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The Valence Electron Excitations and the Optical Properties of Adsorbed Atoms and Molecules on Metal Surfaces E

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The Valence Electron Excitations and the Optical Properties of Adsorbed Atoms and Molecules on Metal Surfaces E THE VALENCE ELECTRON EXCITATIONS AND THE OPTICAL PROPERTIES OF ADSORBED ATOMS AND MOLECULES ON METAL SURFACES E. Burstein, A. Brotman, P. Apell To cite this version: E. Burstein, A. Brotman, P. Apell. THE VALENCE ELECTRON EXCITATIONS AND THE OP- TICAL PROPERTIES OF ADSORBED ATOMS AND MOLECULES ON METAL SURFACES. Journal de Physique Colloques, 1983, 44 (C10), pp.C10-429-C10-439. 10.1051/jphyscol:19831086. jpa-00223544 HAL Id: jpa-00223544 https://hal.archives-ouvertes.fr/jpa-00223544 Submitted on 1 Jan 1983 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNAL DE PHYSIQUE Colloque C10, supplkrnent au n012, Tome 44, d6cembre 1983 page Ci0-429 THE VALENCE ELECTRON EXCITATIONS AND THE OPTICAL PROPERTIES OF ADSORBED ATOMS AND MOLECULES ON METAL SURFACES* E. Burstein, A. Brotman and P. Apellf Physics Department and Laboratory for Research on the Structure of Matter, University of Pennsy Zvania, PhiZadeZphia, PA 19204, U.S. A. ~6sum6- Nous presentons une revue sur les excitations des electrons de valence qui jouent un r81e dans les propridtds optiques, par exemple diffusion Raman, luminescence, etc., des atomes et des mole'cules adsorb& Les propridt6s optiques elles-m&mes fournissent un moyen pour dtudier la structure ilectronique des complexes adsorbat-substrat. Abstract - We present an overview of the valence electron excitations that play a role in the optical p~opertieseg., Raman scattering, lumi- nescence, etc., of adsorbed atoms and molecules. The optical properties can themselves be used as surface-sensitive spectroscopic probes of the electronic structure of the adsorbate-substrate complexes. INTRODUCTORY REMARKS The lack of information about the electronic structure and, specifically about the energies, wavefunctions and widths of virtual bound states and bonding and antibonding states of the adsorbed atoms and molecules, has been a major barrier to efforts to elucidate the key mechanisms, other than surface roughness enhanced EM fields, that play a role in their optical properties, eg., Raman scattering, second harmonic generation, etc. There has been a tendency in the past to view the metal substrate and the adsorbed molecules as separate entities, albeit perturbed by each other's presence, and to attribute the enhanced optical phenomena to enhancement by the metal substrate. Thus, the enhanced Raman scattering by the adsorbed molecules on Ag has been termed llsurface enhanced Raman scattering". It is now clear, from a variety of experimental evidence, that the Raman scattering cross-sections of the adsorbed molecules is due, in part, to contributions from "intermoleculartt (e.g., charge transfer) electronic excitations of the adsorbed atoms and molecules, that are absent in the free atoms and molecules /1,2,3/. The appropriate point of view is that the optical properties of the adsorbed atoms and molecules are those of adsorbate-substrate complexes whose "intermolec- ular" and "intram~lecular'~excitations interact with the electronic excitations of the underlying metal. The enhancement of the macroscopic and local EM fields at an A-S complex by surface roughness, image dipoles, etc., does correspond to an enhancement by the metal substrate. On the other hand, the enhanced optical response arising from "intermolecularlt excitations is not a surface enhancement. It is simply the manifestation of the formation of an A-S complex. We note in this connection that, to observe the Raman scattering (RS) by a monolayer, or submonolayer, of adsorbed molecules on a metal substrate in the absence of any "surface enhance- ment" of the incident and scattered EM fields, it is advantageous to carry out the RS measurements under resonance enhanced conditions /4,5/ (eg., to use excita- tion wavelengths at which intramolecular or intermolecular resonances occur) and, for this purpose, to extend the excitation wavelengths from the visible into the ultraviolet and infrared, Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19831086 JOURNAL DE PHYSIQUE BONDING, ELECTRONIC STRUCTURE AND VALENCE ELECTRON EXCITATIONS To adequately understand the optical properties of adsorbed atoms and molecules on metal substrates, one needs information about the electronic structure and, in particular, information about the electronic excitations, (eg., energies, widths, oscillator strengths, etc.) of the A-S complexes that are formed. The determination of the electronic structure of molecules adsorbed on metal surfaces (and, thereby, the elucidation of the bonding between adsorbates and substrates) has been a major objective of surface physicists and chemists, and there has been considerable experimental and theoretical progress in attaining this objective for a number of adsorbed molecules and metal substrates /6/. We present here an overview of the bonding of atoms and simple molecules at metal surfaces and of the electronic structure and electronic excitations of the A-S complexes that are formed. In doing so, we will limit ourselves to isolated, e.g., low coverage, adsorbates on smooth metal surfaces in ultra high vacuum. Our primary objective is to clarify the nature of the valence electron excitations that play a role in optical phenomena exhibited by adsorbed atoms and molecules on metal surfaces /7/. The metal substrates fall qualitatively into several groups. i) Metals, such as Al, Na and K, which do not have d-bands and are free-electron-like. The covalent bonding of adsorbates by the s,p electrons of these metals is relative- ly weak. ii) Noble metals (eg., Ag, Cu and Au), whose d-bands lie well below the Fermi level, for which the chemisorption of adsorbates via the d electrons is only moderately strong. iii) Transition metals (eg., Ni, Pd, Pt) whose d-bands overlap the Fermi level and, thereby, contribute to a strong covalent bonding of adsorbates. The adsorbates to be discussed also fall into several groups. i) simple aromatic molecules, eg., benzene, pyridine, pyrazine, etc. ii) Diatomic molecules, eg., CO and N2. iii) Atomic adsorbates, eg., hydrogen, halogen, alkali and rare gas atoms. Aromatic Adsorbates The aromatic molecules, eg., benzene, pyridine, etc., are chemisorbed on metal surfaces via covalent bonding of their n electrons with the d electrons of the metal substrate, and generally lie flat on surface. The covalent bonding of benzene is only moderately strong, even in the case of transition metal sub- strates /8/. In the case of pyridine, the plane of the adsorbed molecules may be inclined, or perpendicular, to the surface due to covalent bonding via the lone pair of electrons of the N /9/. As shown by Demuth and co-workers /10,11/ who have determined the electronic excitations of pyridine, pyrazine and benzene adsorbed on Ag(ll1) using electron energy loss measurements, the intramolecular excitations of the adsorbed molecules are only moderately shifted and broadened relative to those of the free molecules. Thus, there is only a moderate mixing of the adsorbate and substrate orbitals. The electron energy loss spectra for pyridine and pyrazine also exhibit a very broad feature with an onset at -2 to 2.5 eV, which Demuth et a1 identify as corresponding to A-S charge-transfer excitations. The energy levels of free pyridine molecules and of pyridine molecules adsorbed on pg, based on the electronic excitations energies reported by Demuth et a1 and on the valence electron binding energies obtained from photoemission measure- ments /12/, are shown schematically in Fig. la. The affinity level of free pyridine lies above the vacuum level (E ) i.e., the negative pyridine ion is unstable. When adsorbed on a free-electron-like metal substrate, the affinity level is lowered by the coulomb interaction of the negative pyridine ion with its image charge in the metal substrate. The affinity level of the adsorbed pyridine lies at an energy -2 eV below EV and corresponds to a "virtual bound state". An electron in the affinity level of the adsorbed pyridine is unstable with respect to the metal substrate and will "hopff into an empty level of the metal. We note that the excited states of the adsorbed (neutral) pyridine molecules lie above the Fermi level (E ) and also correspond to ffvirtual bound states". The fact, that the observed F;intramolecularlpelectronic excitations of adsorbed pyridine are not appreciably broadened, indicates that the contribution to the broadening from the decreased lifetime of the tfunstableflexcited states is not large. There are two possible charge-transfer excitations. One involves the transition of an electron in the metal below E to the affinity level of adsorbed pyridine /13/ with an onset energy ECT = EA* -F~Fequal to - 2 eV. The other involves the transition of an electron from the ground state of adsorbed pyridine to an empty level above EF in the substrate (a process which corresponds to photoemission from the adsorbate into the metal substrate) with an onset energy ECT = EF - EI* equal to - 3 eV. FREE ADSORBED FREE ADSORBED co CO PYRlDlNE PYRlDlNE (b) (a 1 Fig. 1. Schematic energy level diagram for (a) pyridine adsorbed on Ag and (b) CO adsorbed on Ni. Diatomic Adsorbates CO is strongly chemisorbed by transition metals with the C end attached to the metal /11,12/. CO is also chemisorbed by the noble metals with the axis of the molecule normal to the substrate. The covalent bonding is, however, appreciably weaker than on the transition metals /13/. CO is only weakly adsorbed by A1 /14/. The molecule lies flat on the A1 surface and is presumed to be physisorbed.
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