Photoelectron diffraction, surface structure

PED Photoelectron diffraction [107-109] x-rays (40-1500 eV) eject photoelectrons intensity measured as a function of energy and angle Surface structure... 

Such ideal low mean free paths are the basis of FEED, the teclmique that has been used most for detennining surface structures on the atomic scale. This is also the case of photoelectron diffraction (PD) here, the mean free path of the emitted electrons restricts sensitivity to a similar depdi (actually double the depth of FEED, since the incident x-rays in PD are only weakly adenuated on this scale). 

Fadley C S et al 1997 Photoelectron diffraction space, time and spin dependence of surface structures Surf. Rev. Left 4 421-40... 

This chapter contains articles on six techniques that provide structural information on surfaces, interfeces, and thin films. They use X rays (X-ray diffraction, XRD, and Extended X-ray Absorption Fine-Structure, EXAFS), electrons (Low-Energy Electron Diffraction, LEED, and Reflection High-Energy Electron Diffraction, RHEED), or X rays in and electrons out (Surfece Extended X-ray Absorption Fine Structure, SEXAFS, and X-ray Photoelectron Diffraction, XPD). In their usual form, XRD and EXAFS are bulk methods, since X rays probe many microns deep, whereas the other techniques are surfece sensitive. There are, however, ways to make XRD and EXAFS much more surfece sensitive. For EXAFS this converts the technique into SEXAFS, which can have submonolayer sensitivity. 

The second example concerns heptahehcene. Figure 4.17 shows the structure of left-handed M-heptahelicene, where M stands for minus. On Cu(lll) surfaces the M-heptahelicene molecules are found to adsorb in a geometry with their terminal phenanthrene group (the first three carbon rings) oriented parallel to the (111) faces and to successively spiral away from the surface from the fourth ring on, as determined by X-ray photoelectron diffraction experiments (Fasel et al, 2001). 

Two rather different techniques that exploit the same underlying phenomenon of coherent interference of elastically scattered low energy electrons are photoelectron diffraction [5] and surface extended X-ray absorption fine structure (SEXAFS) [6,7]. Figure 1.1. shows schematically a comparison of the electron interference paths in LEED and in these two techniques. In both photoelectron diffraction and SEXAFS the source of electrons is not an electron beam from outside the surface, as in LEED, but photoelectrons emitted from a core level of an atom within the adsorbate. In photoelectron diffraction one detects the photoelectrons directly, outside the surface, as a function of direction or photoelectron energy (or both). The detected angle-resolved photoemission signal comprises a coherent sum of the directly emitted component of the outgoing photoelectron wavefield and other components of the same wavefield elastically scattered by atoms (especially in the substrate) close... 

Figure 1.1. Schematic diagram showing the electron elastic scattering pathways contributing to the techniques of low energy electron diffraction (LEED), backscattering photoelectron diffraction (including the scanned-energy mode - PhD) and surface extended X-ray absorption fine structure (SEXAFS). Black disks represent substrate atoms, grey-shaded disks represent adsorbate atoms.

Allied with the diffraction methods, such as low-energy electron diffraction (LEED) and photoelectron diffraction (PED), which can also be applied in single-crystal research, these advances have led to much better interpretations of the vibrational spectra of chemisorbed hydrocarbons in terms of the structures of the surface species. The new results have in turn led to the possibility of reassessing more reliably earlier interpretations of the infrared or Raman spectra of adsorbed hydrocarbons on the finely divided metal samples (usually oxide supported) that are more closely related to working solid catalysts. Such spectra are more complicated because of the occurrence of a variety of different adsorption sites on the metal particles, with the consequence that the observed pattern of absorption bands frequently arises from overlapping spectra from several different surface species. 

Further pertinent nonspectroscopic determinations of the structure of ethene on single-crystal metal surfaces have recently been published. Using photoelectron diffraction Bradshaw, Woodruff, and co-workers (363) have... 

Over the past 10 years a multitude of new techniques has been developed to permit characterization of catalyst surfaces on the atomic scale. Low-energy electron diffraction (LEED) can determine the atomic surface structure of the topmost layer of the clean catalyst or of the adsorbed intermediate (7). Auger electron spectroscopy (2) (AES) and other electron spectroscopy techniques (X-ray photoelectron, ultraviolet photoelectron, electron loss spectroscopies, etc.) can be used to determine the chemical composition of the surface with the sensitivity of 1% of a monolayer (approximately 1013 atoms/cm2). In addition to qualitative and quantitative chemical analysis of the surface layer, electron spectroscopy can also be utilized to determine the valency of surface atoms and the nature of the surface chemical bond. These are static techniques, but by using a suitable apparatus, which will be described later, one can monitor the atomic structure and composition during catalytic reactions at low pressures (< 10-4 Torr). As a result, we can determine reaction rates and product distributions in catalytic surface reactions as a function of surface structure and surface chemical composition. These relations permit the exploration of the mechanistic details of catalysis on the molecular level to optimize catalyst preparation and to build new catalyst systems by employing the knowledge gained. 

RuHAP was synthesized from a stoichiometric HAP, Ca10(PO4)6(OH)2, with RuCl3nH20. Analysis by powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray (EDX), IR and Ru K-edge X-ray absorption fine structure (XAFS) showed that a monomeric Ru phosphate species is created on the HAP surface. Figure 5.2a shows a proposed surface structure of RuHAP. 

Preliminary models of the surface topography, for example, can be determined by atomic-probe methods, ion-scattering, electron diffraction, or Auger spectroscopy. The chemical bonds of adsorbates can be estimated from infrared spectroscopy. The surface electronic structure is accessible by photoelectron emission techniques. In case the surface structure is known, its electronic structure has to be computed with sophisticated methods, where existing codes more and more rely on first principles density functional theory (DFT) [16-18], or, in case of tight-binding models [19], they obtain their parameters from a fit to DFT data [20]. The fit is not without ambiguities, since it is unknown whether the density of states used for the fit is really unique. 

There has been substantial progress in experimental and theoretical surface analytical methods over the last years. Methods based on X-rays and UV light for diffraction, absorption, or photoelectron spectroscopies benefit from new generation synchrotron light sources. To name a few, surface experimental methods include XPS, AES and SIMS for investigating the surface chemistry A

adsorption energetics and kinetics as well as XPD, RAIRS, HREELS, LEED and STM for molecular and surface structure... 

If we limit ourselves to observed LEED patterns, we find that over the years about 2000 ordered structures have been reported./202/ Among these, perhaps 180 have been structurally solved by various techniques of surface crystallography. Intensity analyses of low-energy electron diffraction have contributed about 150 of these. The remaining 30 structures were obtained primarily with ion scattering (MEIS, HEIS), SEXAFS or photoelectron diffraction (NPD, ARXPS). 

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