dependence on surface indices...... 334 4.1.4. Dependence on temperature ...... 335 4.2. From low coverage to one monolayer ...... 335 4.2.1. Adatom–adatom interaction: dipolar and Lau–Kohn forces ...... 335 4.2.2. Cs/Cu(100) monolayer ...... 337 4.2.3. SHG studies on AM/Cu(111)...... 338 4.2.4. HREELS studies on AM/copper ...... 338 4.2.5. AM/Al(111) ...... 340 4.2.6. AM/Ni(111) ...... 341 4.2.7. AM/Pt(111)...... 342 4.2.8. AM/Mo(100)...... 343 4.2.9. AM/Ru(0001) ...... 343 4.2.10. AM/graphite ...... 344 4.2.11. K/Si(111) ...... 345 4.2.12. K/GaAs(110)...... 346 4.3. AM multilayers: organ-pipe modes ...... 346 5. Binary co-adsorption ...... 348 5.1. Introduction...... 348 5.1.1. General considerations on AM co-adsorption with carbon monoxide ...... 349 5.1.2. General considerations on AM co-adsorption with oxygen ...... 349 5.1.3. General considerations on AM co-adsorption with water ...... 350 5.1.4. General consideration on AM co-adsorption with carbon dioxide...... 350 5.2. On copper...... 350 5.2.1. AM+CO/Cu(111)...... 350 5.2.2. AM+CO/Cu(100)...... 352 5.2.3. AM+O/Cu(111)...... 352 A. Politano et al. / Surface Science Reports 68 (2013) 305–389 307

5.2.4. Na+O/Cu(110) ...... 353 5.2.5. Na+OH/Cu(111) ...... 354 5.2.6. Li+H2O/Cu(100) ...... 354 5.2.7. K+CO2/Cu(110) ...... 354 5.3. On aluminum ...... 355 5.3.1. Cs+H/Al(111) ...... 355 5.4. On nickel ...... 355 5.4.1. AM+CO/Ni(111) ...... 355 5.4.2. AM+O/Ni(111) ...... 356 5.4.3. K+Ethylene oxide/Ni(111) ...... 358 5.5. On platinum ...... 360 5.5.1. K+CO/Pt(111)...... 360 5.5.2. K+OH/Pt(111) ...... 360 5.5.3. K+H2O/Pt(111) ...... 360 5.5.4. K+C2H4/Pt(111) ...... 360 5.6. On ruthenium ...... 361 5.6.1. K+CO/Ru(0001) ...... 361 5.6.2. Cs+CO/Ru(0001)...... 362 5.6.3. Cs+O/Ru(0001)...... 362 5.6.4. AM+H2O/Ru(0001) ...... 363 5.6.5. K+CH3OH/Ru(0001) ...... 363 5.7. On iron ...... 364 5.7.1. K+CO/Fe(110) ...... 364 5.7.2. K+N2/Fe(111) ...... 364 5.8. On cobalt ...... 364 5.8.1. K+CO2/Coð1010Þ ...... 364 5.9. On rhodium...... 365 5.9.1. K+NO/Rh(100) ...... 365 5.9.2. K+dimethyl ether on Rh(111) ...... 365 5.10. On palladium...... 366 5.10.1. Na+CO2/Pd(111) ...... 366 5.10.2. K+NO/Pd(111) ...... 367 5.10.3. K+C2N2/Pd(100) ...... 367 5.10.4. Cs+C2H4/Pd(110) ...... 367 5.11. On gold ...... 367 5.11.1. K+CO2/Au(111) ...... 367 5.11.2. K+nitriles/Au(100)...... 367 5.12. On molybdenum carbide ...... 367 5.12.1. K+CO2/Mo2C...... 367 5.13. On graphite ...... 368 5.13.1. K+O2/graphite...... 368 5.13.2. AM+H2O/graphite ...... 369 5.14. On diamond ...... 370 5.14.1. K+O/C(100) ...... 370 5.14.2. K+CO/C(100) ...... 370 5.15. On silicon ...... 370 5.15.1. AM co-adsorption with organic molecules on Si(100) ...... 370 6. Ternary co-adsorption systems and multilayered substrates ...... 372 6.1. K+CO2+H2O/Cu(111) ...... 372 6.2. AM+CO+O/Ni(111)...... 373 6.3. K+perylene-tetracarboxylic-dianhydride films on Ag(110)...... 374 6.4. K+CO2 on copper films...... 375 6.5. K+CO2 and CO on silver films...... 375 6.6. AM co-adsorption on Mo2C/Mo(100) ...... 375 6.6.1. K co-adsorption with alcohols on Mo2C/Mo(100) ...... 375 6.6.2. K co-adsorption with C3H7 on Mo2C/Mo(100) ...... 376 6.7. AM co-adsorption on Cr2O3(0001)/Cr(110) ...... 376 6.7.1. Na+CO2/Cr2O3(0001)/Cr(110) ...... 376 6.7.2. Na+NO/Cr2O3(111)/Cr(110) ...... 376 6.1. AM co-adsorption with CO on bimetallic surfaces...... 377 6.2. AM co-adsorption on epitaxial graphene ...... 378 7. Conclusions and outlook ...... 379 308 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

Acknowledgments...... 379 References ...... 380

1. Introduction information on AM systems contained therein is used in the present review paper to finely analyze lattice and adsorbate The physical and chemical properties of supported AM dynamics in AM/metal systems. layers continue to attract the attention of researchers in surface Several techniques have been used for studying AM vibrations science [1–41]. At the basis of this interest there are several at metal surfaces: HREELS [1,16,26,190,205,212,270–284], fundamental and practical motivations. Due to their simple SHG [107,272,285–288], RAIRS [289–296], SERS [297,298], electronic structure, AM have been used as model system to HAS [5,73,74,299–309], and, more recently, SE-3He [69,310] describe surface chemical bonds [42–53], the interactions (see Table 1). They provide direct evidence of bond formation or between adsorbates [54–56], the electronic properties of breaking and charge transfers at metal surfaces. metallic layers [13,19,27,57–68] and surface diffusion [69– Similar to the creation of QWS, AM adsorption gives rise to 79]. Moreover, long-lived excited states have been reported for new vibrational states related to the AM overlayer (adatoms) AM overlayers on metal substrates [80–85]. The adsorption of [11,34,69,278,340–342]. In the last decade extensive studies of AM on metal substrates influences electron–electron [86–96] vibrational properties have been performed for AM on metal and electron–phonon [97–108] scattering in excited electron surfaces. Many of these studies have been carried out for low and hole states leading often to a change of the decay coverages of AM in order to reduce the influence of lateral mechanisms of an excited electron or/and hole [109–112]. interactions among adsorbates. A stretch (z-polarized) vibra- AM adsorption may induce a rearrangement of surface tional mode arises [314,343] upon AM adsorption. This mode atoms of the substrate [4,113–124]. This leads to the modifica- does not normally change its energy with increasing coverage tion of the substrate surface electronic states [125–127] and [323], even if its intensity decreases [343]. On the other hand, gives rise to adsorbate-induced electron states [10,128–143]. the frustrated translation (T) mode [300,323], polarized in the The work function of the system [144–159] is significantly plane parallel to the surface, changes its energy with the influenced by the presence of AM. Adsorbed AM produce coverage increase. changes in surface dipoles and electric fields at surfaces [160– Furthermore, significant advancement has been made in our 162] and charge transfers to the substrate [85,163 –175]. The understanding of AM co-adsorption systems which could lead understanding of AM adsorption should also imply a remark- to a theoretical remodeling of AM interactions with reactive able advancement in the comprehension of surface chemical co-adsorbates. bonds and of catalytic reactions occurring in co-adsorption of Moreover, calculations of vibration spectra has been an AM atoms with other chemical species [15,42,51,117,176– important method to determine the adsorption structures by 193]. AM-induced electrostatic fields have a crucial impact on comparison with the experimental spectra. Herein we will the activity and selectivity of a number of technologically attempt to provide a comprehensive guide to all the work in relevant catalytic processes [194]. Hence, systematic studies of this area of which we are aware. co-adsorption systems are essential for a more complete The review is organized as follows. In Sections 2 and 3 understanding of heterogeneous catalysis. Co-adsorption stu- basic concepts in surface vibrational spectroscopies and in the dies of lithium [116,195–200], sodium [201–212], potassium theory of surface excitations are reported, respectively. In [46,57,149,213–246], cesium [149,228,247–262] and rubi- Sections 4 and 5 we discuss AM adsorption and binary co- dium [263–266] with simple molecules (CO, CO2,H2O, NO adsorption on single-crystal surfaces, respectively. The litera- etc.) have been carried out in order to shed the light on the AM ture is organized by substrate and then further sub-divided into promotion effect on surface chemical reactivity. A microscopic specific adsorbate and co-adsorbate systems. The review of understanding of these and related phenomena is important each system begins with a brief discussion of available because of their impact on processes such as vibrational information on the electronic, structural or chemical properties excitations, surface scattering, photochemical reactions and of the system in order to help the reader. In Section 6 we charge and energy transport on surfaces. review available vibrational studies on ternary co-adsorption Several previous reviews [267,268] or monographs [269] and AM adsorption on multilayered substrates. Conclusions appeared on various aspects of this topic. The review by and outlook are given in Section 7. Bonzel [268] is focused on adsorption energetics and kinetics, Here the phonon energies (or the corresponding frequencies) while Kiskinova [267] reviewed the promotional effects of AM are normally given in meV or, in some cases, through the adsorption. The last one (dated 1996) is by Diehl and McGrath corresponding wave-number in cm 1 (1 meV¼8.1 cm 1; [121] which reviewed the structure of AM co-adsorption 1cm1¼0.124 meV). Less used are frequencies in THz systems. No review exists on vibrational studies on AM (1 THz¼4.13 meV), angular frequencies in 1013 rad/s (¼6.55 adsorption and their co-adsorption with molecules on metal meV) and energies in degrees K (10 K¼0.86 meV). Theore- surfaces. In particular, our review is a natural continuation of ticians often give electronic energies in atomic units (1 a. that one by Diehl and McGrath [121]. In fact, structural u.¼2Ry¼1 hartree¼27.2 eV). A. Politano et al. / Surface Science Reports 68 (2013) 305–389 309

Table 1 Adsorption structures and phonons in AM adsorbed systems.

Phonons Systems Coverages and structures Adsorption sites Exp. Calc.

Li/Al(001) c(2 2) Subst. EAM [25] Na/Al(001) c(2 2) Hollow LT EAM [25] Li/Al(111) 0.03–1.0 ML Subst. RT Subst. HREELS [311] EAM [25] (√3 √3) EAM [312] Na/Al(111) (√3 √3) Subst. HREELS [311,313] EAM [312], K/Al(111) (√3 √3) Subst. HAS [5] DFT [314],FC[313], EAM [312], DFT [314] HAS [5] Cs/Al(111) (√3 √3) Top low temp. HAS [5] (2√3 2√3) Subst. RT HAS [5] Li/Cu(001) 0.04–0.8 ML On-surf. HREELS [315] c(2 2) Hollow EAM Na/Cu(001) 0.1 ML Hollow HAS [303] 0.0–0.4 ML HREELS [316] 0.0–0.5 ML HAS [11,300,307] 0.05 ML HAS [55] c(2 2) Hollow EAM Quasi-hex HAS [304,317] K/Cu(001) 0.07 ML Hollow HAS [55] 0.02–0.11 ML HREELS [316] Quasi-hex HAS [318] Cs/Cu(001) 0.08 ML On-surf. HAS [55] 0.27 ML Quasi-hex. HAS [319] Li/Cu(110) 0.04–0.8 ML On-surf., reconstr. HREELS [315] Na/Cu(110) 0.13 ML On-surf., reconstr. HREELS [320] K/Cu(110) 0.02–0.32 ML On-surf., reconstr. HREELS [320] Li/Cu(111) 0.025–0.5 ML On-surf. HREELS [321] Na/Cu(111) 0.075–0.3 Hollow HREELS [321,322] p(3 3), p(2 2) Hollow EAM [323] (√3 √3) (3/2 3/2) 1ML Jellium model [324], DFT [325] K/Cu(111) 0.02–0.27 ML On-top HREELS [316] 0.08–0.4 ML On-top HREELS [321,322] Cs/Cu(111) 2–5 ML Quasi-hex. HAS [299] K/Ni(001) 2–5 ML HAS [299] [326] [327] K/Ni(110) 0.2 ML On-surf. HREELS [328] Na/Ni(111) 0–0.20 ML On-surf. HREELS [329] K/Ni(111) 0–0.20 ML On-surf HREELS [329] Na/Pt(111) 0.05 ML On-surf. HAS [302] K/Pt(111) 0.02–0.15 ML Hollow HREELS [330,331] (2 2) EAM (3 3) TRSHG [332] EAM, DFT [333] Cs/Pt(111) 0.22–0.41 ML On-surf. TRSHG [285] Na/Mo(001) 0.0–0.45 ML On-surf. HREELS [334] Li/Mo(001) (1 1) On-surf. HREELS [335] Cs/Ru(0001) c(2 2) On-top RT 0.03–0.24 ML HREELS [336] 0.08–0.25 ML HREELS [337] Li/W(110) HAS [338] Cs/HOPG(0001) p(2 2) HAS [309,339] K/HOPG(0001) p(2 2) HAS [309,339] Rb/HOPG(0001) p(2 2) HAS [309,339]

2. A brief survey of surface vibrational spectroscopies minimum surface coverage required for the detection of a species ranges from 0.001 to 0.1 of a monolayer. Vibrational spectroscopy is unrivaled by any other methods Adsorbed AM vibrational modes which have polarizations concerning the chemical analysis of surface species due to its perpendicular to the surface have been mainly studied using high sensitivity. As a matter of fact, depending on the vibrational HREELS, while measurements of the modes with polarizations spectroscopy and the nature of the molecular vibration, the parallel to the surface have been investigated by HAS. 310 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

Perpendicular modes have large dynamic dipole components ki=(Ki, kiz)andkf=(Kf, kfz), respectively, and G is a surface so as to render more suitable HREELS, while parallel modes reciprocal lattice vector. Under the kinematic conditions dis- have lower energies which are more accessible with HAS. cussed below no umklapp scattering process is considered (G=0). For planar scattering, Ki=ki sin θi and Kf=kf sin θf,with θ θ fi 2.1. HREELS i the incidence and f the nal angles with respect to the surface normal, conventionally fixed along the z-axis. (Fig. 1). From the In HREELS experiments, a primary electron beam impinges above equations it is possible to calculate the wave-vector with energy E onto a metal or semiconductor surface and the transfer Q for planar scattering as a function of the energy loss: p pffiffiffiffiffiffiffiffiffiffiffi rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi scattered beam emerges with energy Ep Eloss where Eloss is 2mEi Eloss the loss energy, that is the energy lost by the electron towards Q ¼ sin θi 1 sin θf ð4Þ ℏ E elementary excitations of the surface. They include one or i more phonons of the surface or of the adsorbed species, as well According to Mills [347], the differential cross section as single-particle transitions or surface plasmons (see [344– d2S=dωdΩ, is given by 346] for a review). fl 2 For moderate incident energies and small de ections of the 2 2 ? ð þ Þþ ð Þðω jj Þ d S ðmev ? Þ K PðQ; ωÞ v Q Rf Ri i Rf Ri v Q scattered beam from the specular direction the inelastic event is ¼ f dωdΩ π2ℏ5 k 2 ½ð Þ2 þðω Þ22 induced at fairly large distances from the surface. This process 2 iz Q v ? Q vjjQ is called dipole scattering as the long-range electric field of ð5Þ primary electrons interacts with the fluctuating dipolar field where vjj are v ? the parallel and perpendicular components of associated with the induced surface charges. At larger incident the velocity of impinging electrons with respect to the surface, energies the energy loss occurs mainly through impact respectively, and P(Q; ω) is the surface loss function. R and R scattering, which takes place in the close vicinities of ion i f are the amplitude of the complex reflectivity for initial and cores and the scattering intensity is not peaked in the specular final energies. The maximum inelastic scattering occurs for direction. ω ¼ νjjQ, in correspondence of a minimum in the denominator Specular energy loss intensities in HREELS arise mainly from of (5). Such condition corresponds to the interaction of dipole scattering. Hence, HREELS performed in the specular electrons with partial waves with phase velocity νjj ¼ ω=Q. geometry gives basically the same information as RAIRS. For the By defining δ as the deviation from trajectory of electrons dipole scattering mechanism the parallel components of dipole inelastically scattered from specular direction, thus for ℏω{E moments are perfectly screened by their image dipoles on metal i and δ{1, the denominator of (5) may be written as surface. Thus, only vibrations that bear a dipole moment perpendicular to the surface could be excited. In the framework ð Þ2 þðω Þ2 ¼ 2ðδ2 þ Ψ 2 Þ 2θ ð Þ v ? Q vjjQ 4Ei E cos i 6 of group theory, the surface selection rule therefore states that only the modes belonging to the total symmetric representation A′ where Ψ E ¼ ℏω=2Ei. Eq. (6) determines the angular depen- (Cs-group) and A1 (Cnv groups) are active in inelastic electron dence of dipole scattering and its concentrations in a lobe with scattering via the dipole scattering or in surface IR-spectroscopy. semi-amplitude Ψ E along specular directions. Dipole scattering For all practical purposes, the selection rule applies also to dominates for small transfer momenta. As shown in Fig. 2,a semiconductor surfaces. principal maximum exists, corresponding to the condition The inelastic interaction could be treated as a classical ω ¼ vjjqjj, although also a secondary maximum does occur. energy loss of a charged particle reflected from a surface For short-range interactions impact scattering occurs. within the framework of the dielectric theory of inelastic Within this scattering mechanism, electrons are diffused in electron scattering. The system is represented by its complex every possible solid angle, even beyond the incidence plane. dielectric functions ε(ω) or its complex dynamic polarizabil- Both perpendicular and parallel component of the wave-vector ities α(ω), respectively. The loss probability P(Q, ω)is (with respect to the sample normal) are not conserved. As a proportional to consequence of such complexity, theory which describes such ε ðωÞ 1 interactions is not deeply developed. The cross section is PðQ; ωÞp 2 ¼ Im ð1Þ 2 εðωÞþ1 εðωÞþ1 where Q is the parallel wave-vector of the elementary excitation. Conservation of both energy and parallel momentum leads to

Eloss ¼ EiEf ð2Þ

ℏQ ¼ ℏðKiKf þ GÞð3Þ where Ef is the energy of the scattered electron beam, Ki and Kf Fig. 1. Scattering geometry in HREELS experiments. the parallel components of the incident and final wave-vectors A. Politano et al. / Surface Science Reports 68 (2013) 305–389 311 defined as [345] Dispersion of the collective mode, i.e. Eloss(Q) is usually 2 measured by moving the analyzer while keeping the sample ds mEi cos θf 2 ¼ M ð7Þ and the monochromator in a fixed position, but whenever the dΩ 2π2ℏ2 cos θ i analyzer cannot rotate, the SP dispersion is measured through where M is the matrix element for the transition and m is the changing the incident angle. mass of electrons. The cross section presents only minimal The quality of spectra is determined also by the angular changes with scattering angle. acceptance of the spectrometer which in turn affects Q resolution. This is achieved by low impinging energies (below 2.1.1. Low-energy HREELS spectrometers for investigation of 50 eV) and for grazing scattering conditions. An interesting elementary surface excitations possibility to reduce the window in the reciprocal space is HREELS is the most powerful tool for investigating the given by ELS-LEED, which applies the spot profile analysis dispersion and the damping of collective excitations at metal commonly used in LEED to the inelastic signal so as to surfaces (phonons [349–351] and plasmons [352–368]) as well achieve a Q resolution of 103 Å1 [37,372–377]. as the adsorption of chemisorbed atoms and molecules [369,370]. Spectrometers of the last generation have been designed by Harald Ibach [371] and shown in Fig. 3, are 2.2. HAS constituted by a two-step monochromator and by a rotating analyzer with 1511 cylindrical deflectors. This results in an Like HREELS in the impact regime, HAS is a powerful tool ultimate resolution of 0.5 meV and a significant increase of for investigating the dispersion relation of surface phonons intensity in the high resolution range. The basic concept of this [378–395]. Conventional supersonic 4He beams with incident fi spectrometer is a xed double stage monochromator and a energy Ei in the 10 to 100 meV range allow for a resolution of 2 rotatable single stage analyzer. In order to allow the probing of the order of Ei/ΔE10 , which allows to explore the low the largest possible fraction of the surface BZ the maximum acoustic range of surface phonons, hardly accessible to rotation angle of the analyzer stage was increased to 4 901. HREELS [396]. Although atom scattering spectroscopy based The impact energy is variable from 0 to 250 eV. on supersonic nozzle sources at thermal energies is suited to explore the lower part of the phonon spectrum, there is however no conceptual obstacle to produce high-energy neutral atom beams with a good speed ratio, even in the keV range [397–399], for a surface spectroscopy of more energetic excitations [399]. In a phonon creation process the upper limit of the phonon energy detectable by HAS is the incident energy itself; energy transfers as large as 80% of the incident energy are distinctly observable [400]. On the other side of the energy spectrum, the recent development of spin-echo 3He atom spectroscopy allows to investigate slow surface dynamical processes like molecular translational (T) modes and diffusion, with a spectacular resolution of 20 neV [401]. Fig. 2. Schematic plot of the kinematic factor in the dipole scattering probability function, showing that the inelastically scattered electrons are There is another important difference between HAS and confined within two lobes near the specular reflection direction. Adapted HREELS. While electrons in the impact regime are scattered from Ref. [348]. from the high electron density at ion cores, He atoms at

Fig. 3. HREELS spectrometer Delta 0.5, designed by Ibach [371]. 312 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

Fig. 4. Schematic view of the HUGO helium scattering apparatus used in Göttingen [379]. (a) The nearly mono-energetic helium beam is emitted from the nozzle

(1) and after passing through a skimmer (2) and scattering from the sample (4) a part of the scattered intensity enters the time-of-flight chambers (from P5 to P8) and is detected by the mass spectrometer ionizer (5). Rotation of the sample and/or the detector arm allows the angular distribution of scattered He atoms to be mapped out. For time-of-flight studies the beam is pulsed by passing it through a rapidly rotating chopper disc with a few equally spaced narrow slits (3). P1 to P9 are differential pumping stages which provide for a reduction of the helium partial pressure in the detector chamber with respect to the source chamber by over 12 orders of magnitude. The scattering chamber is equipped with various in-situ surface treatment and analysis devices. The inset (b) shows the (in-plane) scattering geometry. An incident beam of wave-vector ki and incidence angle θI with respect to the normal to the surface n is scattered into a final direction of final angle θf with a wave-vector kf. While the initial energy distribution is sharply peaked at Ei the final distribution, besides the elastic scattering peak at Ei, shows inelastic features corresponding to phonon creation and annihilation processes. thermal energies are repelled from a much lower electron 4.2 K. In addition rather unique quantum effects arise because density (in the range of 10 4 a.u.) a few Å outside the surface. of the very weak He–He inter-molecular potential. As a Thus He atoms transmit energy to the phonons of the system consequence inside the adiabatically expanded beam gas the through the phonon-induced modulation of the surface charge scattering cross section increases from about 30 Å2 at RT to density, i.e., through the e–p interaction [378]. In metals, due about 2.6 105 Å2 as the ambient translational temperature of the long range nature of the electron–phonon interaction, He the beam gas (associated with the velocity fluctuations with atoms can perceive the oscillations of deep atoms beneath the respect to the mean translational energy) is cooled to ultra low 3 surface which are transmitted by electron density waves temperatures approaching T1 ¼ 10 K. These large cross through the Fermi sea (quantum sonar), and measure the e–p sections are, in fact, responsible for driving the expansion in its coupling constant for each individual phonon (mode-lambda final stages to much lower temperatures than in other gases. At spectroscopy) [402]. Similar information can now be obtained the same time these same weak interactions also inhibit cluster for AM overlayers via the phonon-induced oscillations of the formation which, because of the heat released, can spoil the QW charge density [403]. expansion and contaminate the atom beam with clusters. These Helium has a number of important advantages for surface two effects, giant cross sections and reduced clustering, both scattering experiments [404,405]. First of all He atoms at explain the extremely low ambient translational temperatures thermal energies have a de Broglie wave-length just compar- in the range of 1 to 10 mK which can easily be achieved inside able with the atomic dimensions (between 0.5 Å and 1.5 Å). the expanded helium gas. Of primary importance for the Moreover it is extremely inert and is not expected to stick to surface scattering experiments are the resulting very narrow 1/2 any surface at temperatures greater than about 5–10 K. Free jet velocity half-widths of about Δv/v E(T1 /T0) E0.5% expansions, which are generally used to produce intense [406]. Thus energy widths of 0.2 meV and 0.08 meV have molecular beams, show particularly advantageous properties been achieved for gas cells operating at liquid nitrogen and in the case of helium [406]. For one because of its weak liquid helium temperature, respectively. interaction potential it has the lowest of all heats of vaporiza- Another important advantage of helium is that it is easily tion and therefore still has a sizeable vapor pressure at 1 bar at detected in a mass spectrometer with an especially low A. Politano et al. / Surface Science Reports 68 (2013) 305–389 313 background signal, which is the reason why it is extensively used in commercial leak detectors. Finally, because of its low mass and the possibility to go to low source temperatures and beam energies, scattering conditions especially favorable to the dominant excitation of only single phonons are easily achieved. The energy of the beam can be calculated by thermody- namics. During the expansion process, energy must be conserved. Since the pressure inside the source chamber is kept constant, in the limit that the velocity perpendicular to the beam direction is vanishingly small, the kinetic energy per atom in the beam is equal to the enthalpy per atom in the source: Mv2 5 E ¼ J ¼ k T ð8Þ i 2 2 B 0 where M is the mass of the atom, v J the axial component of velocity, and T0 the source temperature. A liquid-nitrogen cooled nozzle source yields a beam of 16.6 meV. Fig. 5. (a) The experimental HAS dispersion curves (○) for a monolayer of quasi-hexagonal close packed Cs on Cu(001) in the 〈110〉 direction of the The analysis of the diffraction peak intensities observed in substrate [319]. The data points () intersected by the three scan curves, the angular distribution of scattered He atoms provides the labeled by the respective incident angles θi ¼411,401 and 381, correspond to corrugation function of the surface, which essentially corre- the peaks L, R and S observed in the corresponding energy-transfer spectra (b). sponds to the profile of the atom–surface repulsive potential The diffused elastic scattering peaks E in (b) originate from surface defects. [407,408]. In general a corrugated surface provides the The calculated scan curves correspond to the experimental incident energy E ¼29 meV and to a planar 901 scattering geometry (θ +θ ¼901). The reciprocal surface lattice vectors which allow He atoms to be i i f – branches L and S are assigned to the longitudinal and shear-vertical modes trapped into a surface bound state a process which leaves a of the Cs monolayer, respectively. The R branch tends in the long-wave limit signature in the angular distribution (surface resonances) and to the Rayleigh wave of the clean surface (RW), whereas at short waves, due to permits to determine the attractive part of the atom–surface an avoided crossing with the S branch, converts into a shear vertical Cs mode, potential. The combination of the diffraction data with the while the S branch tends asymptotically to the RW branch. The vertical dash- fi dotted line indicates the zone boundary of the Cs-layer quasi-hexagonal normal surface pro le derived from bound-state resonances Brillouin zone in the 〈110〉 direction; the full lines interpolating the data points yields a detailed knowledge of the surface structure as appears are obtained from a force-constant fit. Adapted from Ref. [319]. at the turning points of the scattered atoms [407]. Useful information on the ordered structures of AM submonolayers where k and k are the final and incident wave-vectors of the on a metal substrate, as well as on the related re-arrangement f i He atoms, K and K are the respective components projected of the surface electron charge density, has been obtained in this f i on the surface plane (Fig. 4(b)), and ℏωðQÞ is the energy of a way with HAS. A few examples are illustrated below in phonon of wave-vector Q defined within the first surface Section 4.2.1. Once the surface structure and the scattering Brillouin zone. The events with ℏωðQÞ>0 and ℏωðQÞo0 are potential have been characterized, the surface phonon disper- referred to as phonon creation and annihilation scattering sion curves can then be obtained from the TOF spectra of the processes, respectively. For the interpretation of the HAS scattered He atoms, after conversion to an energy-transfer scale scan [342,404,409,410]. Also the surface phonon dispersion curves experiments it has proven useful to consider the so-called curves connecting all possible values of ΔE ¼ℏω and for various ordered submono-, mono- and multi-layers of AM ΔK ¼ðQ þ GÞ which are accessible for a given incident atoms on metallic substrates have been measured in this way energy and set of incident and final scattering angles. The (Refs. [299,319] and Table 1). corresponding equation, similar to Eq. (3), is obtained by A schematic view of a typical HAS apparatus is displayed in combining Eqs. (5) and (6) in the form Fig. 4(a), together with a description of the kinematics (inset (b)). The HAS kinematics is similar to that of HREELS, except Δ 2θ Δ 2 E þ ¼ sin i þ K ð Þ that He atoms at thermal energies carry sufficiently large 1 2 1 11 Ei sin θf Ki momentum to allow also for umklapp scattering processes where surface reciprocal lattice vectors G may be exchanged. As shown in the example of Fig. 5 for a quasi-hexagonal In this case the energy and parallel momentum conservation monolayer of Cs on Cu(001) [319], the scan curves corre- laws for one-phonon processes read sponding to the experimental HAS energy-transfer spectra of Fig. 5(b) allow to plot the data point in the (Q, ℏω)-plane ℏ2 Δ ð 2 2Þ¼ℏωð ÞðÞ (Fig. 5(a)) associated with the observed peaks. In this way E kf ki Q 9 2M three acoustical branches and an optical resonance of the overlayer/substrate system can be resolved with a resolution Δ ¼ð þ ÞðÞ K Kf Ki G Q 10 better than 1 meV. 314 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

In the distorted-wave Born approximation (DWBA) the effects like bound-state inelastic resonances being considerably transition probability of a He atom from an initial state of more important in the scattering from corrugated surfaces like wavevector ki to a final state of wavevector kf for a scattering those of closed-shell solids. process from a flat monoatomic surface at T¼0 K, in which Not so on the other extreme of He scattering from metal one phonon of energy εQv, parallel wavevector Q and branch surfaces. In the specific case of free-electron surfaces and, as index ν is created, is given by (up to a constant factor) far as concerns the present review, alkali overlayers on a [411,412] metallic substrate, the conventional HAS theory based on direct atom–atom interactions is irrealistic. Surface atoms are kf 2 Pðki; kf Þp ∑QvjFfiUuQvj δðΔE þ ℏωQvÞð12Þ shielded by a quasi 2DEG. The flying-by He atoms just tickle jk j iz the surface about 0.3 nm above the first atomic layer, and can where uQv is the surface phonon displacement vector. In the therefore transmit vibrational energy to the surface atoms more general case of He-surface interaction extending to mostly via the interposed free electrons, very much in the subsurface layers (labeled by index l>0) of a polyatomic same way effective interionic forces are described in the lattice crystals, uQv is defined in the extended vector space (l,κ,α), dynamics of free-electron metals. In this case the inelastic He where α is a Cartesian index, l labels the atomic layers, and κ scattering intensities are essentially weighed by the electron– the atoms in the crystal unit cell. The force Ffi exerted by the phonon interaction, and the range of interaction of the He atom He atom on the crystal atoms acts in the same space, and the with the crystal atom displacements is that of the electron– scalar product FfiUuQv is the atom–phonon coupling energy. phonon interaction. This fact explains the otherwise surprising It should be remarked that the interaction between the probe ability of HAS to detect in thin metal films also subsurface atom and the phonon atomic displacements has to be treated on phonons eventually localized several layers beneath the surface the same foot as the interactions between the crystal atoms [402,403]. The mechanism is schematically illustrated in the which govern surface dynamics. In closed-shell ionic crystals diagram of Fig. 6: the atomic motion in a deep atomic layer or rare-gas solids, lattice dynamics is based on two-body (phonon Qν) produces, via the electron–phonon interaction, a interatomic forces, only weakly perturbed by ion (atom) charge density oscillation extending up to the surface, which is polarization effects, which may be accounted for by additional perceived by a flying-by atom. As a consequence the atom is valence electron shell coordinates (shell models). Similarly the inelastically scattered from the initial state ki into the final atom scattering from ionic (or rare-gas solid) surfaces is well state kf. described by direct two-body interatomic potentials and the corresponding force ∂ at ¼ ; ; ð ; Þ ; ð Þ Ffi kf z iQ ∂ vlκ Q z ki z 13 z where k; zi are the distorted He-atom wavefunction components for the motion normal to the surface and vlκðQ; zÞ the two-dimensional Fourier transform of the two-body potentialvlκðrÞ between He and the κth ion (atom) in the lth at layer. The coupling force Ffi is fast decaying for increasing l, so that the interaction with insulator surfaces is normally = at restricted to the surface layer (l 0). Moreover Ffi decays exponentially for an increase of both parallel (ΔK) and normal (Δkz) components of the momentum transfer, the faster decay occurring for a softer interatomic potentialvðrÞ. This fact, already established by Zener [413,414] and by Jackson and Mott in the thirties for an exponential soft wall potential [415], causes a similar decay of Pðki; kf Þ at larger energy transfers. The relationship of the decay with the nature of the interatomic potentials is not trivial, however. For example the addition of a van der Waals attraction to the hard-core exponential repul- sion, while extending the range of the potential, yields a slower ð ; Þ Fig. 6. The mechanism by which a He atom impinging on the metal surface decay of P ki kf due to the acceleration of the probe atoms with incident momentum k is inelastically scattered into a final state k by – i f when entering the attractive well (Beeby effect) [411,412,416 creating a phonon Qν several layers beneath the surface. The atomic motion in 418]. In a shell-model picture of surface dynamics the a deep atomic layer generates, via the electron–phonon interaction g, a virtual repulsive interaction of the He atoms with the surface is due electron–hole pair, i.e., an electronic transition around the Fermi level from ′ ′ to the overlap with the surface ion (atom) shell and their state Kn to state K n . The associated charge density oscillation (red and blue contour lines) extends up to the surface and causes the inelastic scattering of a interaction with phonons should be mediated by the shell–core flying-by atom from the initial state ki into the final state kf. (For interpretation force constants (inverse polarizabilities). This does not appear of the references to color in this figure legend, the reader is referred to the web to be necessary in the calculation of HAS intensities, other version of this article.) A. Politano et al. / Surface Science Reports 68 (2013) 305–389 315

Thus the HAS one-phonon transition probability due to this (frustrated translational modes). Frenken, Toennies and Wöll electron-mediated mechanism retains the form of Eq. (12) have shown in 1988 [423,424] that HAS also provides informa- where the He-atom–phonon coupling can be expressed by tion on the microscopic diffusion of single atoms along the [402] surface. The presence of random adsorbates yields a diffuse 2 elastic peak (E-peak in Fig. 5(b)) of intensity proportional to the el U ¼ ðΔ Þλ ð Þ Ffi uQν I E Qν 14 coverage. The non-instrumental part of the E-peak energetic broadening Γ (FWHM) of the is due to the Doppler shift in the the coefficient λ ν is the electron–phonon coupling strength for Q scattering from single moving adsorbed atoms, and its depen- the specific phonon (Q,ν) (the mode-lambda) [419] and I(ε)is dence on temperature and parallel momentum transfer allow to a slowly-varying function of the energy. Their dimensionless determine the adatom diffusion coefficient and its temperature product is given by dependence. This kind of spectroscopy is generally referred to as 2 quasi-elastic HAS (QHAS). IðεÞ λ ν ffi ∑ ∑ 0 g 0 ðK; K þ Q; νÞ Q ð Þε2 Kn n nn N EF The QHAS method is an adaptation to the surface of the 2 n well-known method of neutron quasi-elastic scattering, which 〈f jψ ðrÞψ þ 0 ðrÞji〉 ð15Þ Kn K Qn has been extensively applied to studying diffusion in the bulk – where N(EF) is the density of states at the Fermi energy EF, [425 427]. It should be noted that both techniques do not and measure directly the diffusion coefficient but rather a correla- ΔK Δ 1 2 tion function S( , E). This is the Fourier transform of the g ðK; K þ Q; νÞ¼Vðq Þq Uu νexp½ 〈 q Uu νj 〉 nn′ nn′ nn′ Q 2 nn′ Q time-dependent pair correlation function G(R,t) which pro- ð16Þ vides a complete description of the dynamical behavior of the is the electron–phonon matrix element connecting an electronic ensemble of diffusing particles. G(R,t) is the sum of the self- ψ correlation function Gs(R,t), describing the behavior of a state of band index n, wavevector (K, kn) and wavefunction Kn to specific particle, and a second term Gd(R,t), which describes a state of band index n′, wavevector (K+Q, kn′) and wavefunction ψ ′ the pair correlation between distinct particles. Gs(R,t)isof KþQn′ [420]. V(qnn ) is the Fourier transform of the electron ð ; Þ ½ ð particular interest since it provides information on the micro- pseudopotential and the wavevector qnn′ Q kn kn′ exp i kn scopic tracer diffusion coefficient. The extension of the theory kn′Þzlκ acts on the (l,κ,α)-space, with zlκ the depth of the (lκ)th for the neutron case [426–428] to helium scattering from atomic layer. The electronic wavefunctionD product inside the species diffusing along a crystal surface was first developed by fi i matrix element between the nal f and initial i states of the Levi and co-workers [429] and reviewed by Frenken and He atom acts as an effective scattering potential for the inelastic Hinch [430]. The resulting expression for the transition processes. Thus the inelastic scattering probability for a given probability is given by phonon is weighed by the respective mode-selected electron– ð ; Þp 2∬ ð ; Þ iðΔK U RωtÞ 2 phonon coupling. Due to the particular structure of the effective P ki kf ndF G R t e d Rdt potential, the inelastic scattering intensities may drop at larger 2 pndF SðΔK; ΔEÞð18Þ energy transfers more slowly than for the direct atom–atom collision model. where nd is the surface concentration of diffusing particles and All intermediate cases between closed-shell surfaces and F is a constant atomic form factor. Thus, by measuring the Γ ΔK free-electron metal surfaces, like those of covalent semicon- energetic broadening ( ) for a wide range of wave-vectors, ΔK Δ ductors, transition metal compounds or even transition metals, the Fourier transform S( , E) of the time-dependent pair may require for a better approximation the inclusion of both correlation function is determined directly. Measurements at direct and electron-mediated He-phonon forces: small wave-vector transfers therefore provide information on correlations over long distances ℓ2π=ΔK and, similarly, j U j2 ¼j at U j2 þj el U j2 ð Þ Ffi uQv Ffi uQv Ffi uQv 17 events with small energy transfers ΔE provide information before the advent of DFPT, consistent calculations of surface over long times th/Γ. With the present resolution of a HAS 12 dynamics and inelastic HAS amplitudes could be made for apparatus broadenings down to ΓE10 meV (to4 10 s) 1 noble metal surfaces with the multipole expansion method and and ΔK>0.03 Å (ℓo200 Å) can be explored [431]. its parametrized form known as the pseudocharge model Due to the large scattering cross sections from single AM [421,422]. In this approach both direct and indirect (via atoms on smooth metal surfaces [432], even coverages as low conduction electrons) terms of Eq. (17) were shown to be as 1% can be studied with the QHAS technique. At low R necessary for an accurate description of the HAS intensities. coverages only the self-correlation function Gs( ,t) is important and in this case simple formulas are available relating the Γ fi 2.3. QHAS energetic half-width to the adatom diffusion coef cient D. The half-widths for jump diffusion is given by [428] At a low coverage of adsorbed atoms surface diffusion is an 1 2 ΓðΔKÞ¼4ℏ∑iτ sin ðΔKUai=2Þð19Þ important component of their dynamics intervening in many i important phenomena. Diffusion is triggered by surface vibra- where τi is the inverse of the jump rate in the direction of the tions, mostly by the adatom vibrations parallel to the surface jump length vector aj. Clearly the method is sensitive to the 316 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

Fig. 7. (a) A series of HAS energy-transfer spectra for an incident energy of 11.2 meV showing only the quasi-elastic central peak from a 2.8% monolayer coverage of Na atoms on Cu(001), for increasing parallel momentum transfers ΔK along the symmetry direction at T¼300 K. The open circles show the experimental points. The solid line through the data points is a convolution of the instrument response function (dashed line) with a Lorentian-shaped peak ﬁtting the quasi-elastic broadening of the central peak. (b) The FWHM of the deconvoluted quasi-elastic peak as a function of ΔK illustrates the diffusion dynamics of Na adsorbates on Cu (001) at different temperatures T¼200 K, 250 K and 300 K; the coverage was 0.028 ML for the ΓΜ direction and 0.047 ML for the ΓΧ direction. The solid lines show the result of a Langevin molecular dynamics simulation of Γ(ΔK) as given by Eq. (20). (Adapted from Refs. [301,306].)

τ1 direction of the diffusive jumps since the jump rate i A more detailed analysis of the Na/Cu(001) experiments depends very much on both the jump length and the relied on a molecular dynamics simulation based on the corresponding potential barrier height. For one preferential Langevin equation for an assumed model for the potential jump direction and small momentum transfers in that direction, hypersurface describing the lateral motion of the Na atoms Eq. (19) reduces to within a unit cell [433,434]. The simulations reveal that the data are very sensitive to the detailed shape of the lateral ΓðΔKÞffiℏa2τ 1ðΔKÞ2 ¼ 2ℏDðΔKÞ2 ð20Þ i i potential. The reliability of the potential fitted to the diffusion This corresponds to the continuous diffusion limit [428] and data (Fig. 7) was subsequently confirmed by successfully allows for a direct estimation of the diffusion coefficient simulating both the measured temperature dependence of the ¼ 2= τ ΓðΔ Þ D ai 2 i from the measurement of K . The activation T-mode energy and its width without an additional adjustment barrier for diffusive motion Ediff is then obtained from the of the potential parameters (see Section 4.2) [301,307]. temperature dependence of D expressed by the Arrhenius law: At high temperatures the diffusion barriers measured by QHAS agree with those deduced from other more conventional D ¼ D expðE =kTÞð21Þ o diff methods. In all cases in which the measurements are carried Fig. 7 shows some examples of QHAS measurements for out at RT or below, the activation barriers are consistently the diffusion of Na adatoms on Cu(001) at low coverage, quite smaller than in previous measurements. For example for where a nearly pure single jump mechanism is observed Na on Cu(001) from the dependence of the peak width on the [301,303,306]. The change in width Γ(ΔK) of the quasi- surface temperature between 180 K and 450 K the diffusion elastic peak with the parallel momentum transfer (Fig. 7(a)) is barrier was found to be only Ediff=54 meV. This barrier is obtained by deconvoluting the peak shape measured at the much smaller than the barrier energies of about 1 eV found for given temperature with the peak shape measured at a low other alkali-metal systems [435]. The pre-exponential diffusion temperature, where the diffusion-induced peak broadening is constants Do, Eq. (21), are generally much larger than vanishingly small and only the intrinsic instrumental width previously reported, providing some compensation for the remains [431]. The deconvoluted Γ(ΔK), plotted as function of differences in the barrier energies. These differences to ΔK in Fig. 7(b), illustrates the diffusion dynamics of Na previous results are attributed to the relative insensitivity of adatoms on Cu(001) at different temperatures, for a coverage the QHAS method to the impeding effect of surface defects. of 0.028 ML for the ΓΜ direction and 0.047 ML for the ΓΧ For example, the much smaller value of Ediff for Na on Cu direction. The solid lines show the result of a Langevin (001) was confirmed for potassium on Pd(111) by a photo- molecular dynamics simulation of Γ(ΔK) as given by Eq. (20). electron emission microscopy technique. With this technique it A. Politano et al. / Surface Science Reports 68 (2013) 305–389 317

Table 2 Diffusion barriers of isolated AM adatoms on Cu(100) as derived from QHAS measurements and DFT calculations.

Na K Cs

Bridge: expt. 75 meVa [301] – 2072 meV [69] DFT 79 meV [21] 31 meV [70] 13 meV [21] Top: expt. 84 meV [301] – 2072 meV [69] DFT 143 meV [21] 52 meV [70] 25 meV [21]

aHere the diffusion barrier is 52.5+0.9 meV whereas our fit gave 56 meV. is possible to observe regions on the surface where diffusion occurs on smooth defect free terraces without encountering step edges or other defects [436]. The relative insensitivity of the QHAS method to defects is related to special features of the QHAS technique. For one it is only sensitive to the actual motion of the particles as reflected by the integral over time in Eq. (18). Moreover, since only small distances of the order of E100 Å or less are probed, defects, which are generally more widely spaced, will not significantly influence the results. The important role of defects which generally lead to an increase in the effective barrier energy in the conventional diffusion measurements is, on the other hand, well documented [437]. This explanation is also consistent with the good agreement in Fig. 8. The microscopic potential energy surface V(x,y) for Na/Cu(001) the high temperature measurements. At high surface tempera- determined from quasi-elastic broadening measurements. The Cu atoms are situated at the four points (x,y)=(71.28,71.28) and the minima correspond tures the defects are, on the one hand, much more mobile and, to the hollow sites. The classical barrier for motion along the 〈110〉 azimuths is at the same time, the surface is continually being annealed. of 71 meV, and for motion along 〈010〉 azimuths is of 84 meV. Even the slight In conclusion, QHAS experiments have provided for the minima of the four-fold sites postulated from LEED data are reproduced (from first time a complete microscopic description of the diffusion Ref. [306]). of atoms on nearly ideal terraces of single crystal surfaces. Now that all the vibrational parameters of both the substrate and that of the adsorbate are known for many systems, even high-resolution inelastic neutron scattering spectroscopy [446– 3 more detailed calculations of the microscopic dynamics can be 448]. The HeSE Heidelberg apparatus was however restricted fully justified. Such calculations combined with further refined to incident beam energies of about 3 meV, which is too low for data will lead to a better understanding of the nature and accessing the full range of energy transfer needed for measur- relative importance of the phonon and electronic coupling ing the full surface phonon dispersion curves at least in the between adsorbate particles and the substrate degrees of acoustic region. More recently in Cambridge, UK, and soon at 3 freedom. These latter processes are fundamental for under- Technion in Haifa, a substantial upgrading of the HeSE standing many microscopic processes occurring on surfaces apparatus has been achieved, with an incident beam energy such as diffusion and inelastic scattering of molecules, and which has been by now raised up to about 14 meV [310,449– ultimately for understanding problems of technical importance 451]. This is sufficient for measuring the dispersion curves of such as friction and energy accommodation coefficients. surface acoustic modes in most surfaces, and particularly for With regard to ultra-slow dynamics characterizing the weakly bound adsorbate layers [451]. 3 diffusion or drift of heavier atoms like Cs or AM clusters, HeSE exploits the effective time-reversal imposed to the etc. the 3He spin-echo scattering spectroscopy, illustrated in nuclear spins by a surface reflection. The spin dephasing in 3 the next sub-section, with its much greater resolution of about time of an incident spin-polarized He beam due to unequal 20 neV, holds great promise, and a few QHAS experiments atom velocities, and the consequent loss of magnetization, are have already been published [306,438–443](Table 2). restored by time reversal after reflection (rephasing). If the spin-polarized incident and reflected beams travel through two identical magnetic fields (Fig. 9), the Larmor precession 2.4. 3HeSE respectively encodes and decodes the atom velocities before and after reflection. For elastic reflections no change of On the experimental side further progress in the high- velocity occurs and no change in magnetization is observed resolution surface spectroscopy of low-energy processes was at the detector with respect to the initial incident beam made in the late nineties by the group of DeKieviet in magnetization, whereas any change of magnetization signals Heidelberg with the development of 3HeSE spectroscopy an inelastic scattering process. Here, the time-reversal Larmor [444,445]. 3HeSE is a successful adaptation of the spin-echo precession of the nuclear spins, rather than the actual time of method invented in the early seventies by Ferenc Mezei for flight, works as a “clock”, and the change of magnetization 318 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

less than with the conventional apparatus. To compensate for this a new type of detector with a greatly lengthened ionization region of about 5 cm compared to the short 5 mm ionizer in the TOF apparatus was developed for the Heidelberg apparatus [452]. Another demanding requirement in the construction of a 3HeSE apparatus is the great precision and advanced machin- ing technology in the construction of the hexapoles and the precession magnets [453]. Moreover the use of the much more expensive rare isotope 3He requires a special gas handling and recycling system to keep losses to a minimum. On the other hand the spin-echo technique has the advantage that the resolution is largely independent of the beam velocity, so that experiments with any atomic species carrying a magnetic 3 3 4 n 3 Fig. 9. Schematic diagram of a He spin echo spectrometer. The He atoms moment, like He (2 S1) metastable atoms, as well as o-H2 form a free jet expansion are first polarized on passing through a magnetic and p-D2, and even fermion alkali atoms, can be envisaged. hexapole magnet. In a longitudinal magnetic field the atoms then press several 3 thousand times before being scattered. In a second identical, but oppositely The superior energy resolution of HeSE spectroscopy has poled field, the atoms precess in the opposite direction. The final polarization is been demonstrated in a few remarkable experiments, one on analyzed by a hexapole field mounted in front of the detector. Small selective adsorption [401,454] and the others for quasi-elastic differences in the precession phase provide information on the change in scattering from diffusing alkali metals, notably Na [71],K velocity after scattering. [70], and Cs atoms on Cu(001) [69]. Some notable results for the latter two adsorbates, extracted from Jardine et al. [68] and Hedgeland et al. [69] are illustrated in Figs. 10 and 11, allows to measure the inelastic scattering energy loss with respectively. Fig. 10(a) shows the dispersion curves of the unprecedented resolution. Before starting the clock the 3He frustrated translation (T-) mode for ordered phases of Cs atoms first have to be spin polarized. In the Cambridge adsorbed on Cu(001) at three different low coverages (∎: apparatus this is achieved in a carefully designed 30 cm long 0.056 ML; : 0.044 ML;▲: 0.028 ML) measured with 3HeSE hexapole magnet [449]. The polarized atoms are then lined up along the 〈100〉 direction at a surface temperature of 130 K perpendicular to the flight direction before they start to precess [68]. The full lines are sine-law fits, while the broken line on entering a nearly 1 m long solenoid. The solenoid is represents the RW dispersion curve of the clean Cu(001) designed to provide a homogeneous longitudinal magnetic surfaces. The calculated curves are interrupted below 0.1 field in which the nuclear magnetic moment of the 3He meV because it is not clear from the experiment whether the precesses several thousand times before arriving at the target. experimental dispersion curves terminates at a finite energy for After scattering the atoms are passed through a second ΔK-0, as found for the lighter AM's due to a residual shear oppositely directed but otherwise identical magnetic field, force constant between the AM ion and the surface. Despite where they undergo a similar number of precessions, resulting the superior resolution of 3HeSE the T-modes of the Cs- in an “unwinding” of the polarization acquired in the first adatoms in the ΔK-0 limit can hardly be distinguished from solenoid. Finally the polarization of the beam leaving the the RW dispersion curve. The comparison with the HAS second solenoid is determined by passing the beam through dispersion curves of 1 ML of Cs on Cu(001) reproduced in another hexapole field before the beam arrives at the detector. Fig. 5 illustrates the ability of the 3HeSE technique to measure The resolution achieved with the spin-echo technique is phonon energies one order of magnitude smaller. This is even nearly four orders of magnitude greater than with the conven- more evident from the 3HeSE measurements in the μeV range tional TOF apparatus. In the Cambridge apparatus a precession of the quasi-elastic (QE) peak width plotted in Fig. 10(b) as a phase shift of about 81 (0.14 rad) can be resolved. The total function of the parallel momentum transfer for the 0.044 ML phase amounts typically to about 1.9 104 rad (3000 revolu- of Cs on Cu(001). The data, referring to the 〈100〉 direction and tions). Thus the fractional phase resolution is about Δφ/ a surface temperature of 130 K, show the characteristic de φffi7.4 106. The actual energy resolution is even some- Gennes dip at the G-vector of the periodic adatom lattice (here what more favorable by about a factor 3 [449]. Thus for an at 0.55 Å1) [68]. The fitting curve is calculated from Eq. (19) incident beam energy of, e.g., 8 meV an energy resolution of with the inclusion of only 1st and 2nd neighbor jumps, with 6 about δE/E¼2.5 10 corresponds to a resolvable change in ℏ=τ1 ¼6.9 meV and ℏ=τ2 ¼4.9 meV, respectively; it provides energy of δE¼20 neV [450]. Compared to the best resolvable an apparently better fit than that obtained from a molecular δE¼300 μeV achievable with the TOF technique [431], the dynamics simulation in Ref. [68]. The measured temperature improvement is about 1.5 104. The resolution is largely dependence of the QE peak width for this coverage has independent of the incident beam velocity spread since the provided an effective activation energy of 31+2 meV [68]. winding and unwinding of the spin precession is always the The panels (a) and (b) of Fig. 10 provide a convincing same The dramatic improvement in resolution is acquired, demonstration that 3HeSE spectroscopy can fully characterize however, at a high price. Because of the many stages and large the ultra-slow dynamical processes of weakly bound heavy overall dimensions of the apparatus, the intensities tend to be adsorbates occurring at metal surfaces. A. Politano et al. / Surface Science Reports 68 (2013) 305–389 319

Fig. 11. The quasi-elastic (QE) peak width measured with 3HeSE as a function of the parallel momentum (wavevector) transfer for a 0.018 ML of K on Cu (001) in the 〈100〉 ○ and 〈110〉 (□) directions at a surface temperature of 155 K [69]. The ﬁtting curves are based on Eq. (17) including 1st, 2nd and 3rd neighbor jumps (ℏ=τ1 ¼ℏ=τ2 ¼1.9 μeV, ℏ=τ3 ¼0.63 μeV for 〈100〉 and ℏ=τ1 ¼1.2 μeV ℏ=τ2 ¼1.5 μeV, ℏ=τ3 ¼0.40 μeV for 〈110〉); the temperature dependence of the QE peak width measured in the 〈100〉 direction for 0.056 ML provides an effective activation energy of 26+2 meV.

the equation [506]: Fig. 10. (a) Dispersion curves of the frustrated translation mode for ordered 2 3=2 1=2 ðq=z0Þ ¼ Aβnðz0Þ=½1 þ θ =ð1 þ θÞ ð22Þ phases of Cs adsorbed on Cu(001) at low coverages (∎: 0.056 ML; : 0.044 ML; ▲: 0.028 ML) as measured with 3HeSE spectroscopy along the where q is the effective AM ion charge, A is a constant, β is the 〈100〉 direction at a surface temperature of 130 K [68]; the full lines are sine- inverse decay length of the metal surface charge density n(z), law fits, the broken line is the RW dispersion curve of the clean Cu(001) θ 3 and is the adatom coverage. Clearly for a given q, z0 must surfaces. (b) The quasi-elastic (QE) peak width measured with HeSE as a θ function of the parallel momentum (wavevector) transfer for a 0.044 ML of Cs increase for increasing . It is somewhat surprising that the on Cu(001) in the 〈100〉 direction at a surface temperature of 130 K [68]. The addition of charge to the Cu(001) X surface states, resulting for fitting curve is based on Eq. (19) including only 1st and 2nd neighbor jumps low coverages in the appearance of a small Cu(001) surface ℏ=τ ¼ μ ℏ=τ ¼ μ ( 1 6.9 eV, 2 4.9 eV); the temperature dependence of the QE peak corrugation growing with coverage (see Section 4.2.1), has width for this coverage provides an effective activation energy of 31+2 meV. apparently not been investigated so far with 3HeSE, nor the peculiar effects of the Lau–Kohn forces, as distinguished from the dipolar forces, have been considered in the mentioned The thorough 3HeSE study of K overlayers on Cu(001) studies of AMs on the Cu(001) surface [68–70]. From this 3 performed by Hedgeland et al [69] reports the QE peak width brief analysis HeSE appears to be a choice method for the (Fig. 11) as a function of the parallel momentum transfer along study a slow surface diffusive processes as well as of the the two symmetry directions 〈100〉 and 〈110〉. The plots dispersion relations of low-energy phonon and librational indicate a more complex structure of de Gennes dip which modes of large adsorbed molecules. As concerns AM on requires the inclusion of diffusive jumps up to the third metal surface, a more complete discussion is presented in the neighbor sites in Eq. (19) for a reasonable fit of the QE peak next sections. widths. Also in this case Eq. (19) seems to make a better job than molecular dynamics simulation. 2.5. RAIRS An important piece of information obtained from 3HeSE measurements is the vertical motion of the adsorbates leading RAIRS is a reflection-based technique which can be applied to an increase of the adsorption height z0 (the distance of the also for surfaces in contact with a gas phase, provided that the adatom from the first surface atomic layer) with the increase of density of the gas phase is not so high to block the IR-beam in the coverage. This effect is well accounted for by the Lau– the spectral ranges of interest. In particular, it can be used for Kohn forces originated from the electrons donated by the AM investigating adsorption at metal surfaces. RAIRS involves an atom to the surface and stored in the surface states of the Cu impinging IR beam penetrating the thin film, followed by (001) surface at the four X symmetry points. It is expressed by reflection at a metal surface and the subsequent retransmission 320 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

Fig. 13. Typical experimental apparatus for RAIRS. The experiments are performed by focusing the IR beam from a commercial Fourier-transform infrared spectrometer through a polarizer and a KBr (or NaCl) window onto the sample at grazing incidence. The reﬂected beam passes through a second KBr window and it is refocused onto either a mercury-cadmium-telluride or an indium-antimonide detector. Such conﬁguration for IR in UHV conditions has been developed by several groups [460] with intensities as low as 2–3 106 absorbance units.

Theoretical foundations of RAIRS could be found in Ref. [456]. Since the electric field is perpendicular to the metal Fig. 12. Vibrations of diatomic molecules adsorbed in bridge geometry on a surface, the absorption occurs only in the reflectivity of p- solid surface. Only A modes are dipole active, while B modes are not polarized light. The angular dependence of the change in the observable in dipole scattering. reflectivity due to surface absorption can be calculated by applying the Fresnel-boundary conditions of conventional optics to a three-layer system consisting of vacuum, the through the film, before the beam enters the detector. An adsorbate layer and the metal substrate [456,457]: analysis of the reflected beam provides information on the 8π sin 2θ 1 32π2 sin 2θ molecular vibrations in the surface film, and can be used to Δ ¼ ffi α ðωÞðÞ Rp λ θ dIm ε ðωÞ λ θ nsIm ? 24 identify the surface species. In particular, adsorbed and gas- cos ? cos phase species are distinguished by taking the difference of the where λ is the wave-length, d is the film thickness, ns is the spectral response for s- and p-polarized light. surface concentration of adsorbates while ε┴ and α┴ are the The IR process is governed by a direct absorption of a perpendicular component of the dielectric function and of photon (usually from the vibrational ground level) to an polarizability at the surface, respectively [458]. excited state. As the electric field is perpendicular on a metal The spectral range of RAIRS is limited by window materials surface, only modes with a perpendicular dipole moment (the (such as NaCl which has a lower limit of 75 meV and KBr totally symmetric modes, Fig. 12) can be excited (for a review which has a lower limit of 50 meV). The spectral region up to see Ref. [455]). For the same reason the absorption occurs only 50 meV is therefore difficult to investigate by FT-RAIRS, in the reflectivity of p-polarized light. As a matter of fact, the while infrared spectroscopy has a high inherent resolution for dipole moment μ must change with respect to the normal modes with vibrational energies up to 300 meV [459]. Even coordinate Q during a vibration, that is with the newly available synchrotron light sources the very ∂μ low-energetic frustrated translational modes with typical vibra- a ð Þ tional energies lower than 10 meV are not accessible to this ∂ 0 23 Q technique. In the case of adsorbates on metal surfaces, only vibrational modes with a non-zero component of the dynamic dipole 2.6. SERS moment perpendicular to the surface can be observed. This originates from the screening effect of the electrons of the Raman spectroscopy is a powerful optical tool for providing metal which produces an image dipole within the metal. The information about the vibrational properties of molecules. resulting dipole will vanish if the molecular dipole is parallel However, the application of Raman spectroscopy in biological to the surface. detection is impeded by the relative low efficiency of Raman RAIRS spectra are commonly collected using Fourier scattering due to the small optical cross section of molecules transform infrared spectrometers because of their high-resolu- (typical Rayleigh scattering cross sections of molecules are in tion, high sensitivity, ease of use, and commercial availability the range of 1026 cm2 and typical Raman scattering cross [455]. In a typical RAIRS experiment (the usual experimental sections are in the range of 1029 cm2) [461]. Such problems apparatus for RAIRS is shown in Fig. 13), the IR beam is could be crossed by using the SERS technique which is able to reflected off the crystal surface at grazing angles in order to increase the Raman signals from a molecule by factors of 106– maximize sensitivity. 1012 (a review on SERS could be found in Refs. [462,463]). A. Politano et al. / Surface Science Reports 68 (2013) 305–389 321

incident field: E ðνÞ εε r 3 AðvÞ¼ M 0 : ð26Þ E0ðνÞ ε þ 2ε0 r þ d In this equation A(ν) is the strongest when the real part of the dielectric function ε(ν) is equal to 2ε0. In addition, the imaginary part of ε(ν) should be small. This condition occurs at the resonance excitation wave-length of the surface plasmon and assumes that an embedding medium does not have an imaginary portion in its dielectric constant (i.e., the metal nanoparticle is embedded in a non-absorbing medium). An additional enhancement factor of the order of 10–102 Fig. 14. A simple schematic illustrating the concept of the electromagnetic [462] is represented by a chemical enhancement as a conse- SERS enhancement for a Raman active molecule at a distance d from the quence of electronic coupling. It implies a charge transfer, surface. EM is the field experienced by the Raman molecule and it is a resulting in new electronic transitions between molecules and fi fi combination of the incident eld E0 and the induced dipole eld in the metal the metal surface, so as to create a molecule–surface complex. nanoparticles ESP. Hence, EM ¼E0+ESP.

2.7. SFG This great enhancement of Raman intensities is generally achieved by exciting vibrational transitions in molecules SFG is a nonlinear optical phenomenon that occurs when directly or in close vicinity to a roughened metal electrode. high intensity radiation interacts with a medium that, as a In general, the enhancement of Raman signals through SERS result, radiates at the sum of the frequency of the incident is given from two contributing mechanisms, namely the radiations [466]. SFG can be considered as the conversion of electromagnetic mechanism and the chemical mechanism. two photons with energy ω1 and ω2 into a single photon with The electromagnetic mechanism which is believed to be the energy ω¼ω1+ω2. Symmetry considerations imply that, dominant mechanism responsible for the enhancements found within the electric dipole approximation, SFG is forbidden in in SERS, explains enhancements primarily due to the collec- the bulk of centrosymmetric media, whereas at surfaces or tive electromagnetic resonance (or localized plasmon reso- interfaces the symmetry is broken and SFG becomes allowed nances), which refers to the excitation of collective oscillation [467]. Accordingly, for centrosymmetric media such as Si and of free electrons shared by the material in conduction bands amorphous oxides SFG possesses surface and interface speci- – [462 464]. A strongly localized plasmon resonance, supported ficity of unusual purity and generality. fi by a metallic nanostructure, is followed by modi cations in the In local optics the polarization P(r, t) at the position r and at local electromagnetic density. time t depends only on the field E(r, t) at the same position and This can occur, for example, when a small metal sphere is time. The Taylor-expansion of the polarization in terms of the fl fi in uenced by electromagnetic eld provided that the radius of field is the sphere is much smaller than the wave-length of electro- ð ; Þ¼ε ½∑ χð1Þ ð ; Þþ∑ χð2Þ ð ; Þ ð ; Þ magnetic radiation. Fig. 14 shows that a Raman active Pα r t 0 β αβ Eβ r t βγ αβγEβ r t Eγ r t d molecule placed at a distance, , away from a metal nano- þ ⋯ð27Þ particle of radius, r, will experience a total electromagnetic α β γ field which is the superposition of the incoming field (E0) and where , , denote the components in Cartesian coordinates, χð1Þ χð2Þ the electromagnetic field of dipole (Esp) induced by the metal α;β is the conventional susceptibility tensor, and α;β is the sphere. The electromagnetic field (Esp) on the metal particle's second-order susceptibility tensor. A special case of sum- surface is a surface plasmon which is expressed as [465] frequency generation is SHG, in which ω1¼ω2¼(1/2)ω3. This is the most common type of sum-frequency generation in 3 εε0 r experimental physics as only one input light beam is required. EspðvÞ¼ E0 ð25Þ ε þ 2ε0 r þ d When the photon energy of either the fundamental or the SHG radiation coincides with an optical transition in the When the incident electromagnetic wave (E0) resonates with medium the SHG response is resonantly enhanced. In the electromagnetic of the dipole field induced from the surface Fig. 15 the generation of SHG radiation at the surface and of the nanoparticle (Esp), the incident field and the surface buried interface of a thin film is illustrated. plasmon reinforce each other resulting in a large enhancement Moreover, the SHG technique is a suitable probe for a broad of the electromagnetic field. This increase in the field intensity range of physical properties such as electronic surface and experienced by the molecule will lead to an increase in its interface states (e.g., dangling bonds) [469], surface and Raman scattered signals. interface roughness [470], externally applied and internal The field enhancement factor A(ν) (at a specific frequency electric fields [471], fixed and trapped charge [472], surface ν), for molecule in the vicinity of the sphere at a distance d is and interface strain [471,473], contaminants and adsorbed the ratio of the field at the position of the molecule and the species [469,474], and surface and interface symmetry [468]. 322 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

Fig. 15. Schematic representation of the generation of SHG in a thin film system consisting of a centro-symmetric film and substrate: an incident laser pulse with frequency ω induces the generation of second-harmonic radiation with frequency 2ω at the film surface and the buried film/substrate interface. From Ref. [468].

This broad range of detectable physical properties shows the versatility of SHG and illustrates the high potential of the technique in contributing to an enhanced understanding of surface and interface properties during processing. However, because of the sensitivity of SHG to such a broad range of physical properties the interpretation of SHG experiments can be difficult. In this respect, the combination with linear optical techniques that also probe bulk properties such as spectroscopic ellipsometry is very useful. A comparison between RAIRS, HREELS and SFG spectra has been reported for potassium-doped fullerenes in Ref. [475] (Fig. 16). It should be mentioned that the SHG technique is a suitable probe for a broad range of physical properties such as electronic surface and interface roughness [469] surface and Fig. 16. Comparison of RAIRS (upper panels), HREELS (center panels) and interface roughness [470] fixed and trapped charge [471] fixed SFG (lower panels) spectra recorded in the frequency range 161–186 meV 1 and trapped charge [472], surface and interface strain (1300–1500 cm ). Left panels: α-phase, C60 monolayer on Ag( 111). Central [471,473], contaminants and adsorbed species [469,474], and panels: β-phase, intermediate K-doping. Right panels: γ-phase, saturation surface and interface symmetry [468]. This illustrates the doping. Adapted from Ref. [475]. versatility of SHG and clarifies its high potential. However, the interpretation of SHG experiments is quite complicated and the vibration frequency spectrum is characteristic for the thus it is particularly useful its combined use with linear strength and the type of the bonds [345,458]. The development optical techniques such as spectroscopic ellipsometry, which of experimental techniques for studying surface vibrations was also probe bulk properties. thus essential for the advancement of surface chemistry. AM overlayers enhance the conversion efficiency of SHG by a few orders of magnitude in comparison with clean metal 3.1. Surface phonons surfaces [476]. There are two major origins of the SH enhancement associated with alkali adsorption: interband The vibrational excitations of clean, two-dimensional peri- transitions between surface electronic states and multipole odic surfaces, besides providing essential information for their plasmon excitation [477–479]. Fig. 17 shows the dependence complete characterization as substrates, are also relevant for of the SH intensity of 800 nm (hν¼1.5 eV) photons as a their coupling to the elementary excitations of the adsorbed function of the AM coverage on Cu(111). species as well as for their possible role in adsorption/ desorption phenomena, diffusion and surface chemical 3. Basic concepts in the theory of surface excitations properties. It is worth remembering that the quantized plane-wave Vibrational spectra of adsorbed species on surfaces can solutions of the equation of motion of all atoms in the solid provide important information on surface chemical bonds, as are the phonons of the 3D-solid. A flat surface or interface A. Politano et al. / Surface Science Reports 68 (2013) 305–389 323

Fig. 17. Second harmonic intensity as a function of coverage of (top panel) potassium and (bottom panel) cesium on Cu(111) surface. The excitation wave-length is 800 nm (E¼1.55 eV). Adapted from Ref. [476]. Fig. 18. The surface phonon dispersion curves (heavy full lines) of sagittal polarization for the Cu(111) surface along the ΓΜ symmetry direction, superimposed to the surface-projected bulk phonon bands (light gray area) are shown as a paradigmatic example of a surface phonon structure for a metal surface. The broken lines indicate the expected dispersion curves in the absence of avoided crossing between modes of equal symmetry. Quasi-shear- breaks the 3D-translational symmetry of the solid in one vertical (SV1, SV2) and quasi-longitudinal (L1, L2) modes have the largest direction (conventionally taken as the z-direction), which amplitude in the first or second surface atomic layer, respectively. The portions may give rise to solutions that are localized at the surface in of the phonon branches inside the bulk continuum describe surface resonances. the sense that the vibrational amplitude decays exponentially in Some important modes are conventionally labeled by Sj (S1 is the Rayleigh the interior of the solid. These modes are called surface wave (RW); S2 the longitudinal gap mode; S3 the so-called L resonance). All fi sagittal branches have been measured by HREELS or HAS. (Adapted from (interface) phonons and are classi ed by a parallel wave- Ref. [299]; the HAS data for S have been measured with a room temperature ν 2 vector Q and a branch index . A schematic overview over the He beam of energy Ei¼63 meV (G. Zhang, unpublished)). spectrum of eigenmodes at a surface is shown in Fig. 18. In the limit of a semi-infinite solid or of an infinitely thick slab with two parallel surfaces the bulk modes form a continuum plane. The odd modes are polarized perpendicular to the mirror corresponding, for each parallel wave-vector Q, to all possible plane and are therefore shear-horizontal (SH) transverse modes, values of the third wave-vector component qz contained in the whereas the polarization vectors of the even modes lie in the surface-adapted Brillouin zone of the 3D solid. There may be sagittal plane. Since the surface breaks the symmetry in the z- solutions of the surface dynamical problem whose frequencies direction, even in the symmetry directions where the sagittal fall into the bulk continuum. In this case one has surface plane is a mirror plane the modes are neither perfectly long- resonances, whose eigenvectors can propagate inside the solid itudinal (L) nor perfectly z-polarized (shear-vertical, SV). In but have a strongly enhanced amplitude in the surface region. general the two components are mixed and out of phase so as to In some symmetry direction of the surface there may be give an elliptical polarization in the sagittal plane (sagittal surface solutions inside a band of the bulk continuum whose polarization) However at certain symmetry points where the eigenvector is orthogonal to the eigenvectors of the band group velocity is zero the sagittal modes becomes purely L or modes. In this special case the surface mode is exponentially SV, and the L and SH modes may be degenerate (e.g. at the X decaying inside the solid, thus retaining a surface localized point of a hexagonal surface lattice). Away from these special character, but only along the symmetry direction. Any devia- points the coupling between L and SV displacements remains in tion of the propagation from the symmetry direction transforms most cases fairly weak so that the modes retain a quasi-SV (e.g., the local mode into a resonance. These special solutions are the Rayleigh waves (RW)) or quasi-L character (e.g., the S3 termed embedded surface modes. resonance at long waves). As for bulk modes, surface localized modes and resonances Another important feature of surface modes, and more often are classified according to their character (acoustic or optical) of resonances, is their maximum amplitude in the z-direction: and their polarization, the latter referring to the direction of the this is in general at the top surface layer, but also sub-surface parallel wave-vector Q and to the plane containing Q and the modes with the maximum amplitude in the second or third z-direction (the sagittal plane). When the sagittal plane layer may exist, as a consequence of the surface relaxation. coincides with a mirror plane of the structure (e.g., the [112] This occurs quite often in metals, due to the peculiar direction on the (111) surface of a monoatomic fcc crystal), the oscillating interplanar spacing near the surface, and in deeply surface phonons are even or odd with respect to the sagittal reconstructed semiconductor surfaces. 324 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

Fig. 16 shows the surface phonon spectrum of Cu(111) calculated with DFPT in the high symmetry direction ΓΜ with all surface branches of sagittal polarization in the first and second surface layers on top of the bulk continuum (light gray area). All branches have been measured by either HREELS or HAS and assigned by the theoretical analysis to either quasi- SV modes or to quasi-L modes, both in the first (SV1, L1) and second (SV2, L2) layer. Some modes which are usually found in all surfaces of equal structure have some conventional labels Sj: S1 labels the Rayleigh wave, S2 a gap quasi-L mode, S3 a strong resonance which is quasi-L at long waves and transforms into a SV2 mode at the zone boundary, etc. In the long-wave limit (Q-0) there must be three phonon – branches whose phonon frequencies go linearly to zero the Fig. 19. A schematic representation of the vibrational modes of single three acoustic branches imposed by the translational invariance adatoms, either strongly or weakly bound to the substrate, and of an adsorbed conditions. Besides the S1 and S3 acoustic modes of quasi-SV diatomic molecule. Their frequencies are superimposed to the surface phonon and quasi-L polarization, respectively, there is a SH acoustic dispersion curves and bulk phonon bands of the substrate (light gray area) in fi order to separate the high-frequency localized modes from the low frequency mode, which has in general a bulk character and only at nite modes, which respectively fall above and inside the substrate continuum, the Q may acquire a surface character (S7 mode) [480]. For a more latter acquiring a resonant character. The heavy lines represent the branches of complete information on surface phonons the reader is referred an atomic monolayer. For a strong adatom-surface bond the lateral interaction to the chapters of Ref. [409]. is negligible so as to give two dispersionless branches localized above the substrate continuum. For a week bond the two lowest branches show some 3.2. Vibrational excitations of adsorbates dispersion and an avoided crossing with the surface phonon branches of the substrate.

Other vibrational excitations of interest here are the vibration modes of isolated adsorbed atoms or molecules. Some additional adatom dispersion curves will exhibit avoided examples are depicted in Fig. 19. Single adsorbed atoms may crossing and consequent hybridization with the surface disper- form a strong chemical bond with the substrate or may be sion curves of the substrate of equal symmetry, as shown in bound to the surface by weak forces. Their stretching (S)or Fig. 19 and also in Fig. 5 for the S branch with the substrate frustrated translation (T) modes can alone inform about the RW modes. bonding nature since a strong bond may locate the two modes In the case of adsorbed molecules, the modes associated with above the maximum frequency of the substrate, while the their internal degrees of freedom fall often outside the range of corresponding modes of a weakly bound atom may fall in the substrate phonons due to the strength of the chemical bonds, and low acoustic region of the substrate. In the former case the two are therefore localized modes. Moreover the rotational and modes are localized, in the latter they acquire a resonant translational degrees of freedom of the molecule, being hindered character. It should be noted that the actual position of the by the surface potential and possibly by the formation of a bond, adatom frequencies depends not only on the bond strength turn into vibrational modes. Fig. 19 shows the case of diatomic but also on the ratio of the adatom mass to that of the molecule standing vertically on the substrate surface a forming substrate atoms. with the substrate a bond weaker than the internal one. A When the same atoms form a monolayer, say for simplicity hypothetical distribution of the molecular mode frequencies is a(1 1), the system recovers the translational symmetry of the also shown, where the internal stretching (S) is localized above substrate and the modes of the adsorbed layer contribute three the substrate maximum, whereas the external stretching (S′), the additional dispersion curves associated with the three degrees frustrated rotation (R) and the frustrated translation (T), or of freedom of the adatom. They are schematically shown in rocking (R′) modes are shown as resonances inside the substrate Fig. 19 for the S mode and for the two T modes by two heavy continuum. In case of a molecular monolayer there is a number lines. While at the zone center (Q¼0) the adatoms move all in of additional dispersion curves equal to the number of the phase, at the zone boundary (Q¼π/a with a the lattice spacing) molecular degrees of freedom, exactly as discussed above for the nearest neighbor adatoms move in opposite phase. In the the atomic overlayers. case of strong adatom–surface bond the lateral interaction is Both internal and external modes provide a rich information negligible so as to give two dispersionless branches localized about the chemical state of the molecule and its chemical above the substrate continuum. For a weak bond the lateral processes in the adsorbed phase. For example vibrational interaction may not be negligible and the corresponding spectroscopy can be used for observing the decomposition of branches (the lowest two in Fig. 19) show some weak complex molecules into fragments and the appearance of dispersion. When the lateral interactions among adatoms reaction intermediates on the surface. Moreover, the presence become comparable or stronger than the adatom–substrate or absence of a stretching frequency which characterizes a bonds a substantial dispersion occurs, as, e.g., for the case of a given molecular bond can give unambiguous indication about Cs monolayer on Cu(001) shown in Fig. 5. Moreover the the possible dissociation of a diatomic molecule. A. Politano et al. / Surface Science Reports 68 (2013) 305–389 325

Fig. 20. (a) The substitutional positions of AM adsorbates on Al(111) in the (√3 √3)301 structure as seen in a side view in the ½112 direction and (b) the corresponding SBZ drawn inside that of Al(111) with the respective irreducible parts in gray.

The frequencies of stretching modes also depend on the (a) together with the corresponding SBZ (b), have been adsorption site. As the coverage increases, the vibration modes thoroughly investigated with EAM by Chulkov et al. [312]. of the isolated species may couple with each other giving rise The calculated phonon dispersion curves are displayed in to a dispersion, i.e., to a dependence of the vibrational Fig. 21(b,c, and d) for the Li, Na and K adsorbed phases, frequency on the parallel wave-vector. The lateral coupling respectively, and compared with the phonon dispersion curves between adsorbates can be direct, like for example the dipole– of the clean Al(111) surface, plotted in Fig. 21(a). dipole interaction between polar molecules, as well as indirect It is important to note the smaller size of the SBZ (Fig. 20 through the surface electronic states of the substrate. Examples (b)) for the adsorbate phases (irreducible part: ΓΜ′Κ′) with of indirect interaction among adsorbed species are the Lau– respect to that of the clean surface (irreducible part: ΓΜΚ), and Kohn long-range forces occurring on certain metallic sub- the corresponding folding of the external portion of ΓΜΚ into strates [300]. Both kinds of lateral interactions affect the ΓΜ′Κ′. In particular the symmetry point Κ is folded into the overlayer dispersion curves and cause frequency shifts which zone center Γ, whereas Μ becomes equivalent to Μ′. This is depend on the local environment around the species. Other reflected in the folding of the original surface dispersion curves frequency shifts are caused by the anharmonicity of the of Al(111) at both points Μ′ and Κ′ into branches eventually potential, which can be enhanced at the surface due to lack separated by gaps: this is best appreciated for Na (Fig. 21(c)) of inversion symmetry. The main effect of anharmonicity in which has the smallest mass difference with respect to Al, so vibrational spectroscopy is the reduction of spacing between that the perturbation on the phonon branches is mostly due to two consecutive vibrational levels for increasing quantum the local change of force constants. For Na the lowest branch, numbers. Thus the observation of the overtones of external corresponding to the Rayleigh wave (here labeled S1) near the modes can provide further information on the molecule– zone boundary ðΜ′Κ′Þ is well localized below the acoustic substrate interaction potentials [306,336]. bulk edge and is degenerate with the upper branch R2atΜ′. A third branch just above, also partially localized around Κ′ 3.3. Dynamics of AM on metals with the EAM and having a shear-vertical (SV) polarization, is single. This branch descends below the (S1,R2) pair at the zone boundary for Before the advent of the DFPT [481], various semi- both Li and K adsorbates, acquiring a strong localized character. empirical methods have been suggested in order to account The softening is moderate for Li, though interesting as it for the fundamental role of free electrons in the phonon indicates that the expected frequency increase due to the lighter dynamics of metals and metal surfaces. Among these methods mass is overcompensated by the weakening of the local force one of the most effective and most largely used is the EAM, constants. For K both the mass increase and the force constant for which the reader is referred to the abundant existing weakening, the latter being due to the large outward relaxation of literature [482–484] The EAM, when directly compared to the K ions, concur in giving a very soft and flat SV branch at less DFPT (see for example [378]) qualifies as an expedient tool for than 4 meV. In is interesting to remark that for K at the zone a fast and reliable, albeit semi-empirical, analysis of metal boundary this SV branch is more than twice softer than the surface dynamics, especially for extended surface cells like longitudinal (L) and shear horizontal (SH) acoustic branches. those occurring in low-coverage adsorbate phases on metals, This behavior is also found to occur for K overlayers on which would require a large computational effort with DFPT. graphite [485] or Be(0001) (see below and Ref. [403]), where it has been associated with the formation of a surface quantum 3.3.1. AM/Al(111) well with a regular array of alkali atoms floating on the Lithium and sodium adsorbed on the Al(111) surface in the substrate surface and dipped into an almost uniform free- (√3 √3)R301 phase have the peculiarity of occupying electron gas. This picture does not apply to the substitutional substitutional sites [311]. The surface vibrations for this special configuration of Li and Na, but is approximately valid for K geometry of the adsorbate, shown in Fig. 20 a side view due to the substantial outward relaxation [312,486]. 326 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

Fig. 21. (a) Calculated surface phonon dispersion curves of the Al(111)-(√3 √3)301 clean surface and with substitutional Li (b), Na (c) and K (d) adsorbates. The surface phonon branches are marked by full circles. Adapted from Ref. [312].

The large changes with respect to the clean Al(111) surface √3)301 (coverage θNa ¼0.33) and (3/2 3/2) (coverage θNa¼ of the phonon LDOS projected onto the alkali sublattice and 0.44). The respective hexagonal arrays of Na atoms and corre- on the first and second Al substrate layers are illustrated in sponding SBZ's are shown in Fig. 23 on top of those for the clean Fig. 22 for both z (SV), and x+y (SH+L) polarizations. The Cu(111) surface. Note that the rhombic unit cell for the whole sharp peaks appearing in the z-polarized alkali-projected DOS (adsorbate+substrate) system in the p(3 3) configuration (one Na receive the largest contribution from the zone-boundary atom per unit cell) also holds for the (√3 √3)301 (three atoms localized branches discussed above. As appears from per unit cell) and (3/2 3/2) (four atoms per unit cell) super- Fig. 22, the participation of the substrate atoms of the first structures, so that the SBZ for the whole systems is the same for and second layers in the displacement field of these localized the three coverages. Thus it is convenient to represent the branches is practically null. calculated phonon dispersion curves within the reduced SBZ's of HREELS measurements on Li/Al(111) for coverages ran- the rhombic cells, which are those depicted in Fig. 23(a), valid for ging from 0.03 to 1 [5,311] indicate a SV mode of energy the p(3 3), (√3 √3)301 and (3/2 3/2) superstructures, and between 17.6 and 18.1 meV. This can be better associated with in Fig. 23(b) for the p(2 2) superstructure. the folded SV surface mode at Γ (Fig. 20(b)) than with the With respect to the phonon branches for the clean surfaces of the sharp peak at 21 meV in the calculated LDOS projected on the Cu(111) slab, plotted in Fig. 24(a), the same phonon branches of Li overlayer displayed in Fig. 22. On the other hand the the clean Cu(111) slab represented on the two extended rhombic HREELS data for Na/Al(111) (θNa ¼0.33) [5,311] give a SV cells (Fig. 24(c) and (e)) show a dense folding pattern with a mode at 10 meV, which can be readily associated with the complex array of folded surface branches, all (except the Rayleigh sharp peak in the LDOS for the Na overlayer in Fig. 22, wave) falling onto the surface-projected bulk continuum and taking corresponding to third flat zone-boundary branch of SV therefore a resonant character. When the Na atoms are added at polarization. different coverages (Fig. 24(d,f,g, and h)), new surface phonon branches are introduced, accompanied by a perturbation of the 3.3.2. Na/ Cu(111) intrinsic Cu(111) surface phonon branches. This can be assessed The dynamics of Na overlayers on the Cu(111) has been by a careful comparison of the phonon structure of the adsorbed thoroughly studied theoretical by means of EAM for four different phases to that of the clean surface represented on the same rhombic coverages corresponding to the adsorbate superstructures p(3 3) cell. However the most important feature is the pair of flat and (coverage θNa ¼0.11), p(2 2) (coverage θNa¼0.25) and (√3 almost degenerate (degenerate at Κ′) acoustic branches appearing A. Politano et al. / Surface Science Reports 68 (2013) 305–389 327

Fig. 22. Calculated phonon DOS for the four systems of Fig. 20 Adapted from Ref. [312]. well below the bulk edge for p(3 3) and p(2 2) superstructures parallel (x,y) polarizations for the lower coverage, but a weaker (Fig. 24(d and f)). Unlike substitutional K on Al(111) discussed localization and a substantial dispersion for the higher cover- above, where the flat localized branch has SV polarization, here the age. The latter effects can certainly be ascribed to a larger two flat branches have L and SH polarizations like the lowest mutual interaction of the adatoms. However the striking zone-boundary branches of substitutional Na on Al(111). difference with (√3 √3)301K/Al(111), Fig. 21(d), where On the contrary at the higher coverages (√3 √3)301 and the lowest, sharply localized and dispersionless K branch has a (3/2 3/2) the lowest acoustic branches are only weakly SV polarization is due to other factors. While the K branch on localized as an effect of the increased repulsive interaction Pt(111) is about at the same energy as in Al(111), the heavier between neighbor adatoms. mass of Pt brings the lower acoustic edge at Μ′ down to about All these features are clearly reflected in the Na-projected 6 meV, thus dramatically reducing the localization of the K DOS plotted in Fig. 25 for the four coverages and from their branches. Another, perhaps more important factor, is the comparison with the DOS of the clean Cu(111) surface plotted difference between the surface of a metal with sp electron in Fig. 24(b). The latter is clearly independent of the chosen bands like Al(111) and that of a d-band surface: the latter unit cell representation, and serves as a reference for all four allows for a tighter binding of the K ion to the surface, yielding Na superstructures. a stiffening of the SV mode. This implies that the lowest K HREELS data on Na/Cu(111) [322,343]atθNa=0.3 ML branches have parallel polarizations, whereas the lowest indicate a SV mode at 21 meV which clearly corresponds to branch in Al(111) has a SV polarization. the peak at about that energy for the (√3 √3)301 coverage The phonon LDOS for the two K/Pt(111) superstructures are in Fig. 25. More precisely at this energy there is a flat optical plotted in Fig. 27 and compared with those projected onto the surface branch crossing the whole SBZ (Fig. 24(g)), which surface Pt layer and the bulk DOS. Despite the stronger K–Pt involves to a large extent the SV motion of the Na atoms. binding the LDOS's of K and Pt appear largely decoupled due to mass difference and the strong anisotropy between vertical 3.3.3. AM/Pt(111) and parallel force constants: the LDOS for K parallel-polarized An EAM calculation for two different coverages, p(2 2) modes is sharply peaked at about 6 meV, much below the main and (√3 √3)301, of potassium on the (111) surface of Pt LDOS features, whereas the LDOS of potassium SV modes platinum [487–489], shown in Fig. 26, indicates a fairly good is peaked at the top of the bulk Pt phonon spectrum. The localization of the flat zone-boundary acoustic branches of HREELS spectrum for a K coverage of 0.15 gives a mode at 328 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

Fig. 23. Hexagonal arrays of sodium adsorbed on Cu(111) for increasing coverage θNa and corresponding two-dimensional BZ (gray areas; hatched triangles are the irreducible parts) inscribed into the Cu(111) SBZ: (a) p(3 3), i.e., θNa ¼0.11; (b) p(2 2), θNa ¼0.25; (c) (√3 √3)301, i.e., θNa ¼0.33; (d), (3/2 3/2), i.e., θNa ¼0.44. Note that the rhombic unit cell for the adsorbate+substrate system is the same in (a), (b) and (c) and therefore the SBZ for the whole systems are the same for the three coverages. A. Politano et al. / Surface Science Reports 68 (2013) 305–389 329

Fig. 24. (a) Surface dispersion curves of Cu(111) calculated with the EAM: the open circles describe the surface phonon branches while the continuous lines are the bulk dispersion curves for a 31-layer slab. The corresponding surface-projected phonon density of states are shown in (b) for the SV and the parallel components (L,SH) of surface atom displacements. (c,d) dispersion curves of the Cu(111) slab folded into the p(3 3) unit cell and modiﬁcations induced by the adsorption of one Na atom per p(3 3) unit cell as in Fig. 23. (e,f) same as (c,d) for the p(2 2) structure. (g,h) phonon dispersion curves for the (√3 √3)301 and (3/2 3/2) Na adsorption phases. From [323]. 330 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

Fig. 25. Phonon density of states projected onto the Na adsorbed layer on Cu(111) for the four coverages considered in Figs. 23 and 24 and for SV (full lines) and in-plane (L,SH) polarizations (broken lines) calculated with the EAM. Adapted from Ref. [323].

Fig. 26. EAM calculations of the surface phonon dispersion curves for the p(2 2) and (√3 √3)301 potassium overlayers on Pt(111). Adapted from Ref. [488].

22 meV of SV polarization [330,331] in agreement with the have also been applied to the study of AM on metal surfaces as mentioned flat branch of the SV modes of potassium at regards adsorbate diffusion and local modes at low coverages 20.5 meV calculated with EAM. and surface charge re-distributions (see Section 4.2.1) [69–71], whereas little has been done about their phonon dispersion 3.4. Dynamics of AM on metal surfaces with ab-initio methods curves [305]. HAS studies of metal surfaces revealed a (DFPT) complex dynamic response of the surface charge density to phonon displacements [402]. These effects were found to be Nowadays density functional perturbation theory (DFPT) is pronounced in supported metal multilayers characterized by largely used for ab-initio calculations of the surface–phonon free-electron QW states [304]. A special case is represented by dispersion curves of clean metal surfaces, thanks to develop- alkali overlayers, due to their ability in enhancing certain ments of efficient codes [481]. Density functional methods surface reactions and field emission [121,490]. On a more A. Politano et al. / Surface Science Reports 68 (2013) 305–389 331

Fig. 28. (a) Side view of p(2 2)K/Be(0001) showing the K monolayer and the first three Be atomic layers together with the electron charge density: the charge density associated with the K-overlayer quantum well states fills rather uniformly the empty space between the K ions [403]. (b) Top view of the K overlayer and of the substrate Be atoms: the dotted atoms are on the first surface layer, the other atoms are in the second layer (adapted from [403]).

Fig. 27. Phonon localized density of states of p(2 2) (a) and (√3 √3)301 (b) K/Pt(111) projected on the potassium (top panels) and the Pt(111) surface layer (middle panels) compared with the Pt bulk DOS (bottom panels) for the planar (x+y) and normal (z) polarizations. Adapted from Ref. [488]. fundamental side, in these systems the coupling of the overlayer phonons to electronic transitions between states of the 2DEG allows to study the effects of a quasi-2D electron– phonon interaction [105,491,492]. 2DEG associated with a potassium layer adsorbed on Be(0001) is presently attracting much interest of theoreticians also for the occurrence of collective electronic excitations of acoustic type [493–495] and their involvement in photoemission [134,496], as a natural follow-up to the recent discovery of surface acoustic plasmons in Be(0001) [497,498]. The recent DFPT investigation of the surface phonon structure and e–p interaction of K/Be(0001) complements these studies. Fig. 29. Contour plots of the spectral intensities of the longitudinal (L), shear- Fig. 28 shows a side view (a) with the calculated electron horizontal (SH) and shear-vertical (SV) components of the surface modes of p charge density and a top view (b) of p(2 2)K/Be(0001) (2 2)K/Be(0001) projected onto the alkali overlayer. The upper part of the [403]. The charge density associated with the electronic states spectrum with the substrate modes is not shown since the projection of these of the K-overlayer quantum well ﬁlls rather uniformly the modes on the substrate coordinates gives negligible intensities [403]. empty space between the K ions. The DFPT lowest dispersion curves, associated with longitudinal (L), shear-horizontal (SH) (for planar scattering only for SV and L components). As and shear-vertical (SV) modes of the K overlayer, are shown in shown in Section 2, the intensity of the inelastic HAS from a Fig. 29. The DFPT approach also allows to calculate the HAS phonon of wave-vector Q and branch index ν is proportional to spectral intensities for these modes at different wave-vectors the square of the phonon-induced CDOs at the turning point of 332 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

Fig. 30. Contour plots of the charge density oscillations (CDOs) in the surface region, including the K adsorbed layer and the three first substrate Be layers, as functions of normal (z) and parallel (x) coordinates, induced by frozen-phonon displacements of the low-energy modes of the K layer with SV and L polarizations at the Γ (panels (a) and (c)) and Μ (panels (b)and (d)) symmetry points. The insets indicate the respective phonon energies. The contour lines correspond to CDO values in units of 104 a.u. ranging from 71to7128, each step corresponding to a factor 1/2. Red (blue) lines correspond to positive (negative) modulations. (Adapted from Ref. [338].) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) the scattering He atom. This squared amplitude is in turn space of the 2DEG density all over the BZ may explain the proportional to the electron–phonon coupling strength λQν for negligible dispersion for the K-ion SV branch (Fig. 29). On the that specific mode, which can therefore be directly obtained contrary the 2DEG responds dramatically to the L motion of from inelastic HAS measurements. While no HAS study of K/ the K atoms (Fig. 30(c and d)). Since the L motion of the K Be(0001) is available yet, the DFPT results for the CDOs can ions is opposite to the L motion of the first Be layer, the charge shed light on the puzzling HAS results for the K/graphite density is compressed on one side and is squeezed outward, system [309,339,499]. The calculated CDOs induced by the compensated by the charge depletion on the other side. The alkali SV and L modes at Γ and Μ are plotted in Fig. 30, and strongly modulated outward CDO lobes occurring at Μ are due show quite peculiar properties as the K atom oscillates about to the part of the quantum-well wave-function provided by the its equilibrium position. Be layer, while the smaller lobes near the K ion following its The SV mode at Γ, whose CDO is shown in Fig. 30(a), motion are due to the part of the wave-function provided by actually is a weak resonance with a uniform displacement field the K ion itself. The L mode of the K ion at Γ is instead propagating inside the Be substrate and a strong enhancement strongly localized, almost decoupled from the substrate, and no at the K layer, which however generates no modulation of the modulation of the surface charge density is produced above the surface charge density. For an outward vertical motion of the K overlayer. The important change in the charge density K atoms the valence charge donated to the 2DEG is called response in moving from Γ to Μ is associated with an back, so as to give a positive spot in front of the ion at the appreciable dispersion of this mode (Fig. 29). expense of the delocalized electron gas charge, which shows a The present analysis may be applied to the intriguing case of uniform depletion. Altogether, there is practically no space the inelastic HAS data for the (2 2) alkali overlayers on modulation in the low density region. Of course the inward graphite [309,339,499–501], where only the longitudinal over- displacement of the K atoms yields the opposite change in the layer branch was observed, besides its avoided crossing with 2DEG density. Since the K atom is in the on-top position a the substrate RW. It should be noted however that the downward motion of the K ion attracts electronic charge overlayer density on the honeycomb structure of graphite is between the two ions to reinforce the K–Be bonding and to smaller than on the closed-packed Be(0001) surface. This fact better screen the positive ion charges. combined with the larger inertness of graphite suggests that the Also at the Μ point the SV modes (Fig. 30(b)) produces a effects described above should be emphasized for the graphite modest spatial modulation of the CDO, almost vanishing at the substrate. In particular the CDO associated with the SV alkali K plane, with no change of sign. The small modulation in mode should remain rather small all over the BZ, whereas the A. Politano et al. / Surface Science Reports 68 (2013) 305–389 333

Fig. 31. Vibration energy as a function of the inverse square root of the adatoms mass for (a) the substitutional adsorbed AM/Al(111) and (b) the on-surface hollow- site adsorbed AM/Cu(001). Solid blue and green solid lines represent linear fits for vertical and in-plane modes, respectively. The computed EAM results for AM/Al (111) are taken from [312] and the experimental and DFT data from [5,311,313,314]. The experimental data for AM/Cu(001) are taken from [55,303,315,316,319,502]. The theoretical data for AM/Cu(001) are adapted from Ref. [11]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) alkali L branch, which is less dispersed, should be character- As seen in Fig. 31(a), for the substitutional atoms the ized by a comparatively large CDO for wave-vectors beyond vertical modes fulfill only roughly the M1/2 law, whereas the the region of hybridization with the RW, thus giving the only vibrational energies of adatoms in the hollow site follow relevant feature in HAS time-of-flight spectra. quite well the M1/2 law, indicating for this configuration a substantial equality of the bond strength. The deviations observed for substitutional adatoms cannot however be 4. Vibrational spectroscopy of adsorbed AM atoms explained as exclusively due to the difference in their atomic radii: the exceedingly soft frequency of K can rather be related 4.1. Single-adatom properties to its larger outward position with respect to the ideal substitutional site. This effect is even more pronounced for The few theoretical examples of phonon dispersion curves the in-plane modes. of AM on different close-packed surfaces discussed in the The energies of the in-plane modes of adatoms in the hollow previous Chapter have shown fairly flat dispersion curves for sites (also termed frustrated translations, or T modes) deviate coverages lower than or equal to p(2 2). Thus at sufficiently even more strongly from the M 1/2 law, showing rather an low coverage the mutual direct interaction between adsorbed inverse proportionality to M. As discussed below, an AM atom atoms can be neglected, and their vibrational frequencies can, adsorbed on a perfectly flat surface like that of Cu(001) builds in a first approximation, be viewed as single-adatom proper- its own potential and induces a corrugation by transferring ties. This allows for a first assignment of adatom vibrational electrons into zone-boundary surface state pockets. This frequencies for a given mono-crystalline substrate once their mechanism (equivalent to 2D Friedel oscillations for an general dependence on the adatom mass, adsorption site and isolated adatom), raising a long-range (R 2) interaction surface orientation is qualitatively established. between distant adatoms via surface states (Lau–Kohn forces), can also explain the observed deviations from the M1/2 law 4.1.1. Mass dependence for both T- and vertical-modes. Jacobi et al. [337] and Finberg et al. [5] have shown from the analysis of the experimental data that the energies of the 4.1.2. Dependence on the adsorption site vertical vibration mode are scaled approximately as the inverse AM adatom can be adsorbed in different positions of the square root of the AM adatom mass. In order to analyze the surface lattice, the most common ones being the symmetric role of the mass factor itself, we consider vibrations of hollow, bridge, on-top or substitutional sites. Since each different AM adatoms in the same conditions (adsorption sites, adsorption site of a given surface and for a given AM adatom coverage, and surface orientation). is characterized by its vertical and in-plane vibrational In Fig. 31 the experimental energies of vertical (?) and in- energies, a comparison with theoretical prediction may help plane (||) polarized modes, as well as the theoretical values determining the adsorption sites and their probabilities and to obtained from EAM and DFT calculations, are plotted as distinguish adatom spectra from those of surface defects. For functions of the inverse square root of the adatom mass (M 1/2 example, a calculation by Hannon et al. [331] of the potential law) for the substitutional AM on Al(111) (cf. Section 3.3.1) energy function of K on Pt(111) and of the vertical vibrational and compared to hollow-site AM on Cu(001). frequencies at different sites gives a vertical mode at 17 meV 334 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

Fig. 32. Phonon dispersion curves and LDOS for c(2 2)–Na/Al(001) with sodium atoms in hollow (a) and substitutional (b) adsorption sites (surface states are denoted by open circles). The respective phonon density of states projected onto the Na overlayer are shown in (c) and (d). Adapted from Ref. [11].

Table 3 Experimental vibrational energies of alkali adatoms on different adsorption sites of Al(111). Adapted from Ref. [5].

Adsorbate Structure Mode energy (meV) Polarization

Na On-surface (4 4) (mixed sites) [503] 2.3 In-plane Na Substitutional (√3 √3)R301 [503,504] 1.4 Defect adsorption Na Substitutional (√3 √3)R301 [503,504] 9.2 Vertical Na Intermixed [311,313,505] 4.5 – K On-surface (√3 √3)R301 (adsorption in on-top sites at T=90 K) [503,506,507] 3.1 In-plane K Substitutional (√3 √3)R301, T=300 K [503,506,507] 6.4 Vertical Cs On-surface (√3 √3)R301 (adsorption in on-top sites) [503] 1.7, 2.4 In-plane Cs Intermixed (2√3 2√3)R301 [503] 2.4 In-plane for the hollow site, in excellent agreement with HREELS DOSs projected onto the Na overlayer for the vertical and in- experimental value [331], 19 meV for the on-top site, 12 meV plane components (Fig. 32(c and d)), show indeed that for the for K bonded in a single Pt-atom vacancy, and 15 meV for K hollow position the in-plane modes are softer than the vertical bonded in the center of a three-Pt-atom vacancy island. On this ones, while for the substitutional position the vertical modes basis unambiguous information is obtained that K on Pt(111) are softer that the in-plane modes . at low coverages accommodates in the hollow site [331]. Other examples of experimental vibration frequencies which Fig. 31 also shows that in systems with AM atoms in the permit to assign the adsorption site of different AM adsorbates hollow-site position the vertical vibrations are, as expected, on the Al(111) surface are listed in Table 3. stiffer than the corresponding in-plane vibrations, while the opposite holds for the substitutional adsorption site, due to the 4.1.3. Dependence on surface indices missing nearest neighbors above the vibrating AM atom. Thus For adsorption on multi-faceted surfaces, the dependence of the ratio of the in-plane to vertical energies can also provide vertical and in-plane vibration energies on the surface indices information on the adsorption site through its dependence on is remarkable. However, it should be noticed that no studies on the adsorbate coordination. This is better seen by comparing AM vibrations on high-index surfaces are present in literature. vibration energies of the same AM atom on different sites of While AM adatoms on high-index surfaces would ﬁnd several the same surface. As seen in Fig. 32(a and b), the calculated inequivalent adsorption sites yielding rather different vibra- dispersion curves of a c(2 2) Na overlayer on Al(111) tional frequencies, low-index surfaces (111) and (001) of cubic arranged in either the hollow sites (coordination 3) or the crystals look fairly uniform on the length scale of the adatom- substitutional sites (coordination 9) show important differences image charge distance. While T-modes directly probe the in the Na vibrational branches. The corresponding phonon local site geometry, vertical modes (also termed stretching A. Politano et al. / Surface Science Reports 68 (2013) 305–389 335

Table 4 Summary of S-modes energies for various AM atoms adsorbed on low-index Cu surfaces in the limit of low AM coverage.

Surface AM S-mode energy (meV) Method

Li 38 HREELS [321] HREELS [315,316,508] 21 Na DFT [325] Cu(111) 22 EAM [323] 13 HREELS [322] K 12–13n SHG [509] Cs 7.44 SHG [510]

Li 33–35 HREELS [315] Cu(110) Na 18 HREELS [315] K 13.7 HREELS [315,316]

Na 18 HREELS [316] Cu(100) K 10 HREELS [316]

(S) modes) do not. Thus, S-modes are expected to be insensitive to surface indices. In fact, results summarized in Table 4 for the case of S-modes of AM (in the limit of low coverages) on low-index Cu surfaces indicate a substantial independence on surface indices.

4.1.4. Dependence on temperature Adatom vibrations can be highly anharmonic. Vertical vibrations probe a highly asymmetric potential, since the adatoms are conﬁned below by the rapidly (exponentially) increasing surface charge density and above by the long-range image-charge Coulomb attraction. This implies a robust cubic anharmonic term in the AM-surface potential. In-plane vibrations probe instead more symmetric potentials. For example, the potential of Na in a hollow site of Cu(001) represented in Fig. 8 is perfectly symmetric and has no cubic term. On the other hand, the comparatively low barrier height implies a sizeable negative quartic term causing a substantial decrease of the vibrational energy and a corresponding increase of the spectral line-width for increasing temperature T. In a quasi- Fig. 33. The temperature dependence of the frustrated translation frequency of harmonic approximation the harmonic force constant f is Na on Cu(001) and its spectral width (FWHM) as measured by HAS 0 [301,307]. Eq. (29), expressing the effect of quartic anharmonicity, provide corrected by quartic anharmonicity, represented by the 4th an excellent ﬁt of both the energy and FWHM with the quartic anharmonic 4 4 derivative of the potential d V=dx h, into constant h¼61.4 meV/Å4.

h ε ℏ2 f ¼ f u2 T0 ; u2 ð Þ 0 0coth 0 ε 28 12 2kT 2M T0 4.2. From low coverage to one monolayer where ε is the T-mode harmonic energy and u2 is the mean- T 0 4.2.1. Adatom–adatom interaction: dipolar and Lau–Kohn square displacement at T¼0. The corresponding T-dependent forces vibration energy and line-width (FWHM) are then given by AM adsorption on copper surfaces was found to induce a h ε rearrangement of the substrate via relaxation, reconstruction, ε ðTÞ¼ε ΓðTÞ; ΓðTÞ¼ u4coth T0 ð29Þ T T0 12 0 2kT and faceting, as suggested by STM and LEED measurements [511]. Some of the phenomena can be interpreted as The T-mode of Na on Cu(001) shows indeed a large due to the charge donation of AM atoms to the substrate. At anharmonicity, as resulting from HAS measurements of the low coverage there are however more subtle effects involving vibration energy and FWHM at Q¼0 as functions of the substrate surface states which have been elucidated by temperature (Fig. 33). Eq. (30) provides an excellent ﬁtof HAS phonon spectroscopy. High-resolution inelastic HAS both experimental quantities with h¼61.4 meV/Å4. measurements of the vibrations parallel to the surface for 336 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

Table 5 Summary of T-mode energies and related physical parameters of alkali atoms adsorbed on Cu(001). Adapted from Ref. [55].

2 AM atom θML μ(0) (Debye) Adsorption height (Å) ℏωT (meV) kT (meV/Å )

Li 0.80 2.370.1 1.96 [512] Na 0.50 3.570.2 2.23 5.6 172 K 0.37 7.570.5 2.60 [513] 2.86 76 Rb 0.29 7.370.5 2.75 Cs 0.27 7.570.5 2.94 [513] 0.65 14

Fig. 34. Comparison of the measured dispersion curves for the T-mode along the 〈100〉 azimuth for θ ¼0.05, θ ¼0.07 and θ ¼0.08 ML on Cu(001) at a Na K Cs T surface temperature of 100 K. The dashed lines indicate the dispersion curve of Fig. 35. Comparison of the frustrated translation ( -mode) frequency of Na the Rayleigh mode R. and the longitudinal mode (L) of the clean Cu(001) atoms on Cu(001) measured as a function of the coverage for a parallel ΔKffi 1 T ¼ surface. The inset shows the amplitude A of the dispersion of the T-mode momentum transfer 2.0 Å and surface temperature s 50 K with a calculated as a function of coverage (from Ref. [55]). calculation based on a model with effective long-range forces originating from a charge transfer into intrinsic surface states (full line, for ΔKffi0; broken line, for ΔKffi2.0 Å1). The inset shows the dependence of the T-mode frequency on the separation d between adatoms. low coverages of alkali atoms Na, K and Cs on Cu(001) show a strong fall off in frequencies from 5.6 meV (Na) to 0.65 meV (Cs) which cannot be explained in terms of the mass An example is illustrated in Fig. 35, for Na coverages on Cu difference. It corresponds in fact to a substantial reduction of (001) varying from 0.008 to 0.125 ML, corresponding to an the effective force constant from 172 meV/Å2 for Na to inter-adatom separation decreasing from 31 Å to about 8 Å 76 meV/Å2 for K and only 14 meV/Å2 for Cs (Table 5). The (inset). In this range the T-mode frequency increases almost dispersion of the T-mode energy measured by HAS as a linearly with coverage by about 14%! This unexpected function of parallel wave-vector Q appears to be negligible for behavior together with and the hexagonal ordering of the a coverage of 0.05 ML of Na and 0.07 ML of K, whereas for adatoms even at the lowest coverage point to the existence of a 0.08 ML of Cs a sizeable dispersion is observed (Fig. 34) [55]. long-range interaction despite the lack of dispersion. The A mean-field theory [55] suggests that the T-mode force puzzle was associated with another intriguing observation, constants depend on the adatom size: for small atoms the illustrated in Fig. 36. frequency is determined by the local substrate holding Even at very low coverages (Fig. 36, above) the He potential, whereas in the case of the larger adatom (Cs) the diffraction pattern shows the specular peak and the lowest- main restoring force is the electrostatic adsorbate–adsorbate order diffraction peaks of the substrate surface; the ordering of interaction. A model based on dipole–dipole interaction pre- the Na adatoms is signaled by the appearance of a ring of dicts an increase of the dispersion amplitude with coverage satellite spots around the specular peak. At the largest (inset of Fig. 34) in apparent agreement with observation, but it measured coverage of 0.125 (Fig. 36, below) the adatom is at odds with the observation of an increase with coverage of lattice satellite peaks become as intense as the specular and the Q¼0 T-mode energy. diffraction peaks of the substrate. Since the clean surface It is indeed an intriguing fact, pointed out by Graham et al. of Cu(001) appears perfectly flat to He atoms at thermal [514], that even at Q¼0, i.e., when all adsorbates oscillates in energies and no diffraction is observed within the current phase and lateral interactions do not play any role, the T-mode HAS sensitivity, the unexpected appearance of the substrate frequency shows an unexpected increase with coverage. surface diffraction at very low coverages was associated to an A. Politano et al. / Surface Science Reports 68 (2013) 305–389 337

Table 6 Dynamic dipole moment dμ/dz (in electrons) and energy (in meV) for the low-

lying states of Cu5K and Cu25K cluster models of K/Cu(100). Adapted from Ref. [42].

Cluster Character dm/dz (e) Energy (meV)

Covalent 0.25 11.0 Cu5K Mixed 0.60 11.8 Ionic 0.98 12.5 Ionic 0.90 13.4 Cu25K Ionic 0.90 13.6 Ionic 0.91 13.9

Fig. 36. HAS diffraction patterns of Na/Cu(001) at two different coverages θ¼0.048 (above) and θ¼0.125 ML (below) (from Ref. [514]). The spot rings around the substrate specular and diffraction peaks (arrows) correspond to ordered quasi-hexagonal adsorbate layers with two possible equivalent orientations. The diffraction peak intensities grow with coverage, indicating that Na adsorption also induces a surface corrugation of the substrate. For the clean Cu(001) surface the diffraction peaks would not be discernible on the present gray scale. adatom-induced modification of the surface charge distribution. Another possible mechanism as the formation of a lattice gas was ruled out by the appearance of the satellite peaks indicating an ordering of the adatom lattice. As explained in Ref. [514], the adsorption of Na atoms implies the injection of electronic charge into the four X-point pockets of surface states at the Fermi level of Cu(001), thus building up a surface corrugation with the exact Cu(001) periodicity and Fig. 37. HREELS data for the S-mode energy of Na and K on Cu(001) as the change of the potential well of Na adatoms. The effect is functions of coverage. For Na the HREELS spectra also provide an overtone similar to the formation of surface Friedel oscillations due corresponding to the creation of two quanta (adapted from Ref. [316]). to the surface states at the Fermi level and consequent long- range ðpR2Þ interaction between adatoms, which feel the mass formula: each other through the mediation of the electrons in the ω2 ð Þ T; Na MK Δ surface states (Lau–Kohn forces). This mechanism provided 0 ¼ eG z0 ¼ 4:21; ω2 ðKÞ M a quantitative explanation of the observed increase of the T;0 Na ω2 ð Þ 1 T-mode frequency with coverage and a first clear evidence S;0 Na MK G2z ðNaÞ Δ ¼ 0 eG z0 ¼ 3:88 ð31Þ – ω2 ð Þ 1ð Þ of Lau Kohn forces. S;0 K MNa G 2z0 K The theory provides an expression for the T-andS-mode Â with Δz z ðKÞz ðNaÞ¼0:37 e (Table 6). Both calculated squared frequencies as a function of Q and of the coverage θ 0 0 0 ratios are in reasonable agreement with the experimental for an AM atom on an fcc(001) metal surface [514].AtQ¼0 values derived from the HAS data of Fig. 35 for the T-mode and θ¼0 they are given by and the HREELS data of Fig. 37 for the S-mode. ð Þ ð Þ ω2 ¼ 1 2 AnG z0 ; ω2 ¼ 2 An0 z0 ; ð Þ T;0 G S;0 GG 30 2 M z0 M 4.2.2. Cs/Cu(100) monolayer The phonon dispersion curves of a well-ordered Cs mono- where G is the shortest surface reciprocal lattice vector layer epitaxially grown on Cu(001) show (in addition to the 1 (2.49 Å for Cu(001)), n0ðz0Þ and nGðz0Þ the 2D Fourier Rayleigh mode) a perpendicular resonance near the Γ point component for Q¼0 and G of the surface charge density at the and a longitudinal film mode (Fig. 38). The appearance of a adatom position z0 above the first atomic plane. Both compo- distinct longitudinal mode, which was not observed for the nents depend on z0 as exp(Gz0). A is a constant transforming corresponding monolayer films of Na or K, is attributed to the the charge density into the corresponding potential energy small interface corrugation seen by the Cs atoms and the of the embedded AM atom. In Cu(001) the fitting of the phonon velocity mismatch between the film and substrate. experimental frequencies gives An0ðz0Þ¼0.555 eV and Hence, the lateral motion of the film is effectively decoupled AnGðz0Þ¼0.056 eV. Eq. (31) permits to compare the K-to-Na from the substrate and reveals a quasi-two-dimensional phonon frequency ratios and to explain the important deviations from behavior. 338 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

Fig. 39. Behavior of the initial amplitude of the coherent vibration of the S mode of K/Cu(111) (ﬁlled circles) for a K coverage of 0.63 ML as a function of the excitation photon energy. The solid curve represents the dependence of the number of photo-generated carriers within the Cu substrate [509] on the excitation photon energy. From Ref. [476]

can produce a longitudinally oscillating electron density, which induces a coherent motion of alkali adsorbates. To distinguish the excitation mechanisms, the excitation photon energy dependence of the initial amplitude of the coherent motion of the S mode was measured for K/Cu(111) Fig. 38. (a) Measured phonon dispersion curves of a Cs monolayer along the (Fig. 39) [509]. The excitation photon energy dependence was [1 0 0] and [110] directions. L and R denote the longitudinal and Rayleigh modes, found to be similar to the absorption curve of bulk copper respectively, and P the perpendicular resonance. The dashed–dotted lines indicate the Brillouin zone boundaries for the two domains of the Cs film and the gray (solid curve in Fig. 39). Therefore, the substrate electronic dashed line shows the Rayleigh phonon curve of the undisturbed bare copper excitation is likely responsible for the coherent phonon surface. (b) The phonon density of states of the Cs atoms calculated from a best fit excitation of AM on copper. Similar results have been of the measured dispersion curves for a c(4 2)Cs overlayer on Cu(001) shown in obtained also for Na/Cu(111) [107]. the inset. The calculated vibrational frequency spectrum has been convoluted by a ¼ Time-resolved SHG spectroscopy can observe the coherent Gaussian with FWHM 0.25 meV. The solid and dashed lines represent – displacements perpendicular and parallel to the surface. Cs Cu stretching vibration for a complete single layer of Cs on Cu(111). While the irradiation with ultrafast pulses at both 400 When the Cs coverage is slightly increased above the first and 800 nm generates the coherent Cs–Cu stretching vibration monolayer an entirely different behavior was observed. Instead at a frequency of 1.8 THz (60 cm 1 ), they lead to a distinct of the Rayleigh mode and the longitudinal mode, characteristic pump fluence dependence of the initial amplitude of coherent non-dispersive organ pipe phonon modes at energies of oscillation and to a different initial phase. At 400 nm excita- ℏω¼2.3, 1.4, and 1.0 meV appear for films of 2, 3, and tion, the coherent oscillation is nearly cosine-like with respect 4 ML thickness, respectively (see below). to the pump pulse and the initial amplitude increases linearly with pump fluence. By contrast, at 800 nm excitation, the coherent oscillation is sine-like and the amplitude is saturated 4.2.3. SHG studies on AM/Cu(111) at high fluence. These features are successfully simulated by The AM overlayer at high coverages is characterized by the assuming that the coherent vibration is generated by two presence of an OR located below the L-band gap and a QWS different electronic transitions: substrate d-band excitation at around the Fermi level. These bands correlate to s-like and the 400 nm and the quasi-resonant excitation between adsorbate pz-like bands of a free standing alkali monolayer in the vacuum bands at 800 nm, i.e., possibly from an alkali-induced quantum [515], respectively. Since the QWS is situated inside the L-band well state to an unoccupied state originating in Cs 5d bands or gap, its wave function is localized at the surface. In contrast, the the third IPS (Figs. 40 and 41). wave function of the OR extends more into the substrate, because it is positioned below the lower edge of L-band gap. 4.2.4. HREELS studies on AM/copper For AM coverage ranging from θ¼0.5 to 1.0 ML, transi- Fig. 42 shows HREELS spectra of the clean and AM- tions of OR-QWS, OR-IPSs, and QWS-IPSs are possible covered Cu surfaces. The spectrum of the clean Cu(110) excitations localized at the overlayer [476]. On the other hand, substrate (curve a) is characterized by a surface phonon intraband and interband excitations of s, p, and d bands of bulk resonance at 20 meV [516]. After dosing Na (K) (curves b, are involved in the substrate-mediated excitation in which d, f and c, e, g for Na and K respectively) an intense and broad electrons or holes created by the electronic excitation of bulk peak at about 18 meV (12 meV for K) appears, reaches its bands transiently transfer to the AM-induced electronic state. maximum intensity at about l/6 of the saturation coverage and This results in a modulation of the electron density near AM then decreases without frequency shifts. Loss peaks can be adatoms. In addition, the excitation of the multipole plasmon attributed to the dipole active adsorbate-substrate stretching A. Politano et al. / Surface Science Reports 68 (2013) 305–389 339

Fig. 40. Coherent Cs–Cu stretching vibration at Cs/Cu(111) surface is observed by using time-resolved SHG spectroscopy. Irradiation with ultrafast pulses at both 400 and 800 nm produces coherent Cs–Cu stretching vibration at a frequency of 1.8 THz (60 cm–1). At 400 nm excitation, the coherent oscillation is nearly cosine-like with respect to the pump pulse and the initial amplitude increases linearly with pump ﬂuence. In contrast, at 800 nm excitation, the coherent oscillation is sine-like and the amplitude is saturated at high ﬂuence. From Ref. [510].

Fig. 42. HREELS spectra for AM on Cu surfaces. Spectra are normalized to

the amplitude of the elastic peak. Cu(110): (a) clean surface; (b) θNa ¼0.13 ML; (c) θK ¼0.08 ML; Cu(100): (d) θNa ¼0.18 ML; (e) θK ¼0.01 ML; Cu (111): (f) θNa ¼0.12 ML; (g) θK ¼0.05 ML. Adapted from Ref. [316].

peak, normalized to the intensity of the elastic peak, increases roughly linearly with coverage only for coverages less then l/5 of the saturation coverage. The incoming metallization of the AM layer at higher coverages induces the reduction in the amplitude of the AM- substrate stretch. This behavior can not be explained consider- ing only long range dipole coupling between the alkali atoms with a constant dynamical dipole moment. As the scattering cross section is proportional to the square of the alkali atom dynamical dipole moment [517], the strong coverage dependence of the loss intensities could be due to a large redistribution of the charge in the alkali atoms for increasing coverage. The calculations based on electronic wave-functions for cluster models of K/Cu(100) demonstrated that the Cu–K bond is predominantly ionic [42]. The large dynamic dipole moment Fig. 41. (a) Excitation wave-length dependence of TRSHG traces taken from of ionic K on a metal surface leads to a large observed Cs/Cu(111) at λex ¼800 nm (red open circles) and λex ¼400 nm (blue open intensity of the K vibrations for the frustrated translational squares). The probe wave-length is 565 nm in both cases. The Cs coverage was mode normal to the surface. 0.23 ML and the incident pump �uence was 8.0 mJ/cm2 at λex ¼800 nm; 2 The stretching frequencies of Na and K fall in the continuum 0.25 ML and 3.6 mJ/cm at λex ¼400 nm. Solid lines are the results of the nonlinear least-squares �tting. Dotted lines are over-damped components of the bulk and surface modes of Cu. The coupling between contributed by hot electrons. Inset: Fourier power spectra of the oscillatory overlayer and substrate modes can give rise to a broadening components of the TRSHG traces for λex ¼800 nm (solid) and for λex ¼400 nm and a shift of the stretching peak in the HREELS spectra. In (dotted). (b) Oscillating components obtained by subtracting the over-damped order to evaluate these effects Rudolf et al. [315] have studied ones. Note that initial phases are very different, while the frequencies are very the modes induced by Li on Cu(110), as the Li stretching similar. From Ref. [510]. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.) energy is more than 5 meV above the maximum of the substrate continuum. Li–Cu stretching energy is shifted upwards by 2.5 meV with increasing coverage (Fig. 43), and mode as loss amplitudes are strongly peaked in the specular that this shift is explainable in terms of the dipole–dipole direction. The adsorbate-substrate frequency is nearly inde- interaction and delocalization of the vibrational excitations pendent of the coverage, with the only exception of K on Cu related to the Li stretching mode. Upon reconstruction the (100), where a 3 meV upward shift is observed for θo0.02 energy of the Li–Cu stretching mode shifts downwards by ML. On all the surfaces the intensity of the stretching induced about 2.5 meV. 340 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

Fig. 43. Energies of the stretching peaks in the loss spectra as a function of coverage for Li/Cu(110). Measurements were performed at both 90 (ﬁlled squares) and 300 K (empty circles). Re-adapted from Ref. [315]

On the Cu(110) surface Rudolf et al. [315] found an additional energy loss peak at 11.5 meV, which was attributed to a reconstruction effect of the surface under sodium adsorption. Similar results were obtained for potassium, with the only Fig. 44. A series of HREELS spectra taken at different Na coverages in specular direction. Adapted from Ref. [311]. difference that the energy of the vertical vibration of a K adlayer on Cu(110) was found to be 14 meV in the unrecon- structed case, that is somewhat higher than for (111) and (001) surfaces, where it was estimated as 12 meV. As in the case of sodium, an additional mode at 10 meV was also observed on a reconstructed Cu(110) surface.

4.2.5. AM/Al(111) Vibrational spectroscopy studies on Na and Li adsorbed on Al(111) have been carried out by Nagao et al. [311]. The vibrational frequency of AM has been found to be dispersionless (Fig. 44, 11.6–12.6 meV) indicating a strong screening of the Na–Na interaction due to the substitutional structure. It is worth mentioning that structural difference has been demonstrated to cause different restoring forces at Na atoms [313]. For this system also surfacepffiffiffi phononpffiffiffi measurements have been performed. For the 3 3 R301Na/Al(111) the observed S1 mode (Fig. 45) was assigned [313] to be a transverse surface mode. The surface resonant mode R2 and R1 was identified as an acoustic transverse mode and its back-fold, respectively. Fig. 46 shows the perpendicular motion of Na against the almost fixed substrate which generates the observed dipole active surface resonant mode at the Γ point (R1). For the (2 2)Na/Al(111) surface, the measured surface phonon dispersion curves in Fig. 47 locate very close to Rayleigh mode of the clean Al(111) and its back-fold at ′ Fig. 45. Calculated (R1′, S1′) and measured (R1, R2, S1) phonon dispersions for M point. √ √ 1 – 3 3)R30 Na /Al(111) (with a threefold adsorption geometry are shown. Concerning Li adsorption, a weak Li Li interaction is Two z-polarized modes R1′ and S1′ (thick curves) can be fit to the experimentally deduced by the occurrence of a constant Li stretching vibration observed R1andS1 modes. However, it is impossible to reproduce the strongly (17.6–18.1 meV, Fig. 48). dispersing R2 mode by this model. Adapted from Ref. [311]. A. Politano et al. / Surface Science Reports 68 (2013) 305–389 341

Fig. 48. Behavior of the loss energy for several coverages of Li deposited on Al(111). Adapted from Ref. [311].

Fig. 46. Schematic view of the calculated displacement vectors of the R1 mode. Na atoms have large z-polarized displacement vectors while Al atoms in the second layer move only slightly. The magnitudes of the displacement vectors of the second Al atoms are less than few percent of those of the topmost Na atoms. This vibrational mode is the same mode as assumed in some ab initio frequency calculations. Adapted from Ref. [313].

Fig. 49. Behavior of the Na–Ni stretching energy for a clean Na adlayer and for Na dosed onto a CO-modiﬁed Ni(111) surface at 400 and 300 K, respectively. From Ref. [329].

4.2.6. AM/Ni(111) Very clean layers of AM could be obtained with the Ni(111) surface held at 400 K during both deposition and measurements. Moreover, all spectra were recorded in a few minutes to further reduce contamination. Very likely, the influence of co- adsorbed CO on the A–S bond would be enhanced at low AM coverage, i.e. at a low Na/CO ratio (Fig. 49). Loss spectra of Na deposited on Ni(111) at 400 K are shown in Fig. 50a. The Na–Ni stretching vibration shifted (Fig. 50b) from about 25 meV for a very low coverage (0.01 ML) down to 21 meV for increasing Na coverages (0.06 ML). No further variation of the Na–Ni stretching energy was observed for fi Fig. 47. Measured phonon dispersions for (2 2) Na/Al(111) ( lled circles) higher coverages, at which the adlayer undergoes a metalliza- are shown. Calculated (solid curves) and measured (open circles) phonon dispersions of clean Al(111) are shown together for a comparison. The tion because of the closer distances between Na adatoms. As a resemblance of the observed modes to the Rayleigh mode of the clean Al consequence, dipole fluctuation are screened very efficiently (111) surface is clear. Adapted from Ref. [313]. by the two-dimensional electron gas and the excitation of 342 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

T Fig. 51. (left scale, ﬁlled squares) Stretching energy of the Na–jellium bond

for hexagonal Na overlayers on jellium with rs ¼2 as a function of Na coverage; (right scale, empty circles) Layer binding energy for the same system as a function of Na coverage. Adapted from Ref. [324]

Fig. 50. (a) HREELS spectra of Na layers deposited on Ni(111) at 400 K (loss spectra were acquired at the same temperature). Loss peaks are due to the Na– Ni stretching vibration (b) behavior of the Na-substrate vibration as a function of Na coverage. The intensity of all peaks was normalized to the intensity of the elastic peak. All spectra were multiplied by the same factor. From Ref. [281].

Fig. 52. HREELS spectra of K layers deposited on Ni(111) at 400 K (loss adatom vibrations perpendicular to the surface is no longer spectra were acquired at the same temperature). The inset shows the behavior observable [337]. of the K-substrate vibration as a function of K coverage. The intensity of all Aside from the expectations of Ishida's covalent model peaks was normalized to the intensity of the elastic peak. All spectra were [168,518] for alkali adsorption on metal surfaces, predicting a multiplied by the same factor. From Ref. [281]. softening of the AM-substrate bond as a function of coverage, theoretical calculations for Na/jellium [324] (Fig. 51) and K on

Cu(100) [42] found coverage-dependent downshifts of the θK ¼0.16 ML, due to lateral dipole–dipole interactions AM-substrate stretching energy. In addition, the AM adsorp- between the adsorbed AM atoms (Fig. 53) [330]. For K tion energy decreases largely with coverage [203,519], thus coverages up to θK ¼0.33 ML the frequency decreases to implying a bond weakening for increasing coverage. 19 meV and the loss intensity nearly vanishes, which is Similar conclusions were reached for K deposited onto the attributed to the metallization of the AM layer. Above K – Ni(111) substrate. K Ni vibration energy was found to shift coverages of about θK ¼0.10 ML, the adsorbed K reacts with from 18 down to 15 meV as a function of AM coverage residual water molecules to form KOH, with the K atom (Fig. 52). bonding to platinum. Vibrational modes at 15, 28, 95 and 452 meV are assigned to K–O stretching, the OH bending and 4.2.7. AM/Pt(111) the O–H stretching vibrations of KOH, respectively. The vibrational frequency of adsorbed K adatoms shifts K-adsorbate-induced phonon modes have been also studied from 17 meV at low coverages to 22 meV at a coverage of by TRSHG [332]. The K–Pt stretching mode shows a large A. Politano et al. / Surface Science Reports 68 (2013) 305–389 343

Fig. 53. (a) Loss energy versus K coverage for K/Pt(111). (b) Relative loss intensity versus K coverage for K/Pt(111). The primary energy was 5 eV. Adapted from Ref. [330].

Fig. 55. Coverage dependence of HREELS spectra for Na/Mo(100): (a) 0.007, (b) 0.013, (c) 0.060, (d) 0.100, (e) 0.190, (f) 0.300, and (g) 0.410 ML. All spectra are normalized to the elastic peak intensity. Adapted from Ref. [334].

Fig. 54. TRSHG trace obtained for a Cs coverage was 0.26 ML. The spectrum has obtained by single pulse excitation (bottom) Fourier transformed spectra of the oscillatory parts of TRSHG traces in the top panel. The time-domain data used for the transformation were those for t40.45 ps, The center frequencies for the Cs–Pt stretching and the surface phonon modes at 0.26 ML Cs are indicated by arrows. Adapted from Ref. [285]. anharmonicity via coupling to lateral modes. In contrast, the substrate surface phonon modes do not show any indications of anharmonicity within the laser ﬂuence used in the study in Fig. 56. Coverage dependence of the dynamic effective charge at 320 K, Ref. [332]. derived from the HREELS data using Ref. [520]. Adapted from Ref. [334]. The mode-selective excitation of coherent phonons at Pt (111) surfaces covered with submonolayer cesium atoms has the Na–Mo stretching (Fig. 55). The frequency of this mode been demonstrated by TRSHG [285]. A burst of 150 fs laser does not shift with coverage. The absence of a frequency shift – pulses with the repetition rate of 2.0 2.9 Hz was synthesized is contrary to theoretical expectations [42,324]. by using a spatial-light modulator, and used for the coherent However, the mode intensity is strongly coverage-dependent surface phonon excitation. By tuning the repetition rate, it was as the dynamic charge decreases monotonically with increas- possible to control the relative amplitude of the vibrational ing coverage (Fig. 56). The decrease in the dynamic charge – – coherence of the Cs Pt stretching mode (2.3 2.4 THz, Fig. 54) can be attributed to depolarization interactions between the to that of the Pt surface Rayleigh phonon mode (2.6 or alkali dipoles. 2.9 THz, depending on the Cs coverage).

4.2.8. AM/Mo(100) 4.2.9. AM/Ru(0001) The vibrational and electronic excitations of Na atoms The upward shift of the Cs–Ru stretch frequency by 30% for chemisorbed on Mo(100) have been investigated with 0oθCs o0.19 ML (Fig. 57) is discussed in Ref. [521] in HREELS [334]. An intense feature at 19 meV is assigned to terms of three different effects. The dipole–dipole interaction 344 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

Fig. 57. HREELS spectra for a Ru(0001) surface [521]. The parameter is the

Cs coverage θCs, which is the number of Cs atoms relative to the number of surface atoms. Primary energy EP and sample temperature T are indicated. The incidence angle of the electron beam is 601 with respect to the surface normal. contributes by 7.5%, and the response of the substrate surface layer by 12%. The remaining 10% may be due to an increase of the curvature of the potential-energy surface near to the equilibrium position. This may be due to a lateral squeezing of the screening charge with increasing coverage. For these reasons, in general, a constant or even increasing AM–M stretch frequency is observed (Fig. 58). Metallization starts at Fig. 58. (a) Loss energy of the Cs–Ru vibrational mode versus Cs coverage for θCs ¼0.19 ML. Interestingly, at this coverage a ﬁrst indication Cs/Ru(0001). (b) Intensity of the Cs–Ru stretch vibration relative to the of the 2 2 structure appears, so that the change in shift at intensity of the elastic peak for the same surface. (c) Relative dynamic charge Q/Q0. Adapted from Ref. [521]. θCs ¼0.19 ML may also be put in relationship with the occupation by Cs atoms of the on-top position, which is found in the 2 2 structure at θCs ¼0.25 ML. substrate could be also provided by HREELS [16] experiments (Fig. 60). A weak, but well-resolved dispersionless peak 4.2.10. AM/graphite appears at about 17 meV [16]. The peak reaches its maximum Low-energy (2.3–4.4 meV) vibrational modes have been intensity at θ¼0.30 (inset Fig. 60) and then decreases with observed from K, Rb, and Cs chemisorbed on the basal plane increasing coverage. No energy shift was observed with of graphite using inelastic HAS [486] (Fig. 59). These modes increasing coverage and the loss intensity was strongly peaked are interpreted as phonons propagating in the sagittal plane and in the specular direction. polarized parallel to the surface plane. Modes having perpen- The energy loss continuum at high coverages resembles a dicular polarization were undetectable. This anomalous result typical loss continuum normally seen for metal surfaces, may be due to the coupling of the He atoms to the conduction consistent with the metallic character of the overlayer. The electrons in the surface. lack of an energy shift with coverage is consistent with the Information on the vibrational properties of the K/graphite, essentially linear work function shift in this regime [16], and the metallization of the adlayer at submonolayer coverages, suggests constant charge transfer, and weak depolarization of and the charge transfer from the K adatoms to the graphite the K atoms in this coverage regime. A. Politano et al. / Surface Science Reports 68 (2013) 305–389 345

Fig. 59. HAS measurement of the surface phonon dispersions for (a) the low coverage phase of K/graphite, and the p(2 2) phases of (b) K/graphite, (c) Rb/graphite, and (d) Cs/graphite. The measurements were all taken along the ΓM direction of the graphite using 17.4 meV He beams. The points at higher energies for the p(2 2) overlayers are believed to be overtones. The Fig. 60. HREELS spectra for the clean graphite surface at T¼83 K at different horizontal dashed lines are guides for the eye. DFT calculation points for potassium coverages ranging from 0.17 to 1.13 ML. The HREELS spectra the p(2 2) structure of K/graphite are shown as solid squares. The dashed were obtained in the specular direction using a primary electron energy of curves in the Cs graph indicate avoided crossing behavior near the graphite E ¼1.5 eV, and an incident angle of 601 from the surface normal. Inset: the Rayleigh mode, shown as a solid curve. Adapted from Ref. [486]. i relative intensity of the energy loss at 17 meV as a function of K coverage has a maximum centered around 0.24 ML of K. Adapted from Ref. [16]. 4.2.11. K/Si(111) AM adsorption induces a (3 1) surface reconstruction of the Si(111) surface. Fig. 61 shows the HREELS spectra of the Si(111)(7 7) clean surface (dashed line) and the K/Si (111)(3 1) surface (solid lines), [522]. The long tail of the elastic peak (the Drude tail), which is observed in the spectrum of the Si(111)(7 7) surface (characterized by a metallic behavior) disappears together with the decrease in FWHM of the elastic peak after the K adsorption. The disappearance of the Drude tail indicates the semiconducting electronic features of the K/Si(111)(3 1) surface. This result is in agreement with the previous angle-resolved photoemission studies reporting the semiconducting character of the AM adsorbed Si(111)(3 1) surfaces [523–526]. The energy loss peak observed at 55 meV in the spectrum of the K/ Fig. 61. HREELS spectrum of the clean Si(111)(7 7) surface (dashed line) and the K/Si(111)(3 1) surface (continuous line). The primary electron Si(111) (3 1) surface coincides with the well-known 1 beam energy is 5.0 eV and the incident and scattering angles are both 60 with 55 meV mode of the clean Si(111) (2 1) native surface respect to the surface normal direction. In the inset the dispersion relation of [527] associated with the SV oscillation of the upper edge of the loss feature is shown. Measurements have been carried out along the along the 5-membered rings (5-rings) at the reconstructed surface the ½110 direction, in the ΓAdirection of the (3 1) surface BZ. Adapted [528]. While the side view of the clean Si(111)(2 1) shows from Ref. [522]. an alternate sequence of 5- and 7-rings, where the latter form surface rows of π-bonded chains (Fig. 61(a)), a possible structure for the K/Si(111)(3 1) surface (Fig. 61(b)) clean Si(111)(2 1), where the 5-ring upper edge is presents a sequence of 5-, 7-, and 6-rings, the latter having protected with respect to adsorption of oxygen and it oscilla- the surface cusps passivated by K atoms. For this structure the tion frequency remains substantially unchanged for moderate 55 meV corresponds to the same SV motion of the upper edge exposures [527], also in K/Si(111)(3 1) this mode survives of the surface 5-ring (double arrow in Fig. 62). Similarly to to K chemisorptions. Moreover, as in Si(111)(2 1) [528], 346 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

adsorption of K ions. On the contrary a second feature, falling at about 50 meV and associated with the surface-plasmon polariton, rapidly fades out with K adsorption. This may be + explained as due to the fact that the formation of a positive K layer on the surface is screened by a negative space charge, eventually leading to a degenerate electron accumulation layer As a consequence the HREELS signal from the surface- plasmon polariton, which is determined by the bulk carrier concentration, is quenched while the quasi-elastic peak starts broadening due to low-energy excitations of the free electrons accumulated at the surface. Fig. 62. (a) The structure of the native Si(111)(2 1) surface, with the π- bonded chains (thicker lines) in the topmost atomic plane formed by the upper 4.3. AM multilayers: organ-pipe modes edges of 7-membered rings and the upper edges of the 5-membered rings in the second atomic layer beneath the surface, whose SV motion (double arrow) corresponds to the 55 meV mode observed by HREELS [527,528]. (b) The The phonon dispersion curves have been measured using He adsorption of K on the stable Si(111)(7 /7) surface determines a (3 1) scattering for Na [317] and Cs [319] films on Cu(001), and then reconstruction; this can be modeled as a sequence of 7- and 5-membered rings analyzed within the framework of a force constant model [305]. as in (a) with the insertion of 6-membered rings with cusps saturated by K The strain due to reduced nearest-neighbor distance in film atoms. The 5-ring upper edges retain their SV mode at 55 meV. (3.61 Å) with respect to the bulk value (3.66 Å) leads to a 20% stiffening of the resonance frequencies in film compared with the corresponding 〈110〉 longitudinal acoustic (LA) phonons in the bulk material. An even larger differences occurs for the Raleigh wave velocity along the film surface as compared to that in Na(110). The resonance frequencies for Na films of a given thickness closely follow an odd-integer law like the lower harmonics of an organ pipe with a maximum at the surface and anodeatthefirst substrate layer (organ-pipe modes). The hexagonal sodium overlayers, which grow as bcc (110) films, have one axis oriented along any of the 〈100〉 crystal- lographic directions of the squared substrates. They form in principle a domain structure due to the two possible equivalent orientations. However, the observation of a single and constant RW velocity indicates mono-domain overlayers with an uniform orientation during growth. In Figs. 64 and 65a some selected time-of-flight HAS spectra, converted to an energy-loss scale and taken at incident angles close to the specular direction (451) for various Na coverages, are shown. The RW of the substrate exists also for 2 and 3 layers of Na (broken vertical bar). The RW of Na appears upon increasing Na thickness (solid vertical bar). The transition occurs for 5 layers, for which RW of both substrate Fig. 63. HREELS spectra (incident energy: 10 eV) from a cleaved GaAs(110) and overlayer coexists. The other features, labeled by a fraction surface for increasing K exposure time. While the Fuchs and Kliever (FK) (2n1)/NL (n=1,2,…,NL=number of layers), are associated surface phonon polariton is practically unaffected by K adsorption, the surface with the confined resonances of the film. Their frequencies are plasmon polariton peak (SP) fades out. At 2' exposure the elastic peak starts broadening. Adapted from Ref. [529]. given by: π ω ¼ð Þ vL ð Þ 2n 1 n 32 this mode is dispersionless in the long-period direction due to 2a NL n the good spatial separation of the 5-rings. where νL is the phase velocity and a is an average film interplanar spacing. The plot of the experimental frequencies 4.2.12. K/GaAs(110) (Fig. 65b) for each thickness as a function of n shows indeed The interaction of K with the GaAs(110) surface has been that Eq. (32) is very well satisfied with the same value of the studied by Betti et al. [529]. On the clean surface a feature at phase velocity. From this correspondence it was concluded that 35.5 meV with its first overtone have been recorded and these modes are longitudinal standing waves normal to the n associated to the FK surface phonon polariton [530] surface with wave vector qz ¼π(n(1/2))/a . Their maximum n (Fig. 63). Due to the macroscopic nature of the FK mode, its is at the surface and the nodal plane is at a distance d¼NLa energy and intensity remain practically unchanged upon the below the surface (Fig. 66). A. Politano et al. / Surface Science Reports 68 (2013) 305–389 347

Fig. 64. (a) Inelastic HAS spectra from 2 to 20 ML film of Na on Cu(001), incident energy Ei¼22 meV and a surface temperature of 60 K. For some ML numbers the spectra of different incident angles θi, are shown. The arrows indicate features corresponding to film eigenmodes of vertical polarization with a node at the interface and a maximum amplitude on the first layer (organ-pipe modes) [317].

The organ-pipe mode frequencies for a given value of the just Sezawa waves at zero parallel wave-vector. The Sezawa quantum number n are inversely proportional to the film waves have however a dispersion in their propagation along the n thickness d¼a NL,asshowninFig. 65. For increasing more surface, so that their frequencies depend in principle also on the and more stationary waves are possible in the film, although the parallel wavevector Q. In the case of organ-pipe modes for AM finite resolution of HAS experiments limits the number of the overlayers on metal substrates, their dispersion, shown in Fig. 67 organ-pipe modes which are actually resolved in the TOF for various ML numbers, is, however, fairly small, except in the spectra. The elastic waves which are confinedinthefilm and regions of avoided crossing with either the RW dispersion curve do not propagate into the substrate due to a sufficiently large of the substrate or that of the film, provided the latter is thick acoustic mismatch have been first observed in seismology and enough to sustain a RW. Since the RW penetration length is Λ ¼ð 2 = 2 Þ1=2= are known as Sezawa waves [531]. The organ-pipe modes are approximately given by T 1 vR vT Q [532],wherevR 348 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

Fig. 65. (a) Inelastic HAS spectra from a 10 ML ﬁlm of Na on Cu(001) at three different incident angles θi,, incident energy Ei¼22 meV and a surface temperature of 60 K. The arrows indicate the fetaures corresponding to organ-pipe modes [317]. (b) The organ-pipe mode frequencies at zero parallel wavevector as functions of the wavevector qz in the normal direction for different numbers NL of monolayers. The series of observed quantum numbers n are indicated for each thickness; different symbols correspond to different thicknesses. The experimental points lie along a ﬁlm dispersion curve with a slope which is about 20% larger than that of the corresponding LA bulk branch in the (110) direction [317].

Fig. 66. Schematic diagram showing different possible overtones of organ pipe modes in a 10 ML thin film and a ball and spring model for a row of 10 atoms perpendicular to the plane of the layers. Fig. 67. The organ-pipe frequencies measured with HAS for Na films on Cu (001) [317] are plotted as functions of the film thickness (same symbols as in Fig. 65(b)) and compared with the predicted frequencies of Sezawa waves and vT are the RW and transverse acoustic sound velocities, the cross-over from the substrate to the film RW branch occurs when SWn (full lines) [531]. ΛT,film>d. ThedatareportedinFig. 68 indicate a cross-over at about 5 ML at phonon energies corresponding to the lowest interaction coupling strength (the so-called mode-selected organ-pipe mode (2.5 meV). lambda, λQj), while the depth of the detected phonons It is interesting to note that in the set of HAS dispersion measures its range [402]. This is precious information because curves for 2 ML shown in Fig. 68 there are a few points at the coupling of the substrate phonons to the quantum-well small Q along the anomalous longitudinal surface branch of electrons of the AM overlayer may have important implica- the substrate (dotted line) [378]. The observation of this tions in phonon-assisted surface diffusion and surface chem- branch, in which the substrate atoms move parallel to the istry [533]. surface in the third atomic layer beneath the Na surface and at least 7 Å below the classical turning point of the scattering He 5. Binary co-adsorption atoms is a further demonstration of the basic inelastic HAS mechanism from metal surfaces. As mentioned in Section 3.4, 5.1. Introduction this is based on the phonon-induced surface charge-density oscillations [402] with two important implications: the inelastic AM are effective promoters in many industrially important HAS amplitude from a given phonon of wavevector Q reactions, such as ammonia synthesis, CO hydrogenation or and branch index j is proportional to its electron–phonon catalytic transformation of n-hexane [267,534–537]. Recently, A. Politano et al. / Surface Science Reports 68 (2013) 305–389 349

Fig. 68. Dispersion curves of organ-pipe modes in Na films (from 2 to 20 ML) on Cu(001) [317]. The arrows indicate the predicted positions of the organ-pipe modes. The fractions identifying the horizontal dispersion curve are values of (2n1)/NL where n designates the overtone and NL the number of layers. The directions of the Rayleigh wave branches in the film (full line) and in the substrate (broken line) are also shown. For the 2 ML film the longitudinal resonance of the substrate is also observed at long wavelengths (dotted line).

AM are commonly used to tailor the activity of noble-metal temperature increased [120], as a consequence of the co- catalysts toward hydrocarbon oxidation and to improve the adsorption-induced mutual stabilization of both AM and CO. durability of automobile three-ways catalysts [534,538]. The The general picture of the AM-induced promotion effect is that ability of AM additives to alter the adsorption properties of the intra-molecular C–O bond is weakened, while the metal- transition-metal surfaces is also promising for ﬁnely tuning the CO bond becomes stronger in the presence of AM [563]. working range for catalytic reactions as CO oxidation on The detailed description of the above effects involves platinum. Thus, the co-adsorption of AM with reactive species various models including substrate-mediated charge transfers is a topic of surface science, in view of the fundamental [564,565], direct bond through complex formation [566], interest in understanding the mechanisms of heterogeneous electrostatic interactions [162,567,568], and the nonlocal catalysis and other properties of technological importance such AM-induced enhancement of the electronic surface polariz- as the oxidation processes and the enhanced electron emission ability [2,3]. The presence of AM on the surface induces a rates [47,245,508,539–543]. strong polarization of conduction band electrons of the The catalytic activity of AM promoters is a consequence of substrate toward CO which therefore can overlap more the AM-induced changes in the adsorption enthalpies of efﬁciently with CO accepting orbitals [188]. Moreover, the adsorbates and the activation energies of chemical reactions CO adsorption site was found to change upon co-adsorption involving these species [49,544–550] and of the associated [120,569]. reduction of the work function of the system [551–555].

5.1.2. General considerations on AM co-adsorption with 5.1.1. General considerations on AM co-adsorption with oxygen carbon monoxide AM and oxygen co-adsorption on metal surfaces has been The co-adsorption of AM and carbon monoxide molecules extensively studied for both basic and practical reasons on single-crystal metal surfaces is characterized by a large [57,264,570–574]. For example, AM are used for oxidation decrease of the C–O stretching frequency [278,556–558], an of metal and semiconductor surfaces. AM oxides are widely increase in the heat of adsorption of both AM and carbon used as additives for obtaining low work-function surfaces monoxide [559,560], and changes in core and valence level for photocatodes or for improving catalytic reactivity, such binding energies of CO [561,562]. The CO desorption as, e.g., ammonia synthesis [575]. From a fundamental point of 350 A. Politano et al. / Surface Science Reports 68 (2013) 305–389 view, AM co-adsorption with oxygen is a very challenging 5.1.4. General consideration on AM co-adsorption with subject because of the great variety of chemical and physical carbon dioxide phenomena involved in the reaction. Despite the remarkable Since the start of the industrial revolution the atmospheric interest, only few dedicated spectroscopic studies have been concentration of carbon dioxide is constantly increasing, conducted over the years in order to address the electronic, causing concern about the global climate change and conse- vibrational, and bonding properties of co-adsorbed AM and quent catastrophic economic effects [611,612]. oxygen and, thus, a clear picture has not emerged yet. Despite its relatively low overall atmospheric concentration, DFT calculations by Liu and Hu [178] revealed that the O- CO2 has significant effects since it absorbs and emits infrared substrate bond length increases in the presence of K. This radiation at wave-lengths of 4.26 mm (asymmetric stretching implies an alkali-induced weakening of the bond between vibrational mode) and 14.99 mm (bending vibrational mode), oxygen and the substrate. The nature of the K–O interaction thereby playing a role in the greenhouse effect. Activation of was found to be dependent on their mutual distance. As a the stable CO2 molecule and its conversion into other matter of fact, their interaction is mainly electrostatic when K compounds represents therefore a grand challenge for cataly- is farther away from the O adatom (4–5 Å). On the contrary, sis, also in consideration of the weak and non-dissociative for closer configurations a direct bond occurs. Accordingly, a adsorption on current transition-metal catalysts [613,614]. dependence on alkali coverage is expected. The role of the Since AM promoters are able to negatively charge CO2, the alkali pre-coverage is quite intriguing as the adsorption of AM investigation AM+CO2 co-adsorbed phases is very promising adatoms can significantly modify the bonds between co- for tailoring new catalysts for CO2 sequestration. In particular, adsorbates and the substrate. Moreover, depending on the vibrational measurements showed that reactions occur via a AM pre-coverage, different scenarios exist for the structural chemisorbed anionic intermediate CO2 . This species can evolution of the alkali+O co-adsorption system [576]. either dissociate into CO and O, or dimerize to oxalate 2 2 ðC2O4 Þ, or disproportionate to CO and carbonate ðCO3 Þ, 2 or gain a second electron so as to form CO2 . 5.1.3. General considerations on AM co-adsorption with water 5.2. On copper Water interaction with solid surfaces and interfaces [577– 586] plays a key role in many physical phenomena such as 5.2.1. AM+CO/Cu(111) catalysis, electrochemistry, corrosion, and rock efflorescing. The co-adsorption of Na and CO on Cu(111) has been Improving the comprehension of the reactivity of surfaces investigated with HREELS for low and intermediate sub- toward water could imply important applications in, e.g., monolayer pre-coverages of Na [274,283,508]. HREELS hydrogen production, fuel cells, and biosensors [587]. During spectra at 130 K are characterized by a remarkable increase the past two decades, water adsorption on single-crystal metal of the intensity of the Na-substrate vibration (at 21 meV loss surfaces has been intensively investigated by theoreticians energy) upon CO exposure (Fig. 69). [581,588–593] and experimentalists [582] as a prototype The CO-derived enhancement of the intensity of the Na loss system for understanding chemical bonds in water-solid at 21 meV may be explained by assuming that electron charge interfaces. Nonetheless, a consistent picture of the water–metal interface has not been reached yet [594]. In particular, water dissociation on metal surfaces has attracted a great interest [588,595–601] due to its significant role in many catalytic reactions in the heterogeneous phase [596]. As an electronegative adsorbate, OH intervenes in several electrochemical processes. The chemical reactivity of the products of water dissociation is in principle different from that of undissociated water [602]. The general effect of co-adsorbed AM is to promote water dissociation [585]. AM co-adsorption with water has been already reviewed (including vibrational measurements) by Henderson in Section 4.1 of Ref. [585]. Thus, in the present review we will only update the information in Henderson's review [585] with the recent progress. In particular, in recent years the occurrence of electron quantum confinement and of an increased density of states at Fermi energy has been associated to an enhancement of reactivity [603–610]. Water reactivity in systems exhibiting electron confinement offers the opportunity to put in relationship the nature of the water- Fig. 69. Electron energy loss spectra measured for 0.40 ML Na/Cu(111) and interface bond with the presence of electrons confined into a subsequently exposed to different amounts of CO at 130 K. Adapted from two-dimensional space. Ref. [274]. A. Politano et al. / Surface Science Reports 68 (2013) 305–389 351 is transferred from the Na atoms to co-adsorbed CO molecules. At high Na pre-coverage (such as shown in Fig. 69) the CO adsorption will then effectively bring the AM overlayer back into the ionic regime where the loss intensity in electron scattering is high. On the other hand, the increased loss intensity could be also due to a CO-induced loss of metal character for the Na layer, which will reduce the screening of the field from the incident electrons and thereby produce the observed intensity enhancement. The Na–Cu stretch mode shifts from 22 (Na on the clean substrate) down to 19 meV. On the other hand, HREELS spectra recorded at RT on the Na-modified Cu(111) suggested a fully dissociative CO adsorption, as indicated by the existence of a peak at 36 meV assigned to the O–Na vibration [281,284,369,615– 618]. It should be noticed that the weakening of the Na- Fig. 70. HREELS spectra for 0.06 ML Na/Cu(111) exposed to CO molecules substrate bond is more effective whenever molecular CO is at RT. The S3 acoustic resonance of Cu(111) at 21 meV [384] was not excited present on the sample surface, as in the case of Na+CO/Ni under these scattering conditions in present HREELS experiments. The intensity of all peaks was normalized to the intensity of the elastic peak. All (111) [619]. spectra were multiplied by the same factor. From Ref. [508]. No clear picture has emerged yet to fully explain the AM- induced dissociation on noble-metal surfaces. The adsorption of CO on low-index copper surfaces modified by submonolayers of AM is not dissociative [274,620,621], while CO dissociation was observed on stepped copper surfaces in the presence of potassium [180,238,622]. From the latter measurements, it was suggested that CO dissociation process primarily occurs at the steps. In fact, the presence of steps would cause an electric field with a lateral component which induces a quite high occupation of the anti-bonding 2πn orbitals so as to cause the dissociation of CO molecules in the close vicinities of step [178,623]. The AM promotion effects on CO dissociation were found to be related to both the short-range electrostatic interaction and the direct orbital overlap. In particular, the direct CO–AM bond significantly enhances the efficiency of CO dissociation. Accordingly, the dissociation barrier is lowered only for short AM–CO distances (2–3 Å). A non-dissociative CO adsorption on AM-doped flat copper surfaces [274,620,621] has been reported for studies at temperatures ranging from 100 to 180 K. On the other hand, Fig. 71. HREELS spectra for different amounts of Na deposited onto the Cu HREELS experiments performed at RT by Politano et al. [283] (111) surface and successively exposed to 0.4 L of CO. No critical pre- showed that the dissociation barrier for CO molecules is even coverage for CO dissociation exists. The intensity of all peaks was normalized lowered on AM-modified Cu(111) surfaces compared with to the intensity of the elastic peak. All spectra were multiplied by the same factor. From Ref. [283]. AM-doped transition-metal substrates [624] (Figs. 70 and 71). It is worth mentioning that the sticking coefficient for CO molecules on copper surfaces at 300 K is extremely reduced The reduced reactivity of copper towards CO adsorption compared with on transition-metal substrates (Ni, Pt, Ru). The should imply that CO adsorb only in the close vicinities of AM saturation coverage for CO molecules on clean copper was adatoms, so as to enhance the short-range character of the found to be about zero for temperatures higher than 200 K AM–CO interaction and allow orbital overlap, which plays the [625], while for Ni(111) the surface was fully covered by CO main role in CO dissociation [178]. molecules at 300 K even for small CO exposures [284]. By comparison, the CO+K/Cu(111) system at potassium Accordingly, it is quite expected that CO adsorption on AM- pre-coverages below 0.18 ML has been investigated by Bao modified copper substrates might occur only in the close et al. using HREELS [620]. Two molecular adsorption states vicinity of AM adatoms. HREELS spectra acquired for several of CO were found at low temperature (Fig. 72). One with Na coverages on Cu(111) exposed to 0.4 L of CO (Fig. 71) lower C–O stretch frequency has a stronger interaction indicated that no critical pre-coverage for CO dissociation between the adsorbed CO molecule and co-adsorbed K, while exists, since the AM–O bond is formed even at the lowest AM the other with higher C–O stretch frequency has a weaker pre-coverages. interaction. The two states are occupied sequentially during the 352 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

Fig. 72. HREELS spectra for different coverage of K on Cu(111). The K- modified surface was saturated with CO at 150 K. Adapted from Ref. [620]. Fig. 73. HREELS spectra recorded at 100 K for c(2 2)Li/Cu(100) exposed to 1.2 L of CO (0.21 ML) and further annealed to selected temperatures (200, exposure to CO. Vibrational measurements suggest that the 220, and 260 K). Loss features at 32–36 and 42–45 meV (260–290 and 340– effect of K is not to form K–CO complexes indicated by Yates 360 cm1) were assigned to the Li–Cu and the CO–Cu stretching, and co-workers [295,560] but to form K–O complexes, as also respectively. confirmed by TDS results by Solymosi and Berko [626]. different adsorption sites are occupied by CO molecules: bridge and atop, respectively. 5.2.2. AM+CO/Cu(100) For AM co-adsorbed with CO on Cu(100) the formation of a 5.2.3. AM+O/Cu(111) CO-AM complex was reported [120,566,627]. In particular, Li Loss spectra for 0.07 ML of K on Cu(111) and for (K+O) adsorption on Cu(100) at 300 K induced the (2 1), (3 3) are reported in Fig. 74. For small O2 exposures a new loss and (4 4) structures with increasing Li coverage [628]. The feature at 57 meV arose in the spectrum. For further O2 structure of (2 1)-Li/Cu(100), obtained by Li deposition onto exposures, such peak shifted from 57 to 61 meV and a new Cu(100) at RT with a Li coverage of 0.2–0.4 ML, is called a loss peak appeared at 46 meV. The K–Cu vibrational mode ‘missing-row structure’, where one of every two [001] copper was not influenced by O2 exposure. After an annealing of the atomic rows is replaced by a Li atomic row [629]. The surface at 500 K the intensity of the peak at 57 meV notably chemical properties of this structure are quite interesting for decreased. the one-dimensional copper atomic rows neighbored and The adsorption of atomic oxygen (in the limit of low- possibly promoted by Li atomic rows [630]. pressure O2 dosage) on Cu(111) does not occur at RT (inset of This system offers the opportunity to study the relationship Fig. 74(b)), while a peak at 46 meV was recorded in between the change in surface atomic arrangement and the measurements with the sample kept at 250 K during O2 behavior of vibrational spectra. In particular, a peak at exposures (inset of Fig. 74(a)). Such feature was assigned in 1200 cm1 associated with the formation of a Li–CO complex previous HREELS studies on the same system [631] to over- was not observed on (2 1)Li/Cu(100) at 100 K, in which surface O vibrating against Cu(111). Li atoms aligned one-dimensionally are separated by Cu-atom The feature at 57–61 meV has been assigned to the in-phase rows, whereas its was found after the destruction of the (2 1) vibration of subsurface and over-surface O against the Ag order at 200 K and on c(2 2)Li/Cu(100) at 100 K. The lattice, as occurs for O adsorbed on silver surfaces [632–634]. appearance of the 1200 cm 1 peak could be related to the As expected, the presence of O in a higher coordination site existence of a two-dimensional ensemble of Li atoms on Cu leads to the appearance of a feature with higher vibration (100) with different electronic states from those of embedded energy in the loss spectrum. The population of subsurface sites Li atoms in the (2 1) phase. in oxygenated silver abruptly decreased upon annealing. The existence of features at 228–232 and 251–252 meV However, on Ag the subsurface site for O is metastable (1840–1870 and 2025–2030 cm1)(Fig. 73) reveals that two [632–634], i.e. it is accessible only after the complete A. Politano et al. / Surface Science Reports 68 (2013) 305–389 353

Fig. 75. HREELS spectra of the Na-doped Cu(111) surface and of Na+O/Cu (111). In Na+O/Cu(111) the presence of subsurface O is argued from the appearance of the 58 meV peak. No on-surface species were revealed for Na- doped oxygenated copper. Inset: spectrum for 0.06 ML Na/Ni(111) exposed to

10.0 L of O2. Besides the Na–Ni stretching, only the O–Ni vibration at 65 meV was observed, due to O adatoms in three-fold over-surface sites. The intensity of all peaks was normalized to the intensity of the elastic peak. All spectra were multiplied by the same factor. From Ref. [618]. Fig. 74. HREELS spectra of 0.07 ML of K deposited on Cu(111) at 400 K and successively exposed to O2. Both O2 exposures and measurements were carried out at 400 K. The intensity of all peaks was normalized to the intensity of the elastic peak. All spectra were multiplied by the same factor. All spectra were multiplied by the same factor. Inset: HREELS spectra recorded after exposing to 20.0 L of O2 the clean Cu(111) surface: (a) at 250 K; (b) at RT. From Ref. [618]. occupation of available on-surface sites. Similar conclusions were reached by a theoretical study of O adsorption on clean Cu(100) [635,636]. On the contrary, for K-doped Cu(111) the energetic conditions for the oxygen migration underneath the Cu(111) surface seem to be completely reversed, as shown in Fig. 74. In the first stage of O adsorption, only subsurface sites are occupied. At the saturation, over-surface sites could be populated too. Moreover, spectra in Fig. 74 show that the AM- derived effects on the stability of subsurface O are reduced once O atoms adsorb also in over-surface sites. Upon annealing at 500 K the intensity of the feature assigned to subsurface oxygen (61 meV) drastically decreased. By contrast, only a slight variation in the amplitude of the O–Cu vibration at 46 meV was recorded. The existence of well-defined vibrational frequencies without any shift is another decisive fingerprint for the occurrence of a change in the adsorption site. Subsurface sites are not big enough to accommodate O atoms, hence the penetration of O below the surface is followed simultaneously by the lattice distortion. This is in principle possible for AM/Cu(111) as AM are known to induce a reconstruction of the Cu(111) surface, as evidenced by RAIRS investigation in Ref. [637]. Subsurface oxygen was Fig. 76. HREELS spectra for a saturated coverage of pre-adsorbed sodium and revealed in Na+O/Cu(111) (Fig. 75) but not on Na+O/Ni(111) 0.5 L of oxygen on a well-ordered Cu(110) surface after isochronal annealing (inset of Fig. 75). at the following temperatures: (a) 310 K (unannealed); (b) 390 K; (c) 645 K; (d) 705 K. Adapted from Ref. [205].

5.2.4. Na+O/Cu(110) could be assigned to either a Cu–O stretch (O bonded to Cu The study by Grider et al. [205] shows the effects of atoms with subsurface Na) or to Na–O stretching in Na annealing on the vibrational spectrum in Na+O co-adsorption. superoxide (NaO2) species. However, upon annealing at Up to an annealing temperature of 390 K, the HREELS 645 K sodium peroxide is formed (Na2O2), as evidence by spectrum (Fig. 76) is dominated by a loss at 48 meV, which the appearance of a loss peak at 59 meV. 354 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

Fig. 78. Intensity of the 450 meV peak (O–H stretching) as a function of ﬁlm thickness. The reactivity abruptly increases between 0.20 and 0.44 ML, due to the increase of the Na-covered area, and drops abruptly above 0.60 ML. From Ref. [40].

adsorbates could occur at around 0.44 ML, so as to justify the enhancement of the water splitting efficiency. It is worth mentioning that the decomposition of OH to chemisorbed O and H could be ruled out by the lack of O- and H-derived vibrational peaks. Fig. 77. HREELS spectra for various coverages of Na on Cu(111) exposed to For θNa ¼0.44 ML, water exposure also causes the rise of a 0.20 L H2O at RT. The intensity of all peaks was normalized to the intensity of the elastic peak. All spectra were multiplied by the same factor. From Ref. strong loss peak at 36 meV (Fig. 79) which is due to the [40]. stretching vibration of OH groups against Na atoms. Increasing Na coverage, the peak at 36 meV shifts towards higher loss energies and for θ ¼1.00 ML the OH–Na vibration is at + Na 5.2.5. Na OH/Cu(111) 53 meV. This is exactly the loss energy of the OH–Na Vibrational studies on the co-adsorption of Na and OH on vibration in matrix isolated NaOH molecules [641]. The shift Cu(111) have been reported in Refs. [26,40,277,638]. In order of the Na–OH vibrational frequency is a fingerprint of a charge to verify the reactivity of Na layers, the Na/Cu(111) surface transfer between co-adsorbates. has been exposed to small amounts of water vapour at RT. Fig. 77 shows HREELS spectra for several thicknesses of Na exposed to a common water exposure (0.20 L) in the region of 5.2.6. Li+H2O/Cu(100) the O–H stretching. The energy of such vibration (450 meV) is Vibrational measurements showed that while at small Li the fingerprint of a dissociative adsorption of water molecules, coverages surface hydroxide (LiOH) forms on the surface, at fi also con rmed by the absence of vibrational modes of the H2O high Li coverages surface monoxide (Li2O) forms first, molecule [585]. Only OH groups are present on the substrate. followed by the formation of hydroxide on the monoxide The intensity of the O–H peak is maximum in correspondence [642]. fi of the completion of the rst Na layer, i.e. 0.44 ML (Fig. 78). In particular, HREELS spectra of 0.15 ML Li+H2O/Cu(100) Such measured quantity is proportional to the average adsorp- showed an intense peak at 74 meV (assigned to the Li–OH tion probability and thus it is indicative of the chemical stretching) and small features at 136, 161 and 446 meV [270]. reactivity of the adlayer. A linear triatomic molecule of LiOH which sits on the surface Recent theoretical findings [599] support these conclusions. In upright with Li down is formed in the reactions among co- fact, at one monolayer Na coverage, the water molecule shows a adsorbates. The reaction scheme is expressed as Li(a)+H2O maximum ratio between the adsorption energy and dissociation (a)-LiOH(a)+H(a), where (a) denotes ad-species. The intense barrier, favoring the catalytic water splitting reaction [599].In loss-peak indicates the LiOH molecule formed on Cu(001) has correspondence of this coverage, well-definedNaQWSexistinthe an ionic-bond character. In fact the effective dynamic charge overlayer [106,107,129,137,138,141,599,639,640] which shifted in of the Li–OH stretching vibration is estimated to be 0.5e, and energy and disappeared upon increasing Na thickness. this is larger than that of the Li stretching vibration in the Li/ These results demonstrated that the water reactivity is Cu(001) system, i.e. 0.3e. enhanced in systems exhibiting electron confinement. Water at surfaces forms chemical bond with metal electrons, especially with those whose wave-function describes confined 5.2.7. K+CO2/Cu(110) states in a two-dimensional space. Hence, electron quantum Fig. 80 shows the HREELS spectrum for the co-adsorption confinement could be used for tailoring water reactivity. of K and CO2 and Cu(110) reported in the work by Onsgaard Moreover, the minimum of the work function in Na/Cu(111) et al. [643]. The frequency and the assignment of the various has been revealed for about one Na physical layer [552]. This vibrational peaks is reported in Table 7. Physisorbed carbon means that the largest charge transfer from the interface to dioxide desorbed upon annealing at 173 K. Heating also A. Politano et al. / Surface Science Reports 68 (2013) 305–389 355

Fig. 80. HREELS spectra acquired for 0.4 ML K/Cu(110) after the exposure to

10 L of CO2 at 107 K (bottom spectrum) successively annealed to selected temperatures (173, 233, 400, and 500 K). Adapted from Ref. [643].

co-adsorbed with AM on aluminum surfaces. The TPD results clearly indicate that there are three types of hydrogen in the H/ Cs/Al(111) system [257,650]: the hydrogen adsorbed at the Cs-effect-free Al sites (α-H), the hydrogen adsorbed at the Al sites perturbed by the presence of Cs (β-H), and the hydrogen strongly interacting with Cs (γ-H). 1 Intense loss peaks ω1 at 99 meV (800 cm ) and ω2 at 205 meV (1650 cm1) are present in the HREELS spectrum (Fig. 82) obtained for Cs+H co-adsorption on Al(111) [257]pffiffiffi. They are H-induced vibrations as indicated by the 1= 2 isotopic shift for Cs+D/Al(111). Both the ω1 and ω2 peaks are associated with γ-H. In fact, ω1 and ω2 are clearly observed for the H/Cs/Al(111) system with a Cs coverage of 0.36 ML, for which only the γ-H exists on the surface. The loss peaks ω1 and ω2 have been assigned to a bending mode and a stretching mode of a cesium aluminum dihydride (CsAlH2) complex, respectively. This complex is stable on the surface up to about 450 K. A strong charge transfer from Cs to AlH2 makes the Al–H bond ionic resulting in the larger loss intensities for the Fig. 79. HREELS spectra of Cu(111)and of Na-covered Cu(111)for several Al–H vibrations of CsAlH2 compared to those for H on clean coverages. Each coverage was exposed to 0.12 L of water vapor at RT. The Al(111). intensity of all peaks was normalized to the intensity of the elastic peak. All Furthermore, another weak loss feature observed around spectra were multiplied by the same factor. From Ref. [277]. 305 meV has been also ascribed to hydrogen-associated vibration, in particular to a combination mode (ω1+ω2). At induced the appearance of CO species (213 and 256 meV), lower Cs coverage (0.04 and 0.07 ML), shoulder features are which desorbed at 233 K. observed on the higher frequency side of the ω1 and ω2 peaks. Whenever H is pre-adsorbed on the sample surface Such shoulders are associated with α-H sites due to the (Fig. 81), HREELS spectra indicate the formation of formate correspondence with the frequencies attained for hydrogen through the reaction on clean Al(111) [649,651]. δþ δ K þ H þ CO2-K UHCOO ð33Þ The observation of the formate is a step in elucidating the 5.4. On nickel role of alkali promoters for the synthesis of methanol on + copper based supported metal catalysts (Table 8). 5.4.1. AM CO/Ni(111) A remarkable modification of the loss spectrum of Na/Ni (111) has been obtained upon CO exposure (Fig. 83). The Na– 5.3. On aluminum Ni stretching frequency shifted from 22 down to 13 meV. The C–O stretching energy, initially at 194 meV (0.07 L of CO), 5.3.1. Cs+H/Al(111) shifted upward to 218 meV (for higher CO exposure). As a HREELS experiments reported in Refs. [648,649] suggested comparison, the C–O stretching frequency is 235 meV that hydrogen atoms form alkali hydrides when hydrogen is (0.5 ML of CO) for CO on Ni(111) [281,619]. Interestingly, 356 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

Table 7

Vibrational features of K+CO2/Cu(110) and assignment as a function of annealing temperature. From Ref. [643].

ΔE (cm1) Assignment 107 K 173 K 233 K 400 K 500 K

362 ν(CO-substrate )

627 ν(CO2,phys) 1045 1045 1045 1045 νs(C–O) in carbonate 1375 vsðCO2 Þ, νs(CO2,phys) 1440 1512 1512 1512 1512 νs(OCO) in carbonate 1704 1704 ν(C–O) in K-modiﬁed Cu 2050 2050 ν(C–O) in K-free Cu

2356 νa(CO2,phys)

As already mentioned, the C–O bond is weakened by AM co-adsorption and it could also be broken. The AM-promoted CO dissociation was found to be strongly dependent on AM coverage, and therefore on the average Na–CO distance. Depending on temperature an AM critical pre-coverage for CO dissociation was found to exist. At RT such critical coverage was found to be 0.40 ML, and 0.10 ML at 400 K. On the contrary, CO adsorption on a Na-modified Ni(111) surface is always dissociative for temperatures higher than 430 K [283]. Na vibration was still present in the loss spectrum of Fig. 85 even with the sample kept at 600 K, while the C–O and the O–Na stretching modes disappeared. Hence, the results found in Ref. [624] are in contrast with old results in literature Fig. 81. HREELS spectra for CO /H/0.75 ML K/Cu(110) at 200 K (lower 2 based on the assumption of a concurrent AM and CO curve), the CO2/K/Cu(110) surface annealed to 250 K, a H/CO2/K/Cu(110) interface annealed to 350 K before H adsorption, and a difference spectrum desorption [559,560,654]. (upper curve) between the last two mentioned spectra. Adapted from Similar results have been obtained for K+CO/Ni(111) [655] Ref. [644]. and are well reproduced by DFT calculations [245]. The latter also indicates that the energies of both K–Ni and C–O stretching modes are practically independent of K and CO – – the co-adsorption process affected both the Na Ni and the C adsorption sites, respectively. O vibrations and their frequencies were found to depend on the Na/CO local ratio. Likewise, the K–Ni stretching energy was found to shift from 15 down to 10 meV upon CO exposure. 5.4.2. AM+O/Ni(111) This finding is in excellent agreement with the observed Several vibrational studies on Na co-adsorption with O on increasing of the AM-substrate bond length in the K+CO Ni(111) have appeared in recent years [279,281,282,616]. co-adsorption on Ni(111) and Ni(100) [652,653]. They have put in evidence a certain dependence of the The Na–CO interaction could be investigated with more vibrational properties of the AM adlayer on the sequential details by adsorbing Na on the c(4 2)CO/Ni(111) surface order of deposition of co-adsorbates. (Fig. 84). The CO–Ni and the C–O modes were measured at For a p(2 2)O pre-covered surface, the loss spectrum 50 and 235 meV, respectively. The CO–Ni mode strengthened shows (Fig. 86) the O–Ni mode at 70 meV and an O-activated and shifted to higher loss energies up to 72 meV, while a new phonon at 33 meV [616,656] corresponding to the S2 gap feature arose at 220 meV close to the C–O vibration. Both phonon at the M-point of the Ni(111) surface [657]. After the modes (at 220 and 235 meV) merge into a single feature at deposition of sodium onto the O-modified surface, the Na–Ni 220 meV for a Na coverage of 0.22 ML. Increasing Na stretching appears at 25 meV, while the O–Ni vibration shifts coverage up to 0.37 ML made such peak to shift down to from 70 down to 64 meV. For Na coverages between 0.11 and 205 meV. The existence of two distinct C–O frequencies for 0.30 ML, a new feature appears at 34 meV and the Na–Ni peak Na coverage between 0.07 and 0.13 ML indicated the presence shifts to 21 meV. The feature at 34 meV was assigned to the of two different species of adsorbed CO molecules. One is Na–O stretching mode. Accordingly, a direct interaction essentially unaffected, the C–O stretching energy being at between O and Na arises with the formation of a bond between 235 meV; the other is instead strongly influenced by Na atoms, co-adsorbates. No other relevant changes were observed for as indicated by the C–O stretching at 220 meV. The line-shape higher Na coverages. of the CO–Ni loss broadened as a consequence of different The weakening of the O–Ni bond may be interpreted as due local [Na]:[CO] stoichiometries. to a charge transfer from the metal surface to the anti-bonding A. Politano et al. / Surface Science Reports 68 (2013) 305–389 357

Table 8 Vibrational frequencies for various bonds, expressed in cm1. Adapted from Ref. [644].

Bond CO2/H/K/Cu(110) [644] HCOOH/Cu(110) [645] HCOOK [646] HCOOH/K/Co(1010) [647]

HCOO δ(OCO) 750 764

υs(OCO) 1385 1350 1357 1366 υa(OCO) 1600 1597 1614–1637 υ(CH) 2795 2950 2808 2780

CO3 υs(OCO) 1050–1480 CO υ(CO) 2030a K–OH υ(OH) 3710

a1710 In the presence of K.

Fig. 83. HREELS spectra for 0.05 ML Na/Ni(111) at 400 K for different CO exposures at RT. The intensity of all peaks was normalized to the intensity of the elastic peak. All spectra were multiplied by the same factor. Adapted from Fig. 82. HREELS spectra for H/Cs/Al(111) for various Cs coverages, Ref. [278]. successively saturated with hydrogen. The primary energy was 4.1 eV. Adapted from Ref. [257]. intensity of the component at 19 meV and an increase of the peak at 25 meV (Fig. 87(b)). states of adsorbed oxygen atoms, as found by theoretical An effective charge Q can be then calculated from μ¼Q 1⧸2 calculations for the K/O/Rh(111) system [178]. In fact, the (ℏ⧸2Μrω) , with Mr the oxygen reduced mass. Since the O electric ﬁeld by alkali adatoms would affect the O-substrate stretching frequency varies with Na coverage θ in a range well bond by shifting the electronic states. In particular, the anti- above the maximum phonon frequency of Ni, the substrate bonding orbitals with O 2pz character become partially cannot vibrate and Mr is approximately constant and equal to occupied in the presence of co-adsorbed AM. the oxygen mass. The dynamical charge of O–Ni can then be Important information could be provided by reversing the derived from μ which is in turn derived from the experimental sequential order of adsorption with respect to spectra in intensities in dipole scattering conditions. The dynamic dipolar Fig. 87, i.e. sodium before of oxygen. Upon O2 exposures, charge Q relative to the value Q0 for θ¼0 was found to two different Na–Ni stretching vibrations were revealed at 19 decrease exponentially for increasing Na exposure according and 25 meV. The peak at 25 meV is assigned to Na adatoms to the best ﬁt curve (Fig. 88) interacting with oxygen. Very likely oxygen adsorption – = ¼ð þ ρ CθÞ=ð þ ρÞðÞ induces a notable strengthening of the Na Ni bond. Increasing Q Q0 1 e 1 34 oxygen exposure, the number of Na atoms in close contact with oxygen gradually increased causing a decrease of the where ρ¼0.97 and C¼67. 358 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

Fig. 85. HREELS spectra of 0.08 ML Na/Ni(111) exposed to 0.4 L CO as a function of the sample temperature. Na deposition and CO exposure were Fig. 84. HREELS spectra of c(4 2)CO prepared at 200 K and after made at the same temperature. CO adsorption was found to be partly deposition of different amounts of Na at the same temperature. From Ref. [281]. dissociative at 400 K and completely dissociative for temperatures higher than 430 K. The intensity of all peaks was normalized to the intensity of the elastic peak. All spectra were multiplied by the same factor. From Ref. [624]. Similar results should be obtained when depositing another AM, e.g. potassium. HREELS spectra taken for 0.10 ML O/Ni (111), 0.03 ML K/Ni(111), and the co-adsorbed phase 0.10 ML O+0.03 ML K on the same substrate are reported in Fig. 89. The comparison of the last spectrum with the other two shows how the K–Ni and O–Ni stretching vibrations change upon co-adsorption of the two species. It is seen that the K stretching energy increases from 15 to 18 meV, whereas that of the O–Ni vibration decreases from 70 to 66 meV. The presence of CO on the surface inhibits the AM-induced population of O 2pz anti-bonding orbitals, as demonstrated in Ref. [617]. Hence, measurements taken on very clean alkali overlayers are needed in order to verify experimentally the predicted alkali-induced softening of the O–Ni bond. In fact, in the HREELS study of the K+O co-adsorption on Ni (111) [658], potassium layers were affected by the presence of CO contamination on the surface and no weakening of the O–Ni bond was revealed.

5.4.3. K+Ethylene oxide/Ni(111) The stabilization of ethylene oxide is essential for epoxida- tion chemistry [659–663]. Ethylene oxide (Et–O) is stabilized in K-modiﬁed Ni(111) for sample temperatures up to 450 K Fig. 86. Electron energy loss spectra of p(2 2)O prepared at 400 K and after [664,665]. In Fig. 88 shows HREELS data for Et–O and deposition of different amounts of Na at the same temperature. The p(2 2) O spectrum shows also the weak O-activated 33 meV S2 mode of the Ni(111) acetaldehyde on clean and 0.36 ML K-covered Ni(111) are surface (↓) [656], which is however quenched by the disordered Na adsorption. shown [236]. The loss spectrum for Et–O on the clean Ni(111) The intensity of all peaks was normalized to the intensity of the elastic peak. All surface, reported in Fig. 90, is dominated by the intense Et–O spectra were multiplied by the same factor. Adapted from Ref. [619]. A. Politano et al. / Surface Science Reports 68 (2013) 305–389 359

Fig. 89. HREELS spectra of: (a) 0.03 ML of K deposited at 400 K onto the Ni (111) surface, (b) 0.10 ML O/Ni(111) at the same temperature, and (c) 0.03 ML K co-adsorbed at 400 K with 0.10 ML of O on the Ni(111) surface. The intensity of all peaks was normalized to the intensity of the elastic peak. All spectra were multiplied by the same factor. From Ref. [616].

Fig. 87. (a) HREELS spectra of Ni(111) and 0.16 ML Na/Ni(111) exposed to

O2 molecules at 400 K. (b) Each curve was obtained by subtracting an exponential background. The resulting curve was ﬁtted by two Lorentzian line- shapes. The intensity of all peaks was normalized to the intensity of the elastic peak. All spectra were multiplied by the same factor. Adapted from Ref. [279].

Fig. 88. The dynamic dipole charge Q/e (e is the electron charge) of O as a function of Na exposure for the system Na+O/Ni(111). Adapted from Ref. [616]. Fig. 90. HREELS spectra for Et–O and acetaldehyde on the clean Ni(111) and for the same surface modiﬁed by the adsorption of 0.36 ML. (a) 2 L Et–O 1 dosed on 0.36 ML K/Ni(111) at 100 K, (b) the same upon annealing at 500 K, ring deformation mode at 105 meV (850 cm ). Further losses and (c) 0.36 ML K/Ni(111) exposed to 2 L acetaldehyde. The sample has been are due to CH2 wagging and twisting modes at 158 meV heated at 500 K. (d) 2 L acetaldehyde dosed on Ni(111) at 100 K. Adapted 1 1 (1275 cm ), CH2 scissor at 184 meV (1480 cm ) and C–H from Ref. [236]. 360 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

Fig. 91. HREELS spectra for saturation CO exposures with various potassium coverages. From Ref. [218]. Fig. 92. Energy loss spectra at 310 K before and after exposing H2O (a and b) and D2O (c and d) to a K-precovered (θK¼0.23 ML) Pt(12 12 13) surface. The 1 stretching at 377 meV (3040 cm ). A drastic change is time between the deposition of K and the recording of the spectra is given by observed upon co-adsorption with K (deposited at 100 K) the curves. Adapted from Ref. [329]. and annealing to 500 K. This loss spectrum presents losses at 67, 105, 141 and 175 meV (544, 850, 1140 and 1410 cm 1) with also a splitting of the C–H stretching at 343 and 379 meV [330]. For K coverages above 0.10 ML, the adsorbed K reacts (2770 and 3060 cm1). with residual water molecules to form KOH, with the K atom A nearly identical HREELS spectrum has been recorded bonding to platinum. Vibrational modes at 120, 230, 770 and upon co-adsorption of acetaldehyde with 0.36 ML K on Ni 3650 cm 1 (Fig. 92) are characteristic of this compound and (111). Identical peak positions and intensities demonstrate the are assigned to the Pt–K, the K–O stretching, the OH bending formation of the same surface species from both adsorbates. and the O–H stretching vibrations, respectively. The experi- Actually the analysis of HREELS data suggests the formation ments on K+D2O co-adsorption further support the validity of of aldehyde-like or acetaldehyde polymerization products. their conclusions.

5.5. On platinum 5.5.3. K+H2O/Pt(111) 5.5.1. K+CO/Pt(111) By maintaining the sample temperature at 110 K, water No CO dissociation has been found in HREELS study of K molecules do not dissociate, as indicated by the presence of H– +CO co-adsorption on Pt(111) [218] [271]. A change in the O–H bending. The influence of K on the vibrational properties CO adsorption site from atop to bridge has been shown to of adsorbed water molecules at 110 K has been investigated in occur with increasing K coverage (Fig. 91). The CO stretching Ref. [666]. Co-adsorbed water induced the disappearance of vibrational energy continuously decreases with either increas- the frustrated translation (O–O) stretch and of the Pt–O ing potassium coverage or decreasing CO coverage [271]. stretching (Fig. 93). All frequencies are reported in Table 9 Stretching energies as low as 171 meV (1380 cm1) are together with their vibrational mode assignment. reported in Ref. [271]. The analysis of the vibrational frequency indicates a considerably weakened C–O bond and it is consistent with an increased adsorption energy. 5.5.4. K+C2H4/Pt(111) Windham et al. [243] showed that addition of potassium 5.5.2. K+OH/Pt(111) induces a significant charge rearrangement at the ethylene/Pt Water dissociates on K films on Pt(111) and Pt (12,12,13) (111) interface. In the absence of K, the C–H stretching mode surfaces at RT, as evidenced by the HREELS study in Ref. at 363 meV (2930 cm1), which is representative of hydrogen A. Politano et al. / Surface Science Reports 68 (2013) 305–389 361

Fig. 93. Vibrational spectra for (a) H2O/Pt(111) and (b) K+H2O/Pt(111). Adapted from Ref. [666] for several coverages of H2O. Sample was kept at 110 K during measurements.

Table 9

Vibrational frequencies of H20 and D20 adsorbed on clean and K-doped Pt(111). Adapted from Ref. [666].

1 1 Frequency H2O (cm ) Frequency D2O (cm ) Mode assignment

240 240 Frustrated translation of H20, O–O stretch in solid ice 560 Pt–OH2 stretch, (frustrated z-translation) 695 510–580 Frustrated rotation 1630 1195 H–O–H symmetric bending (“scissor”) 3430 2560 O–H stretch, due to H-bridge bonding 3680 2700 O–H stretch, due to free –OH groups Co-adsorption with 0.06 ML of K 110 110 Pt–K stretch 330 260 Frustrated rotation (Ry “rocking”), low coverage, supports model (a) 485 375 Frustrated rotation (Rx “wagging”), higher coverage 1525 1525 Impurity CO (stretch) 1590–1630 1200 Symmetric bending

3225 2350 O–H stretch, with hydrogen bonding to Pt, only at very low H2O coverage 3395 2550 O–H stretch, H bridge bonding to H2O, only at high coverage 3620 2685 O–H stretch, due to free –OH groups, only at very low coverage

bonded to an sp3 hybridized carbon atom [667,668], and 5.6. On ruthenium the frequency of the C–C stretching mode at 131 meV (1060 cml) indicate that ethylene adsorbed on clean Pt(111) 5.6.1. K+CO/Ru(0001) is di-s-bonded to the surface. The foremost effect of co- The adsorption of CO on a K-pre-covered surface Ru(0001) adsorbed potassium on the C2H4 spectrum (Fig. 94)isto surface is non-dissociative [669–671]. The observation of a generate weakly interacting π-bonded ethylene species, which 22 meV (180 cm1) shift of the ﬁrst overtone of the K- are characterized by new loss peaks. The population of the π- perturbed C–O stretching with respect to twice the funda- bonded ethylene increases with K coverage. mental frequency (Fig. 95) indicated a strong anharmonicity of For weakly chemisorbed species, the strongest modes are the C–O potential well as an effect of K pre-adsorption [670]. those with a dipole moment perpendicular to the molecular Both strength and range of the interaction are found to vary plane [345]. The loss peak at 108 meV (870 cm1) has been with the ratio of CO:K coverages. At low CO coverage, a assigned to the CH2 wag mode of weakly bonded ethylene strong, short-range interaction between K and CO occurs and species with a double bond CQC parallel to the Pt surface. CO molecules adsorb in a side-on bonding mode. With The other features are the CQC stretching at 201 meV increasing CO coverage (at constant K pre-coverage) CO (1620 cm1), the CH bending at 169 meV (1360 cm1) and adsorb in bridge sites, with their molecular axis oriented the C–H symmetric stretching at 379 meV (3060 cm1). perpendicular to the surface. 362 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

Fig. 94. HREELS spectra of saturation coverages of ethylene adsorbed at 100 K for various potassium pre-coverages on Pt(111). Adapted from Fig. 96. HREELS spectra for saturated CO layers on the Cs-pre-adsorbed Ru Ref. [243]. (0001) surface at 85 K as a function of the Cs coverage. The spectra were measured after annealing the surface at 210 K and recorded in the specular geometry with an incidence angle of 601 from the surface normal. LEED pattern changes are also shown at the right side of the ﬁgure. Adapted from Ref. [558].

Co-adsorption of a small amount of Cs (0.06 ML) gives rise to a new C–O stretching mode at 223 meV (Fig. 96) and an extra LEED pattern of p(2 2) in addition to the "compres- sion" pattern arising from the incommensurate hexagonal CO lattice with a compressed unit cell [679]. The appearance of the p(2 2) pattern at this low Cs pre-coverage indicates formation of Cs+CO islands with the p(2 2) lattice. Upon increasing Cs coverage the low-frequency peak increases in intensity, though it shifts to lower frequency. Furthermore, the p(2 2) spots become sharp and intense with Cs coverage. These observations indicate two-dimensional growth of the Cs Fig. 95. HREELS spectrum for (a) 0.25 L CO adsorbed at 80 K on 0.10 ML + – K/Ru(0001) (b) the same after annealing to 950 K. Adapted from Ref. [670]. CO islands. On the other hand, the C O stretching mode the metal-CO stretching mode (at around 55 meV) due to the CO 5.6.2. Cs+CO/Ru(0001) species adsorbing outside the islands reduce in intensity and ﬁ Recently, the occurrence of a predominant long-range nally disappear at around a Cs coverage of 0.25 ML, where character of the alkali-induced effects has been invoked by the islands cover the whole surface and the sharpest p(2 2) different researchers [2,3,212,560,669,672–677]. He and LEED pattern is observed. In particular, the disappearance of Jacobi [336,678] found a signiﬁcant weakening of the C–O metal-CO stretching is due to the screening by the nearby stretching energy even if CO is adsorbed on the Ru(0001) AM atoms. surface far from the Cs atoms. They discussed the long-range or nonlocal effects in the following picture: the CO outside the 5.6.3. Cs+O/Ru(0001) (Cs+2 CO)2 2 islands reduces the amount of charge back- The interaction between Cs and O on Ru(0001) is char- donated to the CO within the island. Moreover, in the co- acterized by a strong interaction among co-adsorbates with the adsorption of K and CO on Ru(0001) an upward shift of the formation of an AM–O bond [262]. The peak at 24 meV and C–O mode for increasing CO exposures was revealed [669] the shoulders at 35 and 42 meV were assigned to Cs–O and attributed to a long-range substrate-mediated interaction vibrations. In agreement with matrix isolation spectroscopy between co-adsorbates. [680,681], the latter two frequencies are typical of Cs–O A. Politano et al. / Surface Science Reports 68 (2013) 305–389 363

stretching in CsO and CsO2Cs. On the other hand, the peak at 5.6.4. AM+H2O/Ru(0001) 24 meV has been ascribed to a Cs–O bending rather than to a HREELS measurements showed that potassium hydroxide is stretch mode. formed when water is adsorbed at 80 K on a Ru(0001) surface Concerning the O–Ru stretch, it has been found to shift from pre-covered with potassium. Heating the surface to 500–600 K 55 to 75 meV upon increasing O coverage (Fig. 97). At higher induces KOH decomposition and desorption of the dissocia- O coverage and low sample temperature (95 K), also di- tion products. The decomposition pathway has been found to oxygen species are revealed. The peak at 98 meV is assigned depend strongly on the potassium pre-coverage [683]. On the 2 to O–O stretching in O2 . For sake of comparison, its energy other hand, vibrational measurements on H2O adsorption on is 133–148 meV in O2 . The low intensity of the O–O Cs-pre-dosed Ru(0001) surfaces [684] showed that for vibration is typical of homonuclear molecules, in which no θCs ¼0.08 ML undissociated H2O adsorbs at 90 K. H2O reacts dipole exists. However, under impact or resonance scattering with the metallic Cs layer for θCs ¼0.25 and 0.33 ML. While conditions, also O–O stretching is observable. On the other for θCs ¼0.25 ML dissociation into adsorbed H and OH hand, for 0.25 and 0.33 ML of Cs a superoxo ðO2 Þ species species is observed, complete dissociation into 2H(ad)+O(ad) can be stabilized at 90 K [682], characterized by its overtones occurs for θCs ¼0.33 ML. at 278.4, 414.0 and 546 meV and the O2 –Ru vibration at 15 meV. From the overtone energies the dissociation energy of + O2 is derived as D¼3.970.3 eV close to the gas-phase 5.6.5. K CH3OH/Ru(0001) value (Table 10). Vibrational measurements showed that the presence of potassium inhibits O–H bond breaking and thus methoxy formation. The methanol molecule does not interact directly with the potassium atom (Fig. 98). No evidence is found for K–O related vibrations at 25–37 meV (200–300 cm1). The only low frequency vibration observed at temperatures below

Fig. 97. HREELS spectra for Cs co-adsorption with O. Spectrum a is related to

the clean Cs monolayer. Spectra b, c, and d are related to O2 exposure at 220 K with a dose of 0.8, 1.0, and 3.2 L, respectively, on the Cs-saturated Ru(0001)

surface. Spectrum e is related to O2 exposure of 1.3 L at 95 K on the Cs- Fig. 98. Vibrational spectra of methanol adsorbed on a 0.10 ML K/Ru(0001) saturated Ru(0001) surface. Adapted from Ref. [262]. surface after annealing to indicated temperatures. Adapted from Ref. [686].

Table 10 � Vibrational energies from HREELS and mode assignments for a monolayer of Cs (θCs¼0.33) at Ru(0001) as a function of O2 dose ( ) indicates weak intensity. From Ref. [682].

+ O2 dose (L) Cs–O2 (meV) Cs–O (meV) Ru–O (meV) (O–O) (meV) 2(O–O) (meV) Cs–O Ru–O (meV)

0.08 25 0.23 28 56 84, 96 0.38 28 64 94 0.53 15 28 66 140 94 0.75 15 26 71 140 278 95 1.13 15 25 74 140 278 97 2.30 15 25 75 140 278 98 364 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

Fig. 100. Left: the notable softening of the N–N bond can be explained by the * signiﬁcant electronic charge transfer from metal d-orbitals to the N2 π -orbital. Right: In the vicinity of K, a strong electron donor, the d–π* back-donation is even more enhanced. This results in further weakening of the N–N bond. Adapted from Ref. [688].

Fig. 99. HREELS spectra for 0.04 ML K/Fe(110) exposed to CO. Adapted from Ref. [687].

500 K (i.e. prior to methanol desorption) is the Ru–K mode at 17 meV (140 cm1) [669]. The red-shift of the ν(O–D) mode from 305 meV (2460 cm 1) on clean Ru(0001) to 287 meV (2315 cm1)(Fig. 96) [685] indicates bond weakening due to interaction with the potassium-modiﬁed surface. The observation of O–D stretch and the out-of-plane O–D bend at 65 meV (525 cm1)isaﬁngerprint of the presence of undissociated 15 methanol. Fig. 101. HREELS spectra for the saturation exposure of N2 on a Fe(111) surface pre-covered with K. Adapted from Ref. [688].

5.7. On iron 5.8. On cobalt 5.7.1. K+CO/Fe(110) 5.8.1. K+CO /Coð1010Þ The co-adsorption of CO and K on Fe(110) has been studied 2 – Toomes and King studied by infrared spectroscopy the by Zhu et al. [687]. Three adsorption states with distinct C O reactions of carbon dioxide at a saturated K monolayer on stretch frequencies have been found. Such states, sequentially Coð1010Þ [690]. Vibrational measurements (Fig. 102) shed the occupied upon CO exposure and named as α1, α2 and α3, have α α α light on the pivotal role of the chemisorbed CO2 as strong ( 1), intermediate ( 2) and weak ( 3) interaction with intermediate in the surface chemical reactions. On a K co-adsorbed K, respectively (Fig. 99). The distance between monolayer at RT the favoured reaction products are chemi- the CO molecule and the nearest K neighbor is the shortest for sorbed CO and O. At 160 K, adsorbed CO and CO2 have the α1 state and the longest for α3 (6 Å). been observed. The interaction of CO2 with multilayer K at 160 K leads to CO2 and surface oxalate formation. In + addition, disproportionation of CO2 yields adsorbed CO 5.7.2. K N2/Fe(111) 2 2 and carbonate, CO3 . The amount of CO3 formation can The interaction of N2 with K on Fe(111) surface was be substantially increased by pre-adsorbing oxygen, which investigated by HREELS [688,689]. Co-adsorbed K atoms 2 enhance the efficiency of the back-donation process from metal inhibits CO and oxalate formation. A well ordered CO3 πn overlayer can be formed by annealing to 600 K. Adsorbed d-orbitals to the N2 -anti-bonding orbital (Fig. 100), due to 2 the K-promoted decrease of the work function which alters the CO3 can also be formed on the saturated monolayer of K on local electrostatic potential. the Co(1010) surface, but with a perpendicular orientation. The HREELS spectra recorded after the saturation exposure Vibrational modes in Fig. 102 are reported with their assignment in Table 11. For the sake of comparison, in the same of N2 on K/Fe(111) are shown in Fig. 101. Two components at 174 and 180 meV (1405 and 1450 cm 1) have been observed Table 11 the normal modes of CO2 and CO2 are reported. and assigned to the N–N stretch vibration of K-coordinated and Fe-coordinated N atoms, respectively. The K-coordinated 5.9. On rhodium N atoms are characterized by a higher desorption energy and a lower stretch frequency with respect to those ones of N atoms 5.9.1. K+NO/Rh(100) n far from AM. This implies a strengthening of the N2–Fe In the gas phase, NO has one unpaired electron in the 2π interaction and a weakening of the N–N bond strength. anti-bonding orbital, unlike CO whose 2πn orbital is A. Politano et al. / Surface Science Reports 68 (2013) 305–389 365 unoccupied. Hence, co-adsorption of NO with AM could shed It should be also mentioned that the saturation NO coverage light on the role of 2πn states in AM co-adsorption. The increases monotonically with K coverage from 0.4 ML with a adsorption of NO on Rh(100) is characterized by two K-free surface to 0.8 ML for 0.41 ML K. molecular species, the side-on bonded (or highly inclined) α -NO species, whose N–O stretch is revealed at 115 meV 1 5.9.2. K+dimethyl ether on Rh(111) (panel (a) of Fig. 103) and the vertically bonded α2-NO species (N–O stretch at 200 meV). On the clean Rh(100) surface NO There is a considerable interest in the production of DME α which is a key compound in the conversion of methanol to initially adsorbs as 1-NO but with increasing NO coverage – α -NO starts to adsorb and α -NO reorients to α -NO [549]. gasoline [688 702]. For surface science studies DME repre- 2 1 2 sents a challenging compound since its activation and dis- The NO dissociation reaction is strongly dependent on cover- fi age on both K-free and pre-covered surfaces. The promotion of sociation on metal surfaces are very dif cult. Therefore only NO dissociation on Rh(100) by K can be accounted for by the few studies have been devoted to its bonding, structure and α reactivity on metals. stabilization of the 1-NO species, which is a precursor for NO fl dissociation. The in uence of potassium on the interaction of DME with The analysis of the vibrational spectrum of NO/0.30 ML K/ Rh(111) could be put in evidence by the analysis of HREELS Rh(100) (panel b of Fig. 103) indicates that the stretch experiments [703]. DME adsorbs molecularly on K-dosed Rh frequency of the α -NO species depends on the [K]:[NO] (111) at 90 K as indicated by the spectral features character- 2 ized for DME. Depending on K coverage, a fraction of stoichiometry. In particular, the N–O stretch of α2-NO species is strongly red-shifted in the K-modified surface for low NO adsorbed DME decomposes to adsorbed H and CO during coverage. However, by increasing the content of NO on the annealing. Analysis of HREELS spectra suggested the surface, the frequency of the N–O stretch undergoes a blue- shift up to reach the values observed for the clean substrate.

Fig. 103. (a) HREELS spectra upon adsorption of NO/Rh(100) at 100 K.

Fig. 102. RAIRS spectra acquired for CO2 dosed on multilayers of potassium, Panel (b) shows the recorded HREELS spectra for several coverages of NO grown at 160 K and successively annealed at selected temperatures (200, 250, adsorbed onto a Rh(100) surface modiﬁed by the pre-coverage of 0.30 ML K. 550, and 660 K). Adapted from Ref. [691]. Adapted from Ref. [549].

Table 11 Vibrational frequencies of CO2 and CO2 recorded on several K-promoted surfaces.

Sample δ(CO2) δðCO2 Þ vsðCO2 Þ vaðCO2 Þ νs(OCO) monodentate CO3 νa(CO2) Ref.

Rh(111) 640 840 1340 1630 1440 2350 [692] Pt(111) 640 660 835 1380 1645 1440 2350 [693] Pd(111) 645 744 1210 1530 1452 2350 [540,694] Ru(0001) 666 884 1240 16251708 1467 2357 [695]

Mo2C/Mo(100) 660 750 1250 16281670 1445 2355 [696] Co(1010) – 883 1133 1600 1438 – [690] Au(111) – 957 1350 1620 ––[697] 366 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

Table 12 Characteristic desorption and HREELS data for the DME+K/Rh(111) system. From Ref. [703].

TPD peaks (K) HREELS losses (cm1) Products from DME+K/Rh(111) θK ¼0.09 θK¼0.36 θK ¼0.09 θK ¼0.36

DME+K/Rh(11) 180 120 890 γs(CO) 890 γs(CO) 1180 γ(CH3) 1150 γ(CH3) 1450 δ(CH3) 1470 δ(CH3) 2910 ν(CH3) 2930 ν(CH3) H2/DME+K/Rh(111) 410 480 660 – H2/K/Rh(111) 450 520 –– CO/DME+K/Rh(111) 640 690 1480 ν(CO) 1490 ν(CO) CO/K/Rh(111) 640 690 500 ν(metal–C) 1410 ν(CO) 1500–1770

CH3O/DME+K/Rh(111) ––1010 1030 CH3O/CH3OH+K/Rh(111) ––– 180 ν(metal–C) 1050 ν(CO)

2780–2900 ν(CH3)

Fig. 104. Series of HREELS spectra for several coverages of Na on Pd(111) Fig. 105. IRAS spectra recorded for the K/Pd(111) surface after a saturation exposed to 2 L of CO2 at 90 K. Adapted from Ref. [704]. exposure to NO at 320 K. Spectra have been collected for K coverage ranging from 0 to 0.54 ML. Adapted from Ref. [293]. transitory formation of methoxy species which is stabilized by 150 meV (symmetric stretch). The peak around 190 meV potassium. Strong CO vibration losses at 206 meV 1 1 1 (1530 cm ) might be either attributed to the asymmetric (1660 cm ) for ΘK ¼0.03 and 185 meV (1490 cm ) for δ stretch of CO2 or to an AM-inﬂuenced C–Ostretch. ΘK ¼0.36 are observed as a signature of K-promoted dissociation of DME (Table 12). At high Na coverage (2 ML) losses at 83 and 293 meV (670 and 2363 cm1) were recorded and ascribed to physisorbed linear CO2. In the region between 105 and 220 meV several 5.10. On palladium losses were recorded, which were attributed to the vibrational modes of mono- and bidentate carbonate [705,706]. 5.10.1. Na+CO2/Pd(111) The co-adsorption of Na and CO2 on Pd(111) has been studied in Ref. [704]. For low Na coverage CO2 molecules 5.10.2. K+NO/Pd(111) dissociate into CO and O even at 90 K. At intermediate Na RAIRS has been used by Nakamura et al. [293] to δ coverage the reaction proceeds via a bent anionic CO2 species investigate NO adsorption on the K/Pd(111) surface. The which dissociates at higher temperatures into CO and O. The adsorption of NO on the clean Pd(111) surface was purely δ chemisorbed bent CO2 is characterized (Fig. 104)byvibra- molecular and reversible, whereas the decomposition of NO tional losses at 35 (metal–CO2 stretch), 92 (bending mode) and proceeded only on the K/Pd(111) surface. Fig. 105 shows the A. Politano et al. / Surface Science Reports 68 (2013) 305–389 367

RAIR spectra measured after the saturated adsorption of NO with a desorption temperature of Td ¼124 K. Adsorption of on the K/Pd(111) surface (θK ¼0–0.54) at 320 K. The vibra- CO2 on K-dosed Au(111) leads to its stabilization and to the tional feature due to NO adsorbed on the threefold hollow site formation of the CO2 anion radical. The reaction pathway of of Pd was observed at 192 meV for the clean Pd(111) surface CO2 species depends on K coverage. It undergoes dissocia- [707]. For the K/Pd(111) surface, three N–O stretching tion at low coverage, while a stable carbonate is formed at high features were observed at 167, 174, 186 meV. The existence K coverages. of three N–O spectral components is a clear fingerprint of HREELS spectra of the CO2+K co-adsorbed layer on Au different local chemical environments for NO molecules, thus (111) at different potassium coverages are shown in Fig. 106. suggesting short-range interactions among co-adsorbates. In The presence of potassium induces the appearance of vibra- particular, the latter peak at 167 meV has been assigned to NO tional modes at 119, 167, and 201 meV (957, 1350, and adsorbed on K islands on Pd(111) [293]. 1620 cm1), which were assigned to the bending, symmetric, and asymmetric stretching modes of CO2 [614], respectively. 5.10.3. K+C2N2/Pd(100) HREELS data in Ref. [708] show that ionic KCN forms 5.11.2. K+nitriles/Au(100) from the dissociative co-adsorption of C2N2 and K deposited The interaction of two different nitrile molecules, acetoni- on the Pd(100) surface. HREELS of this phase shows a surface trile (CH3CN; ACN) and benzonitrile (C6H5CN; BCN), with a mode due to bulk KCN and a C–N stretching frequency of K-modified (5 20)-reconstructed Au(100) surface was stu- 256 meV. Upon heating to 800 K, a new species of KCN, with died in Ref. [710]. HREELS measurements on K+BCN vibrational losses at 17 and 232 meV, is observed. This species (Fig. 107) revealed characteristic blue shifts of ν(C N) in is identified as covalently bonded species of KCN, where the the presence of co-adsorbed K. This is attributed to a removal reduced ionicity of KCN is due to its interaction with the of the slightly anti-bonding charge density at the nitrogen end underlying Pd substrate. of the C N group.

+ 5.10.4. Cs C2H4/Pd(110) 5.12. On molybdenum carbide Vibrational investigations [709] showed that ethylene on the Pd(110)(l 2) Cs surface is more weakly bonded to the 5.12.1. K+CO2/Mo2C surface as compared with ethylene on the clean Pd(110) The adsorption of CO2 on K-dosed Mo2C [696] for K surface. Moreover, the de-hydrogenation and CQC bond coverages above 0.18 ML induced the appearance of peaks scission processes are promoted by the pre-adsorbed Cs atoms. characteristic of negatively charged CO2 at 93, 120, 182, and 201 meV (750, 965, 1470 and 1625 cm1)(Fig. 108). For 5.11. On gold lower K coverages, the vibrational modes of CO2 are not inﬂuenced by the presence of AM atoms. Very likely, CO2 is + 5.11.1. K CO2/Au(111) formed in a direct interaction between potassium and CO2 Potassium adsorption could convert the neutrally adsorbed δþ δ KðadÞ þ CO2ðgÞ ¼ K CO2ðadÞ ð35Þ CO2 into a more reactive negatively charged CO2 due to a charge transfer from K-dosed Au(111) to an empty π orbital of through a charge transfer from the K-dosed Mo2C into the n CO2.CO2 adsorbs only weakly on a clean Au(111) surface, empty π orbital of CO2.

Fig. 106. (a) Effects of potassium coverage on the HREELS spectra of adsorbed CO2 on Au(111) at 95 K. The CO2 exposure was 8.0 L. (b) Effects of CO2 exposure on HREELS spectra of Au(111) at θK ¼1.0 ML. Adapted from Ref. [697]. 368 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

5.13. On graphite

5.13.1. K+O2/graphite The system K+O2/graphite was investigated by HREELS experiments in Ref. [32]. Besides the presence of the vibrational transition of O2 at 190 (v¼0–1) and 380 (v¼0–2) meV (Fig. 109), also another mode at 140 meV with its overtone at 280 meV has been revealed. It has been ascribed to O2 , as the 2Π vibrational frequency of the g state of the O2 molecule in the gas phase is 133 meV [711]. The coexistence of ionic and neutral oxygen species is a ﬁngerprint of a local charge transfer between co-adsorbates. Moreover, the analysis of vibrational peaks in the work by Chakarov et al. [712] suggests the occurrence of continuous transformations between different [K]:[O] stoichiometries, with also the coexistence of different KOx complexes on the surface.

Fig. 107. HREELS spectra recorded in specular geometry (451) with an impinging energy of 3 eV for: (a) BCN multilayers on the clean Au(100) surface; (b) a nearly complete monolayer of BCN on Au(100). The spectrum (c) was recorded after a (i) pre-dosing of 0.06 ML K, (ii) exposure to BCN up to saturation, and (iii) selective desorption of the molecular BCN species which Fig. 109. HREELS spectrum from co-adsorbed K and O2 on graphite at are not in direct contact with K (as indicated by their known desorption To30 K, adapted from Ref. [32]. The scattering conditions are shown in temperatures). Adapted from Ref. [710]. the inset.

Fig. 108. (a) Effects of K coverage on the HREELS spectra of adsorbed CO2 at 100 K. Effects of CO2 exposure on the HREELS spectra of K/Mo2C system: (b) ΘK¼0.1 ML; (c) ΘK ¼1.0 ML. Adapted from Ref. [696]. For the assignment of the vibrational modes, the reader is refereed to Table 11 of this review. A. Politano et al. / Surface Science Reports 68 (2013) 305–389 369

Fig. 110. HREELS spectra of H2O and H2O co-adsorbed with Na on graphite (0001) surface at 95 K: (a) 0.65 ML H2O; (b) 0.1 ML Na+0.65 ML H2O; (c) 0.22 ML Na+0.65 ML H2O. Adapted from Ref. [713].

Fig. 111. HREELS spectra for 0.25 ML H2O on graphite for different 5.13.2. AM+H2O/graphite potassium coverages. The spectra are taken at 85 K, which was also the The HREELS spectra in Fig. 110 show the effect of Na co- deposition temperature. In all cases the ﬁrst co-adsorbate deposited on the graphite surface was water. Adapted from Ref. [217]. adsorption with H2O on graphite [713] for two typical values of θNa. When co-adsorbed with Na, the H2O molecules are only partially dissociated as it is evident from the OH stretch vibration at 416 meV and the HOH bending vibration at 210 meV. The partial dissociation and probable formation of NaOH is further indicated by the O–H stretch in OH that is seen around 446 meV. The appearance of the collective modes between 30 and 105 meV is due to hydroxide and the undissociated water being incorporated in an H-bonded network at the graphite surface. In fact, it was found that co-adsorption with NaOH results in increased clustering of the H2O molecules at the graphite surface [585]. Similar results have been obtained for K co-adsorption with water [217]. The transition from non-dissociative to dissociative co-adsorption as the K coverage is increased as evidenced by several changes in the vibrational spectra (reported in Fig. 111): (i) the disappearance of the H2O collective modes; (ii) the frequency shift of the O–H stretch (from 415 to 454 meV); (iii) the intensity variation and the frequency shift of the frustrated rotation mode νR (8878 meV), and (iv) the appearance of K–O and K–H losses at 33–55 and 106– 126 meV, respectively. The development of these features with increasing K is shown in Fig. 112, for a constant water coverage of 0.25 ML. Water dissociation does not occur for potassium coverages below 0.3 ML. For 0.4 ML of K water dissociation is complete.

5.14. On diamond

5.14.1. K+O/C(100) Chemisorption of the O2 and CO molecules on the K- modiﬁed C(100) surface has been studied by HREELS at 300 K [714]. Although O2 does not react with the clean C(100) surface, it readily reacts with the K-modiﬁed surface at 300 K. Fig. 112. A summary of the HREELSobservations for potassium-induced Two different chemisorbed species are observed which are water dissociation. Water coverages vary between 0.25 and 0.4 ML. Σ(diss. characterized by the loss peaks at 150 and 214 meV (Fig. 113). products) shows the appearance and growth of K–O and K–H vibrations at 33– The loss peaks at 150 and 214 meV are ascribed to the C–O 55 and 106–126 meV, respectively. Adapted from Ref. [217]. 370 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

Fig. 113. HREELS spectra in the specular mode of: (a) the clean C(100) 2 1 surface characterized by its own phonon modes; (b) the clean surface exposed to 1 ML of K; and (c–g) K-modiﬁed surface exposed to O2 with increasing exposure at 300 K. All spectra are normalized by the elastic peak intensities. The primary energy is 6.7 eV. Adapted from [714]. stretch of C–O–C (ether) and CQO (carbonyl) species, which s π are formed by breaking both and bonds of the surface Fig. 114. HREELS spectra recorded for: (a) the clean C(100) 2 1 surface; (b) dimer, respectively. the same surface covered by 1 ML of K; and (c,d) K-modiﬁed surface exposed The large decrease of the work function of C(100) due to the to 0.3 and 15.0 L of at RT. The primary energy is 6.7 eV. Adapted from Ref. K adsorption (3.2 eV at 1 ML K) facilitates the electron [714]. n transfer to the 2π anti-bonding orbitals of O2 resulting in the dissociative adsorption of O2 molecules. In contrast to what happens in Si(100) (see below), the K-induced oxidation occurs mainly at the top layer of C(100).

5.14.2. K+CO/C(100) While the CO molecule is known not to adsorb on the clean C(100) 2 1 surface, the adsorption of CO occurs on the K- modified C(100) surface. The adsorbed species are characterized by the loss peak at 154 meV with a shoulder at 192 meV 2 + Fig. 115. Possible products formed by the CO exposure on the K-modified C (Fig. 114), assigned to C–O stretch in (C2O2) 2K and 2 + (100) surface. Adapted from Ref. [714]. (C4O4) 2K complexes, respectively. The possible products formed upon CO exposure on the K-modified surface are shown in Fig. 115. spectrum could provide important information on surface chemical bonds at the interface between organic molecules 5.15. On silicon and the Na-modified silicon surface. It is worth remembering that molecules, whose primary 5.15.1. AM co-adsorption with organic molecules on Si(100) functionality is a carboxylic acid, such as benzoic acid, adsorb Vibrational spectroscopy is also a powerful tool for obtaining by cleavage of the O–H bond and its replacement by a single information on structural properties. An example is represented covalent O-Si bond [732]. In the case of 4-aminobenzoic acid by HREELS experiments on AM co-adsorption with organic on Na/Si(100)2 1 [716] the species were bonded to the molecules on Si(100). The deposition of a sodium template on substrate through the carboxylate group in bi-dentate coordina- the Si(100)2 1 surface significantly modifies the electronic tion leaving the amino group unperturbed leading to an amino properties of the substrate [715,716]. In order to understand and terminated surface. thereby to control the interface between organic molecules and Fig. 116 shows the vibrational spectra of 4-aminobenzoic inorganic substrates, it is important to study model systems in acid adsorbed at RT on (a) Si(100)2 1 and (b) Na/Si detail. In particular, the interaction of organic molecules with (100)2 1. Table 13 provides the frequency assignment of silicon surfaces is attracting great attention [717–729] due to the the observed features by comparison with ab initio calcula- enormous possibilities of Si-surface functionalization for mole- tions. Vibrational data indicate that 4-aminobenzoic acid cular devices, primarily hybrid structures made from organic adsorbs on clean Si(100)2 1 through the carboxylate molecules on silicon surfaces. group. The observation of the ν(CQO) stretch indicates The co-adsorption of Na and 4-aminobenzoic acid [716], species bonded to Si(100)2 1 in mono-dentate coordina- benzoic acid [276,730] and glycine [731] have been studied on tion. The bonding geometry of 4-aminobenzoic acid to Si Si(100) by Richardson's group. The analysis of the HREELS (100)2 1 is dramatically modified by co-adsorbed Na A. Politano et al. / Surface Science Reports 68 (2013) 305–389 371 atoms. The sharp feature observed at 176 meV (1420 cm1)in Upon adsorption of benzoic acid on Na/Si(100)2 1, the HREELS spectrum of 4-aminobenzoic acid on Na–Si benzoate species are found in a bi-dentate coordination and (100)2 1, shows clearly that bonding occurs exclusively tilted with respect to the surface normal [730]. HREELS via the carboxylate group, leading to 4-aminobenzoate species spectra in Fig. 117 show an intense νs(OCO) stretching in bi-dentate coordination. The amino group related bands vibration, which is characteristic for benzoate aligned perpen- observed at 46, 201 and 429 meV (370, 1620 and 3460 cm1) dicular to the substrate surface. In contrast, benzoate bonds to indicate that this adsorption geometry leads to an amino Si(100)2 1 through a Si–O–(CO)–C connection. The terminated surface with NH2 groups pointing out into the observation of SiH species on both surfaces shows that the vacuum. chemisorption of benzoic acid occurs through a de-protonation HREELS measurements support a structural model in which of the acid. the 4-aminobenzoate species adsorb on Na/Si(100)2 1 through the formation of a delocalized bond between the de- protonated carboxylate group and the adsorbed Na atom.

Fig. 117. HREELS spectra of benzoic acid adsorbed on Na–Si(100)2 1 (spectra (a) and (b)) and on Si(100)2 1 (spectra (c) and (d)). Spectra (a)

Fig. 116. HREELS spectra of 4-aminobenzoic acid adsorbed on (a) clean Si and (c) were recorded in specular (θi ¼θs¼451) geometry while spectra (b) and (100)2 1 and (b) on Na/Si(100)2 1. The spectra were recorded in (d) in off-specular (θi ¼651, θs ¼451) scattering conditions. The primary specular scattering geometry (θi ¼θf ¼451) and an electron energy of Ei ¼8 eV. electron energy was 8 eV. A schematic representation of the benzoate surface Adapted from Ref. [716]. species is also given. Adapted from Ref. [730].

Table 13 Comparison of the experimental HREELS results with the predicted vibrational spectrum of 4-aminobenzoic acid (ABA) on the clean and Na-precovered Si(100) 2 1 surfaces. Adapted from Ref. [716].

Gaussian 98 (cm1) Observed in HREELS (cm1) Assignment

ABA/Na/Si ABA/Si ABA/Na/Si ABA/Si

3512 3540 3513 3513 νas(NH2) 3408 3490 3460 3460 νs(NH2) 3180 3110 3120 3120 νas(CH) 3050 3000 3060 3060 νs(CH) – 2090 2075 – ν(SiH) – 1780 – 1705 ν(C¼O)

1685 1690 1620 1620 δ(NH2) 1495 –– – νas(OCO) 1404 – 1420 – νs(OCO) 1213 1300 1290 1290 ν(CO) 1156 1190 1150 1180 β(CH)

854 860 825 830 δ(CO2) – 800 – 750 ν(SiO) – 620 – 630 Ring 643 – 610 – ν(CN) 429 480 495 490 Amin/carbox

364 – 370 – β(NH2) 372 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

Fig. 120. Vibrational spectra of different alkali compounds obtained during annealing of the surface: (a–c) formate at 200, 250, and 375 K; (d) oxalate at 475 K; and (e) carbonate at 600 K. Adapted from Ref. [733].

high stability of formate and its sequential and selective Fig. 118. HREELS spectra recorded for (a) glycine and (c) d5-glycine conversion to oxalate and carbonate. adsorbed on clean Si(100) 2 1 and (b) and (d) on Na/Si(100) 2 1. The IR spectrum (a) of Fig. 120 is characterized by several Spectra were acquired in specular scattering geometry (θi ¼θf¼451) and for an impinging energy of 8 eV. Adapted from Ref. [731]. vibrational bands, which are due to potassium formate. The band at 345 meV (2785 cm1) is assigned to the C–H stretch and a weak band at 1388 cm1 to the CH bending mode. The bands at 201 and 170 meV (1619 and 1367 cm1) correspond to the asymmetric and symmetric OCO stretch, respectively, while the band at 95 meV (766 cm1) is that of OCO bending mode. The remaining band at 2695 cm 1 is the overtone of the CH bend 2δ(CH), whose intensity is enhanced by a Fermi resonance with the C–H stretch. Annealing from 200 (spectrum (a)) to 250 K(spectrum (b)) produces small shifts and sharpening of the vibrational modes Fig. 119. At temperatures below 200 K potassium pre-covered Cu(111) of formate. promotes the hydrogenation of carbon dioxide to potassium formate. At C–H bands due to formate survive up to 425 K and increasing temperature a sequential transformation of formate into oxalate and – carbonate is observed. The processes can be monitored spectroscopically as disappear at higher temperature. The shift in the C O stretch shown in Fig. 119. Adapted from Ref. [733]. modes is a fingerprint for oxalate formation. Upon heating above 500 K, asymmetric and symmetric C–O stretches are no longer seen. The occurrence of an intense band at 181 meV As regards glycine and deuterated glycine co-adsorption (1460 cm 1) and a weaker line at 102 meV (822 cm 1) with Na [731] (Fig. 118), HREELS measurement showed that indicates the transformation into carbonate, which is stable glycine bonds to Na/Si(100)2 1 through the carboxylate on the surface up to 675 K. group in bi-dentate coordination. In contrast, mono-dentate species were found after the exposure of Si(100)2 1to 6.2. AM+CO+O/Ni(111) glycine. Studying the co-adsorption of CO and O in the presence of 6. Ternary co-adsorption systems and multilayered AM allows to follow step-by-step the reaction pathways for the substrates formation of carbonates [704] or carbon dioxide [245]. The presence of CO molecules has a strong influence on the 6.1. K+CO2+H2O/Cu(111) oxidation process of AM atoms exposed to oxygen [617].In order to investigate the effects induced by CO adsorption on Hoffmann et al. [733] have presented a time-evolved IRAS the surface chemical bonds of Na and O, oxygen was co- and TPD study of the reaction of CO2 and H2O in thin adsorbed with Na onto a CO-modified Ni(111) surface [617] potassium layers on Cu(111). Potassium promotes the forma- (Fig. 121). The co-adsorption of Na atoms (third spectrum of tion of formate species by reduction of carbon dioxide and by Fig. 119) caused the disappearance of the CO–Ni stretching at dissociation of water, as sketched in Fig. 119. Water splitting 43 meV (as common in co-adsorption of CO and AM [619]) serves as a source of hydrogen for CO2 hydrogenation. and a softening of the C–O stretching energy down from 226 Conversion of carbon dioxide utilizing protons from water down to 197 meV. An oxygen exposure of 8.0 L caused decomposition is likely to provide a sustainable source of fuels remarkable changes in the vibrational spectrum, thus suggest- and chemicals in the future. Thermal stability and decomposi- ing the occurrence of striking modifications of surface chemi- tion pathways studies by TPD and IRAS point to a relatively cal bonds. The appeareance in the loss spectrum of a feature at A. Politano et al. / Surface Science Reports 68 (2013) 305–389 373

34–36 meV indicated the formation of the Na–O bond [279– For a CO-modiﬁed Ni(111) surface (by predosing the 282,284,508,617,618]. In the meantime, the energy of the C–O substrate with the same CO exposure of 1.0 L), the C–O stretching shifted from 197 to 209 meV. Further O2 exposures stretching energy as a function of O2 exposure was found to cause a shift of the C–O peak up to 214 meV. follow a power law (Fig. 122) depending on the initial Na pre- coverage:

B vCO ¼ E0 þ ALO ð36Þ

where LO is the exposure to oxygen, E0 the C–O stretch energy at vanishing oxygen exposure, A, and the exponent B are the fitting parameters, whose values are reported in Table 14. Only small variations of A and B with respect to the Na coverage were found for the AM pre-coverage varying over an order of magnitude, so as to suggest that only E0 is affected by the initial amount of adsorbed AM. This supports the conclusion that the C–O stretch frequency versus O2 exposure obeys the same power law over a wide range of AM pre-coverage, whose effect only consists in a shift of E0 [734]. As expected, the energy of the C–O loss peak is strictly related to the Na/CO ratio. Exposing to O2 a CO-modified Ni (111) surface with a Na coverage beyond 0.20 ML, the 2 formation of carbonates CO3 occurs, as inferred by the appearance of loss peaks at 92 and 108 meV. This is a direct Fig. 121. HREELS spectra of a CO-modified 0.06 ML Na/Ni(111) exposed to consequence of the CO-induced change of the electronic oxygen at RT. The intensity of all peaks was normalized to the intensity of the charge of AM adatoms. Such results indicate that oxygen elastic peak. All spectra were multiplied by the same factor. From Ref. [284]. interacts strongly with charged AM species and less with neutral adatoms. Furthermore, it should be noticed that also the CO-induced blocking of the population of O 2pz anti-bonding orbitals in the AM+O co-adsorbed phase may play a key role in the microscopic mechanisms leading to AM oxidation. For Ni(111), the efficiency of the AM oxidation via CO dissociation [282] or via the CO-promoted ionization of AM adatoms [617] depends on the local stoichiometries between AM and CO, on sample temperature and on CO and AM pre- coverages. We want to point out that the method reported in Ref. [282] implies a dissociative adsorption for CO, while AM oxidation obtained by the CO-induced ionization of AM necessitates pre-adsorbed CO molecules. CO exposure was found to cause a remarkable increase of the intensity of the Na–Ni peak [278]. Such finding is a clear fingerprint of the occurrence of a local charge transfer between co-adsorbed species, i.e. a CO-induced partial ionization of AM adatoms. In fact, the intensity of vibrational modes is expected to be higher for ionized species rather than for strongly polarized but essentially neutral adatoms. In Fig. 123 the behavior of the dynamical charge of Na as a function of CO exposure is

Fig. 122. Behavior of the C–O stretching energy as a function of the O2 shown. As the amount of CO deposited on the surface exposure for different Na pre-coverages. The inset shows an approximate linear increased, the dynamical charge Q of Na decreased almost dependence of E0 on the Na pre-coverage. linearly, giving hints on the charge status of AM atoms. Hence,

Table 14 Values of the coefﬁcients of the ﬁt procedure using Eq. (36) of curves presented in Fig. 120. From Ref. [617].

Na coverage (ML) E0 (meV) AB

0.02 205.070.5 5.270.7 0.3770.05 0.06 197.570.3 5.370.5 0.3670.04 0.20 179.970.3 5.470.4 0.3670.02 374 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

Fig. 123. The dynamic dipole charge Q/e (e is the electron charge) of Na as a function of CO exposure. From Ref. [328].

Fig. 125. HREELS spectra as a function of K coverage for PTCDA/Ag(110). Fig. 124. HREELS spectrum acquired for 0.10 ML K deposited onto a Spectra were recorded with a primary electron beam energy of 2.5 eV. Adapted 0.10 ML O-modified Ni(111) surface (spectrum a). Reversing the order of from Ref. [738]. adsorption, the alkali–O bond is formed. Similar results were obtained by substituting K with Na (spectrum b). The intensity of all peaks was normalized to the intensity of the elastic peak. All spectra were multiplied by the same is nearly identical to that from undoped films (see Fig. 125 and factor. From Ref. [617]. Refs. [740,741]). Above a concentration of 0.7 K/PTCDA five modes at 48, 66, 129, 150, and 191 meV (marked in Fig. 125 as the occurrence of a charge transfer between co-adsorbates is at A–E) appear and their intensity increased with K coverage. The the basis of the formation of the AM–O bond. C–H out-of-plane mode at 108 meV, indicative of PTCDA Measurements were also performed (in the absence of CO) multilayers [742], disappears at K concentrations above 1 K/ by reversing the sequential order of the adsorption, i.e. oxygen PTCDA. This mode is known to be very sensitive to changes in adsorbed before the AM. In these conditions, AM oxidation is the environment of the PTCDA molecule. The suppression with readily observed, as suggested by the appearance of the O–K increasing K-dose in the present case is therefore attributed to the (28 meV) and O–Na (34 meV) vibrational modes in HREELS structural and chemical changes occurring in the presence of K. spectra (Fig. 124). The direct bond of atomic oxygen with the In agreement with Ref. [740] the first two modes (A and B substrate modifies the electronic properties of the system and it in Fig. 125) are assigned as C–CQO and C–O–C bending should be responsible of the observed behavior. Very likely, modes in the anhydride group. The next two (C and D) involve the charge state of O atoms plays a decisive role. C–O– C stretching and C–H bending vibrations, accompanied by ring deformations. The fifth marked mode (E) is attributed 6.3. K+perylene-tetracarboxylic-dianhydride films on Ag(110) to C–H bending and ring deformation vibrations. The K– PTCDA interaction leads to a remarkable charge transfer and Doped molecular organic semiconductors are of increasing hence to the formation of new inter-molecular chemical bonds, interest as promising materials for novel molecular electronic accompanied by structural changes. At about 1 K/PTCDA the and photonic devices, such as light-emitting diodes, lasers and effect is most pronounced leading to a partially filled (“metal- thin-film transistors [735,736]. The vibrational properties of K- lic”) molecular valence band. For high K-coverages the doped ultrathin films of the organic semiconductor 3,4,9,10- valence band is filled and the molecular layer becomes again perylene-tetracarboxylicacid-dianhydride (PTCDA) (for a a semiconductor. review, see Ref. [737]) deposited on Ag(110) have been studied in Ref. [738]. HREELS indicated that K-doping leads 6.4. K+CO2 on copper films to a semiconductor–metal phase transition for low K doping and to a metal–semiconductor one for higher K doping, as also SERS has been used for investigating the co-adsorption of observed for AM doping of fullerenes [739]. CO2 on AM-doped copper surfaces [742]. On potassium- For small amounts of K (up to approximately 0.5 K/PTCDA, doped copper films, monodentate carbonate as well as carbon i.e. 0.5 K atoms for PTCDA molecules) the vibrational spectrum monoxide has been observed in addition to the two carbon A. Politano et al. / Surface Science Reports 68 (2013) 305–389 375

CO2 is with its molecular plane perpendicular to the surface and bonded to the AM through the O atoms. Experiments with varying potassium coverage imply that the þ K CO2 pair is not randomly distributed on the surface but aggregated into islands, as supported by a Monte-Carlo simulation. Experiments on Li- and Cs-dosed Ag surfaces led to similar conclusions. The same authors studied CO adsorption on K- modiﬁed silver [298]. Therein the formation of a KOC surface complex was proposed.

6.6. AM co-adsorption on Mo2C/Mo(100)

6.6.1. K co-adsorption with alcohols on Mo2C/Mo(100) As noted in Section 5, metal carbides have been demonstrated to be efficient catalysts [743–750]. Mo2C is a particularly active material for converting methanol and ethanol into more valuable compounds [751,752]. An important feature of Fig. 126. SERS spectra of 20 L CO2 adsorbed at 86 K onto a copper film and doped with 3.6 ML K at the same temperature. Spectra have been recorded as a Mo2C catalyst in these reactions is its high thermal stability function of temperature. For the assignment of the various peaks the reader is and long life. The catalytic activity of Mo2C is further refereed to Table 1 of Ref. [742]. improved by adding potassium to Mo2C, which considerably enhanced the rate and the yield of the hydrogen formation [751,753]. The influence of potassium on the nature of the adsorption, on the dissociation pathways, and on the decomposition of adsorbed alcohols (methanol and ethanol) on Mo2C/Mo(100) surface has been studied in Ref. [753]. Potassium stabilizes the adsorbed alcohols on the Mo2C/Mo (100) surface, which decompose only at higher temperature yielding H2, CO and CH4 (for methanol) and H2, CO, C2H4, CH3CHO and CH4 (for ethanol). Vibrational measurements provided evidence for the ruptures of both O–H and C–O bonds in the adsorbed alcohols and the possible formation of CH3O–K, CH3 (for methanol) and C2H5O–K, C2H5, species (for ethanol), respectively. The relationship between adsorption sites and the experimental vibrational spectrum has been also studied by theory. For methanol adsorbed on a K top site, Pistonesi et al. [273] – – Fig. 127. SERS spectra of CO2 on a K-predosed surface at 50 K with (a) found that the C O distance increases and C O frequency 0.15 ML of K and (b) 0.40 ML of K, respectively. Adapted from Ref. [297]. presents the maximum decrease when compared with the methanol adsorption in on-top site of Mo. The C–O bond is – dioxide species. When co-adsorbing CO and atomic H on softened and O H bond is strengthened. This suggests a 2 – such a substrate, the formation of a surface-bound formate hypothetical dissociating mechanism in which a C O bond species seems to occur when hydrogen is adsorbed before breakage on K-covered surfaces could be more favorable. carbon dioxide (Fig. 126). Formate is stable on clean copper surfaces at 200 K, but it completely dissociates when co- 6.6.2. K co-adsorption with C3H7 on Mo2C/Mo(100) adsorbed with AM. HREELS spectra obtained after an exposure of 8.0 L of C3H7I on clean and K-dosed Mo2C at 100 K showed no substantial difference [754] with respect to those recorded for 6.5. K+CO2 and CO on silver films molecularly adsorbed C3H7I on different solids [755–757]. A completely different picture was, however, obtained by low SERS experiments have been carried out by Maynard and C3H7I exposure (0.4 L), for which new features at 170, 336, Moskovits [297] on silver films dosed with submonolayer 365 meV appeared. Based on the XPS measurements, these amounts of potassium. They found that CO2 reacts to form the new spectral features are very likely due to the vibration of þ C2v isomer of a K CO2 surface complex. Bands at 94 and propyl fragment formed in the dissociation of C3H7I. This 151 meV (755 and 1220 cm1) observed with K-modified result suggests that the K-promoted effects are short-ranged surfaces (Fig. 127) were assigned to the stretching and bending and they are evident only for a reasonably high [K]:[C3H7I] vibrational modes of CO2 . Very likely, the orientation of stoichiometry. 376 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

The analysis of the HREELS spectra also provided evi- it leads to the formation of N2O, which desorbs just above dences for the formation of KI (formed from the reagents K 100 K. and C3H7I present on the sample surface) upon annealing at Fig. 129 shows RAIR spectra recorded after a saturation 207 K, as indicated by the appearance of FK modes of exposure of NO at 90 K of the Na-modified surface. The adsorbed KI at 46 and 81 meV [758]. intense N–O stretching mode recorded at 218 meV 1 (1760 cm ) on clean Cr2O3(0001) is completely absent. 6.7. AM co-adsorption on Cr2O3(0001)/Cr(110) Instead, for the lower Na coverages two weak features near 161 meV (1300 cm1) and another weak band at 278 meV 1 6.7.1. Na+CO2/Cr2O3(0001)/Cr(110) (2243 cm ) were observed. For higher Na coverages three CO2 adsorption and reaction on Cr2O3(0001) is considerably intense bands with maxima at 107, 132, and 163 meV (864, modified by the presence of Na on the surface. RAIRS has 1064 and 1315 cm 1) were recorded. been used in [759] for investigating the dependence of the The band at 278 meV can be identified as the N–N- catalytic properties on Na coverage (Fig. 128). At low cover- stretching vibration of weakly bound N2O molecules which, age Na adsorbs by transferring an electron to the Cr2O3(0001) therefore, already forms at very low temperature on the surface favoring the formation of bent CO2 (carboxylate) surface. Bands near 160 meV are indicative of the existence + upon CO2 exposition, which has different vibrational features of Na NO salts in different chemical environments. The (see Table 11) with respect to physisorbed CO2 (characterized disappearance of the signal at 500 K is compatible with the by an intense peak due to asymmetric stretch at 292 meV, thermal decomposition of Na2N2O2 with the release of N2O 1 2357–2359 cm ). NaCO2 salts form and also Na2CO3 can be according to: observed (asymmetric stretch at 179 meV, 1440 cm1). Car- Na2N2O2-Na2O þ N2O↑ ð38Þ bonate forms via disproportionation of two CO2 molecules into carbonate and CO, with the latter being released into the gas phase: 2 CO2 UCO2-e -2CO2 -CO3 þ CO↑ ð37Þ 6.1. AM co-adsorption with CO on bimetallic surfaces

Recently, chemical reactions at bimetallic surfaces have attracted a notable interest [761–770] for both fundamental 6.7.2. Na+NO/Cr O (111)/Cr(110) 2 3 interest and technological applications (fuel cells, heterogeneous For NO adsorption on Na-modiﬁed Cr O (0001)/Cr(110), 2 3 catalysis, chemical sensors, pollution). AM co-adsorption on vibrational spectra indicate the formation of the alkali nitrosyl bimetallic surfaces has been scarcely investigated. In the recent salt Na N O at lower temperatures and the formation of an 2 2 2 work by Politano et al [1]. the co-adsorption of AM atoms (Na, alkali salt of the (NO) dimer, i.e. sodiumhyponitrite Na N O 2 2 2 K) with CO on Ag/Cu(111) and Ag/Ni(111) has been studied. at higher temperatures. N O is released via decomposition of 2 The existence of QWS within the Ag adlayer is proven for both the latter. NO dissociation occurs even at low temperature and Ag/Cu(111) and Ag/Ni(111) [61] systems. However, due to the absence of a gap in Ni(111), the character of the QWS in such

Fig. 128. Series of IRAS spectra of CO2 (30 L at 90 K) adsorbed on clean and Fig. 129. IR spectra of NO adsorbed (30 L at 90 K) on a sodium covered Na pre-covered Cr2O3(0001)/Cr(110) surfaces. Na coverage was prepared with Cr2O3(0001) surface as a function of sodium coverage. The sodium layer was the sample at 90 K. Adapted from [759]. prepared at 90 K. Adapted from Ref. [760]. A. Politano et al. / Surface Science Reports 68 (2013) 305–389 377

Fig. 131. HREELS spectra acquired for 0.15 ML of K deposited onto 10 ML Ag/Ni(111) at RT.

below 0.40 ML Na. For such coverage two loss features at 36 and 55 meV appeared (Fig. 130, spectrum b), thus indicating the occurrence of CO dissociation. Na doping determines the activation of a local charge transfer from the Ag/Ni(111) interface to the 2πn anti-bonding orbitals of CO leading to a – Fig. 130. HREELS spectra acquired after having exposed 0.4 L of CO at RT softening of the C O bond up to the dissociation of CO on (a) 0.06 ML Na/Cu(111), (b) 0.40 ML Na/Ni(111), (c) 0.06 ML Na/Ni molecules. Such charge transfer is very likely due to the n (111), and (d) 0.06 ML Na/1 ML Ag/Ni(111). From Ref. [1]. hybridization between the CO 2π states and the QWS of Na- promoted Ag/Ni(111). A large hybridization, and probably an efficient charge two systems is substantially different [771]. QWS on Ag/Cu transfer, is possible only whenever the Na–CO distance is (111) are characterized by standing wave patterns, which on the short, as also found in theoretical calculations performed for contrary are not supported on Ag/Ni(111). Such significant CO adsorption on Na quantum dots on Cu(111) [772]. More- dissimilarities in the electronic properties between these two over, recent theoretical results performed for CO adsorption on bimetallic surfaces could in principle lead to quite different Al/Fe(100) [773] have demonstrated that the activation barrier catalytic properties. Accordingly, they may be used as model for CO dissociation is strongly reduced in the bimetallic systems for a comparative study on the chemical reactivity of surface with respect to the clean Fe(100) surface. On the basis bimetallic surfaces. of the latter result, it can be suggested that the joint action of To verify the effects of the adsorption of submonolayers of AM AM doping and the existence of a bimetallic Ag/Ni(111) on the chemical reactivity of the above bimetallic surfaces, interface should further reduce the activation barrier so as to 0.06 ML of Na deposited on a single layer of Ag on Ni(111) produce an enhanced CO dissociation rate. On the other hand, were exposed to 0.4 L of CO (Fig. 130, spectrum d). For the sake the ratio between the intensity of the O–K vibration (recorded of a comparison, the same experiment was performed for 0.06 ML at 27–29 meV) with respect to the C–O stretching indicates of Na deposited onto the clean Ni(111) and Cu(111) substrates that in the K-doped Ag/Cu(111) surface the activation barrier (Fig. 130, spectra a and c). for CO dissociation is higher than on K/Cu(111). The occurrence of CO dissociation was found for 0.06 ML The remarkable differences towards the promotion of CO Na/1 ML Ag/Ni(111) (Fig. 130, spectrum d) while only dissociation in AM-modified Ag/Cu(111) with respect to the molecular CO adsorption was observed on 0.06 ML Na/Ni case of AM-doped Ag/Ni(111) could be ascribed to the (111) (spectrum c). The partial dissociation of CO molecules different electronic properties of these two systems. Due to was inferred from the appearance of the stretching vibration of the absence of a gap on Ni(111), hybridization effects between Na against O atoms at 36 meV coming from dissociating CO QWS and substrate states are enhanced on Ni(111). This effect molecules. As concern the C–O internal vibration, its energy is very important for a single layer of Ag [774]. Such was recorded at 215 and 232 meV for 0.06 ML Na/Ni(111) hybridization enhances the efficiency of the charge transfer (spectrum c) and at 186 meV in the 0.06 Na/1 ML Ag/Ni(111) to 2πn anti-bonding orbitals of CO, so as to soften the C–O (spectrum d), respectively. Interestingly, at a fixed Na pre- bond and decrease the activation barrier for CO dissociation. coverage (0.06 ML) the C–O bond is much more softened in the On the other hand, the surface obtained by depositing bimetallic surface with respect to the clean Ni(111) substrate. 0.15 ML of potassium on 10 ML Ag/Ni(111) was found to The weakening of the C–O bond leads to CO dissociation and to be strongly reactive towards water molecules arising from chemical reactions of the derived species, i.e. the Fischer- residual gas in the UHV chamber. Vibrational spectra Tropsch process, CO oxidation, carbonate formation and so on. (Fig. 131) revealed the adsorption of partially dissociated On the other hand, for Na+CO co-adsorption on clean Ni water, as suggested by the presence of O–H stretching peaks (111), CO dissociation was not revealed for AM pre-coverages at 435 meV (water molecules) and 450 meV (OH groups) 378 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

[40,585]. Another clear fingerprint of adsorbed molecular water is the appearance of HOH bending at 199 meV [585]. Interestingly, water adsorption was not allowed for AM- modified 1 ML Ag on both Cu(111) and Ni(111) substrates. The enhanced reactivity of the AM/bimetallic surfaces towards water adsorption could be ascribed to the presence of well- defined Ag 5sp-derived QWS within the adlayer for 10 ML compared to 1 ML [775]. In fact, water reactivity was found to be enhanced in systems exhibiting electron quantum confinement [40,277,588]. Such finding may be of great interest as water interaction with solid surfaces is useful for understanding a wealth of physical and chemical phenomena such as pollution, corrosion, energy production, and heterogeneous catalysis.

6.2. AM co-adsorption on epitaxial graphene

The vibrational spectrum of the co-adsorbed phase of potassium with water on graphene/Pt(111) has been studied by Politano and Chiarello [776] for a sample temperature of 100 K. In the absence of alkali dopants, water on graphene/Pt(111) adsorbs undissociatively at 100 K, as also reported in Ref. Fig. 132. HREELS spectra for 0.08 and 0.25 ML of K adsorbed on MLG/Pt [370]. Water molecules on graphene/Pt(111) at 100 K are (111) and successively exposed to 1 L of H2O at 100 K. The inset shows a fi physisorbed and they desorb from graphene by heating the magni cation of the outermost spectrum (0.25 ML K). surface at 130 K. To investigate the effects of K on water adsorption, the MLG/Pt(111) surface precovered by 0.08 capability of storing hydrogen. In particular, the presence of (bottom spectrum of Fig. 130) and 0.25 ML (top spectrum of metal catalysts on graphitic systems enhances the hydrogen Fig. 130) of K, respectively, was exposed to 1 L of water at uptake of the system via spillover [780]. 100 K. Loss spectra (Fig. 130) indicate that pre-adsorbed K Reversing the order of adsorption, i.e. water before of has dramatic effects on the adsorption of water. Contrary to the potassium, could reveal other aspects of the water-alkali results for pristine graphene in Ref. [370], there is no evidence interaction at epitaxial graphene. HREELS spectra for of adsorbed molecular water. Vibrational bands centered at 0.01 ML of potassium adsorbed on 1.5 L H2O/MLG/Pt(111) 177, 364, and 445 meV were observed in both loss spectra. surface are shown in Fig. 131a. The analysis of HREELS Interestingly, the bands at 177 and 364 meV indicate the spectrum indicates that low K coverage has negligible effects occurrence of water dissociation as they are a fingerprint of the on the water adsorption and no dissociation occurs. On the formation of C–H bonds [777]. Their energy corresponds to contrary, significant changes in the loss spectrum have been the bending (177 meV) and stretching vibrations of C–H observed at 0.20 ML of K. Vibrational features related to (364 meV). The energy of the C–H stretching indicates the molecular water disappear and the loss spectrum is dominated formation of a sp3 bond. In fact, such mode is expected around by the K–OH vibration at 42 meV. Weak modes due to C–H 375–380 and 360–365 meV for the sp2 and sp3 hybridization, bending and stretching and to C–OH stretching are observed respectively [667,778]. The band centered around 445 meV is (Fig. 133b). Again, water dissociation occurs provided that the assigned to the O–H stretching of residual adsorbed hydroxyl [K]:[H2O] stoichiometry is enough. C–H and C–OH are groups [40,585] while the peak at 42 meV in the 0.25 ML K/ weakly bonded to the graphene surface. Heating the surface – H2O/MLG/Pt(111) is due to a vibration of K atoms against OH to 160 K induces a reduction of the K OH peak by three times fi groups in K(OH)x complexes. (Fig. 133a) and nally coadsorbates completely desorb by No K-graphite vibrational features were found in the range annealing at 200 K. 10–20 meV and this indicates that no agglomeration of potassium in clusters occurs, in agreement with theoretical 7. Conclusions and outlook results [601,779]. Results in Fig. 132 provide a novel mechanism for attaining We have presented an extended survey on the adsorbed a partial graphene hydrogenation by water dissociation as a phases of alkali atoms on metal surfaces and their role in the consequence of the promotional role of K dopants in the heterogeneous catalysis of several fundamental chemical dissociation process [178,283,624]. processes, which have relevance in the area of renewable The partial hydrogenation of graphene induced by the energy. We specifically addressed the characterization of presence of K could be driven by the spillover phenomenon. structures and processes by means of vibrational spectroscopy. The decoration of graphene with alkali atoms can improve the The wide arsenal of spectroscopic methods available today, A. Politano et al. / Surface Science Reports 68 (2013) 305–389 379

co-adsorbed phases, as well as in surface catalytic processes, as free charge reservoirs. Thus low-energy plasmon and ASP spectroscopy would represent a further characterization tool of great potential in surface chemistry as well as in applied plasmonics. On the optical side, TRSHG spectroscopy is proving to be a very versatile tool in the investigation of coherent phonon dynamics at surfaces. This method appears to be ideally suited for alkali overlayers because of the marked enhancement of SH intensity. So far applications of TRSHG to other adsorption systems or to co-adsorbed phases have been very limited, mainly because species other than AM lack such a large enhancement in SH intensity. This obstacle can be removed if SH intensity is enhanced by tuning the photon energy of probe pulses to an electronic resonance of adsorption systems. In addition, using much shorter pump pulses extends the applic- ability of TRSHG to coherent surface phonons and adsorbate vibrations at higher frequencies. The recent progress made in these respects opens a further window on the investigation of alkali metals on metal surfaces and their applications in surface catalytical processes of relevance in materials science for renewable energy.

Acknowledgments

One of us (AP) acknowledges the hospitality at the Donostia Fig. 133. (a) HREELS spectrum recorded for a MLG/Pt(111) surface pre- International Physics Center (DIPC) for the ﬁnalization of the dosed with 1 L of H2O at 100 K and successively covered with different amounts of potassium. The obtained surface was annealed to different sample present review. GB acknowledges IKERBASQUE for support temperatures (160 and 200 K); (b) Magniﬁcation of the loss spectrum in the (project ABSIDES). We are indebted to Professor Renée D.

100–600 meV range for 0.20 ML K/H2O/MLG/Pt(111). Diehl (Penn State University) for the complete information on her and her coworkers' work on AM's on metal surfaces and ranging from the inelastic scattering of electrons (EELS, graphite and for very useful discussions. We also thank Prof. J. HREELS) and atoms (HAS, QHAS, 3HeSE), to optical P. Toennies (Max-Planck-Institut für Dynamik und Selbstor- methods like surface-sensitive infrared (RAIRS), Raman ganisation (MPI-DS) Göttingen) for useful discussions on (SERS) and sum frequency generation (SFG) spectroscopies, HAS experiments on AM/metal surfaces, especially during a allows to determine the bonding nature and strength of visit of one of us (GB) at MPI-DS in the framework of the Re- adsorbed atoms and molecules with the substrate and their invitation Program of the Alexander-von-Humboldt Founda- mutual interactions. The latter are of great relevance for co- tion, which is gratefully acknowledged. adsorbed phases. Besides being a vehicle of information on the bonding References structure, vibrations play an important role in surface chemistry not only collectively in thermal activation, but also [1] A. Politano, V. Formoso, G. Chiarello, Journal of Physical Chemistry C individually through the modulation of the distances among 113 (2009) 316. reactants and catalysts. As shown in recent years, surface [2] T.S. Rahman, S. Stolbov, F. Mehmood, Applied Physics A 87 (2007) 367. phonons may selectively induce chemical reactions via [3] S. Stolbov, T.S. Rahman, Physical Review Letters 96 (2006) 186801. phonon-induced surface charge density oscillations [533], the [4] C. Ramirez, R. Adelung, R. Kunz, L. Kipp, W. Schattke, Physical latter being governed by the mode-selected electron–phonon Review B 71 (2005) 035426. interaction [401,781]. As discussed in Section 2.2, such [5] S.E. Finberg, J.V. Lakin, R.D. Diehl, Surface Science 496 (2002) 10. interaction can be directly extracted for conducting surfaces [6] S. Moré, A.P. Seitsonen, W. Berndt, A.M. Bradshaw, Physical Review B 63 (2001) 075406. from inelastic HAS amplitudes. Moreover the high sensitivity [7] M. Wilde, I. Beauport, K. AlShamery, H.J. Freund, Surface Science 390 of helium atom scattering to surface charge oscillations, (1997) 186. especially those originating from the surface electronic states [8] T. Yang, X.D. Wen, Y.W. Li, J. Wang, H. Jiao, Surface Science 603 which are the most peripheral and the ﬁrst being probed by the (2009) 78. impinging atoms, bears great promises for the spectroscopy of [9] E.G. Michel, P. Pervan, G.R. Castro, R. Miranda, K. Wandelt, Physical – – Review B 45 (1992) 11811. ASP [364,494,496 498,782 785] in the low meV regime, [10] S. Achilli, M.I. Trioni, E.V. Chulkov, P.M. Echenique, V. Sametoglu, presently inaccessible to electron energy loss spectroscopies N. Pontius, A. Winkelmann, A. Kubo, J. Zhao, H. Petek, Physical discussed in Section 2.1. Alkali atoms work in adsorbed and Review B 80 (2009) 245419. 380 A. Politano et al. / Surface Science Reports 68 (2013) 305–389

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