X-ray photoemission

APD, ARPEFS Azimuthal PED, angle- Same as PED resolved photoemission extended fine structure Spectroscopy of Emitted X-rays or Photons Same as PED... 

Figure Al.3.15. Density of states for silieon (bottom panel) as ealeulated from empirieal pseudopotential [25], The top panel represents the photoemission speetra as measured by x-ray photoemission speetroseopy [30], The density of states is a measure of the photoemission speetra.

At a surface, not only can the atomic structure differ from the bulk, but electronic energy levels are present that do not exist in the bulk band structure. These are referred to as surface states . If the states are occupied, they can easily be measured with photoelectron spectroscopy (described in section A 1.7.5.1 and section Bl.25.2). If the states are unoccupied, a teclmique such as inverse photoemission or x-ray absorption is required [22, 23]. Also, note that STM has been used to measure surface states by monitoring the tunnelling current as a fiinction of the bias voltage [24] (see section BT20). This is sometimes called scamiing tuimelling spectroscopy (STS). 

A number of surface-sensitive spectroscopies rely only in part on photons. On the one hand, there are teclmiques where the sample is excited by electromagnetic radiation but where other particles ejected from the sample are used for the characterization of the surface (photons in electrons, ions or neutral atoms or moieties out). These include photoelectron spectroscopies (both x-ray- and UV-based) [89, 9Q and 91], photon stimulated desorption [92], and others. At the other end, a number of methods are based on a particles-in/photons-out set-up. These include inverse photoemission and ion- and electron-stimulated fluorescence [93, M]- All tirese teclmiques are discussed elsewhere in tliis encyclopaedia. 

Figure Bl.25.1. Photoemission and Auger decay an atom absorbs an incident x-ray photon with energy hv and emits a photoelectron with kinetic energy E = hv - Ej. The excited ion decays either by the indicated Auger process or by x-ray fluorescence.

Ultraviolet photoelectron spectroscopy (UPS) [2, 3 and 4, 6] differs from XPS in that UV light (He I, 21.2 eV He II, 40.8 eV) is used instead of x-rays. At these low excitmg energies, photoemission is limited to valence electrons. 

McFeely F R, Morar J F and Himpsel F J 1986 Soft x-ray photoemission study of the silicon-fluorine etching reaction Surf. Sc/. 165 277-87... 

Other techniques in which incident photons excite the surface to produce detected electrons are also Hsted in Table 1. X-ray photoelectron Spectroscopy (xps), which is also known as electron spectroscopy for chemical analysis (esca), is based on the use of x-rays which stimulate atomic core level electron ejection for elemental composition information. Ultraviolet photoelectron spectroscopy (ups) is similar but uses ultraviolet photons instead of x-rays to probe atomic valence level electrons. Photons are used to stimulate desorption of ions in photon stimulated ion angular distribution (psd). Inverse photoemission (ip) occurs when electrons incident on a surface result in photon emission which is then detected. 

The short-range order in a material is important in determining optoelectronic properties. For instance, x-ray and electron diffraction experiments performed on amorphous siHcon (i -Si) and germanium (a-Ge) have revealed that the nearest neighbor environments are approximately the same as those found in their crystalline counterparts (6) photoemission experiments performed on i -Si show that the DOS in valence and conduction bands are virtually identical to the corresponding crystal with the exception that the singularities (associated with periodicity) present in the latter are smeared out in the former. 

X-Ray Photoelearon Spectroscopy X-Ray Photoemission Spectroscopy Electron Spectroscopy for Chemical Analysis X-Ray Photoelectron Diffraction Photoelectron Diffraction Kinetic Energy... 

Thus in all X-ray photoelectron spectra, features appear as a result of both photoemission and Auger emission. In XPS, the Auger features can be useful but are not central to the technique, whereas in AES (see Sect. 2.2), Eq. (2.2) forms the basis of the technique. 

Fig. 11. Composition dependence of the resistivity p x) for thick films of Cbo doped with Na, K, Rb, and Cs. Points indicate where exposure to the alkali-metal source was stopped and x-ray and ultraviolet photoemission spectra were acquired to determine the concentration x. The labels indicate the known fulleride phases at 300 K. The minima in p x) occur for stoichiometries corresponding to NaQCeo, K.iCeo and Cs,.,.Cfio[ll3].

Binary silicate-based glasses, x-ray photoemission spectroscopic... 

Waste storage, nuclear x-ray photoemission spectroscopic study of... 

Figure 2.41. X-ray photoemission spectra of Fe foil after CO hydrogenation at 548 K in CO/H2=1 20 at 1 bar total pressure and varying reaction times, (a) C Is spectra from K-free Fe. (b) K 2p and C Is spectra from K-covered Fe, Ok -OA 28 Reprinted with permission of the American Chemical Society.

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