Table 1.1 Surface Science Techniques (page 19-28) Acronym Name Description Primary Surface Information Adsorption or Atoms or molecules are physisorbed into a Surface area, selective porous structure-such as zeolite or a adsorption site chemisorption (1) sample of coal-or on to a surface and the concentration amount of gas adsorbed is a measure of the surface area available for adsorption. Chemisorption of atoms or molecules on surfaces yields surface concentration of selected elements or adsorption sites. AD Atom or helium Monoenergetic beams of atoms are Surface structure diffraction (2-13) scattered from ordered surfaces and detected as a function of scattering angle. This gives structural information on the outermost layer of the surface. Atom diffraction is extremely sensitive to surface ordering and defects. AEAPS Auger electron A monoenergetic beam of electrons is used Chemical appearance potential to excite atoms in the near surface region. composition spectroscopy (2-4, As the beam energy is swept, variations in 14-17) the sample emission current occur as the beam energy sweeps over the energy of an Auger transition in the sample. Also known as APAES. AES Auger electron Core-hole excitations are created, usually Chemical spectroscopy (2-4, by 1 to 10-keV incident electrons; Auger composition 14, 16, 18-29) electrons of characteristic energies are emitted through a two-electron process as excited atoms decay to their ground state. AES gives information on the near-surface chemical composition. AFM Atomic force Very similar to scanning tunneling Surface structure microscopy (30-37) microscopy (STM). In this technique, however, the attractive van der Waals forces between the surface and the probe cause a bending of the probe. This deflection is measurable by variety of means. Because this technique does not require a current between the probe and the surface, nonconducting surfaces may be imaged. APAES Appearance potential See AEAPS. auger electron spectroscopy APXPS Appearance potential The EAPFS excitation cross section is Chemical X-ray photoemission monitored by fluorescence from core hole composition spectroscopy (2-4, decay (also known as SXAPS). 16) ARAES Angle-resolved auger Auger electrons are detected as a function Surface structure electron spectroscopy of angle to provide information on the (38) spatial distribution or environment of the excited atoms (see AES). ARPEFS Angle-resolved Electrons are detected at given angles after Surface structure photoemission being photoemitted by polarized extended fine synchrotron radiation. The interference in structure (38-40) the detected photoemission intensity as a function of the electron energy » 100-500 eV above the excitation threshold gives structural information. ARPES Angle-resolved A general term for structure-sensitive Electronic structure, photoemission photoemission techniques, including surface, surface spectroscopy (3, 24, ARPEFS, ARXPS, ARUPS, and ARXPD. structure 41-44) ARUPS Angle-resolved Electrons photoemitted from the valence Valance band ultraviolet and conduction bands of a surface are structure, bonding photoemission detected as a function of angle. This gives spectroscopy (3, 42, information on the dispersion of these 45-48) bands (which is related to surface structure) and also gives structural information from the diffraction on the emitted electrons. ARXPD Angle-resolved X-ray Similar to ARXPS and ARPEFS. The Surface structure photo-electron angular variation in the photoemission diffraction (3, 38, 39, intensity is measured at a fixed energy 49-51) above the excitation threshold to provide structural information. ARXPS Angle-resolved X-ray The diffraction of electrons photoemitted Surface structure photoemission from core levels gives structural spectroscopy (3, 38, information on the surface. 39, 49, 50) CEM Conversion electron A surface-sensitive version of Mossbauer Chemical Mossbauer spectroscopy. Like Mossbauer environment, spectroscopy (4, spectroscopy, this technique is limited to oxidation state 52-55) some isotopes of certain metals. After a nucleus is excited by g-ray absorption, it can undergo inverse b-decay, creating a core hole. The decay of core holes by Auger processes within an electron mean free path of the surface produces a signal. Detecting emitted electrons as a function of energy gives some depth-profile information because the changing mean free path. DAPS Disappearance The EAPFS cross section is monitored by Chemical potential variations in the intensity of electrons composition spectroscopy (2-4, back-scattered from surface. 16) EAPFS Electron appearance A fine structure technique (see EXAFS). Surface structure potential fine Core holes excited by monoenergetic structure (3, 56) electrons. The modulation in the excitation cross section as the beam energy is varied may be monitored through absorption, fluorescence, or AUGER emission. ELNES Electron energy loss Similar to NEXAFS, except Surface structure near edge structure monoenergetic high-energy electrons » 60-300 keV excite core holes. ELS or Electron energy loss Monoenergetic electrons are scattered off a Electronic structure, EELS spectroscopy (3, 4, surface, and the energy losses are surface structure 20, 23, 41, 57-60) determined. This gives information on the electronic excitations of the surface and the adsorbed molecules. ESCA Electron Now generally called XPS. Composition, spectroscopy for oxidation state chemical analysis (2-4, 16, 22, 61-63) ESDIAD or Electron (photon)- Electrons or photons break chemical bonds Bonding geometry, PSD stimulated ion in absorbed atoms or molecules, causing molecular orientation angular distribution ionized atoms or radicals to be ejected (2-4, 8, 64-69) from the surface along the axis of the broken bond by Coulomb repulsion. The angular distribution of these ions gives information on the bonding geometry of adsorbed molecules. Ellipsomerty (70) Used to determine thickness of an Layer thickness adsorbed film. A circular polarized beam of light is reflected from a surface, and the change in the polarization characteristics of the light gives information about the surface film. EXAFS Extended X-ray Monoenergetic photons excite a core hole. Local surface absorption fine The modulation of the absorption cross structure and structure (3, 8, 71-77) section with energy at 100-500 eV above coordination numbers the excitation threshold yields information on the radial distances on the neighboring atoms. The cross section can be measured by fluorescence as the core holes decay or by attenuation of the transmitted photon beam. EXAFS is one of the many “fine- structure” techniques. EXELFS Extended X-ray Monoenergetic electrons excite a core Local surface energy loss fine hole. The modulation of the absorption structure and structure (3) cross section with energy 100-500 eV coordination numbers above the excitation threshold yields information on the radial distances to the neighboring atoms. The cross section can be measured by fluorescence as the core holes decay or by attenuation of the transmitted photon beam. FEM Field emission A strong electric field (on the order of Surface structure microscopy (2-4, 11, volts/angstrom) is applied to the tip of 20, 22, 78, 79) sharp, single-crystal wire. The electrons tunnel into the vacuum and are accelerated along radial trajectories by Coulomb repulsion. When the electrons impinge on the fluorescent screen, variations of the electric field strength across the surface of the tip are displayed. FIM Field ionization A strong electric field (on the order of Surface structure and microscopy (2-4, 11, volts/angstrom) is created at the tip of a surface diffusion 20, 22, 79, 80) sharp, single-crystal wire. Gas atoms, usually He, are polarized and attracted to the tip by the strong electric field, and are ionized by electrons tunneling from gas atoms into the tip. These ions, accelerated along radial trajectories by Coulomb repulsion, map out the variations in the electric field strength across the surface, showing the surface topography with atomic resolution. FTIR Fourier transform Broad-band IRAS experiments are Bonding geometry infrared spectroscopy performed, and the IR adsorption spectrum and strength (81-83) is deconvoluted using a Doppler-shifted source and the Fourier analysis of the data. This technique is not restricted to surfaces. HEIS High-energy ion High-energy ions, above » 500 keV, are Surface structure scattering scattered off a single-crystal surface. The spectroscopy (3, 11, “channeling” and “blocking” of scattered 84, 85) ions within the crystal can be used to triangulate deviations from the bulk structure. HEIS has been especially used to study surface reconstructions and the thermal vibrations of surface atoms. See also MEIS and ISS. HREELS High-resolution A monoenergetic electron beam, » 2-10 eV Bonding geometry, electron energy loss is scattered off a surface; and the energy surface atom spectroscopy (2, 3, losses between » 0.5 eV to bulk and vibrations 86-88) surface phonons and vibrational excitations of adsorbates are measured as a function of angle and energy (also called EELS) INS Ion-neutralizations Slow ionized atoms, usually He+, strike a Valence bands spectroscopy (2, 3, surface, where they are neutralized in a 89) two-electron process that can eject a surface electron-a process similar to Auger emission from the valance band. The ejected electrons are detected as a function energy, and the surface density of states can be determined from the energy distribution.