Core level photoemission line shapes

The changes in BE and line shapes of Ir 4f core level photoemission with increasing Ir coverage on perylene red whiskers clearly demonstrate the existence of reacted Ir + valence state at low Ir coverage (Fig. 22.6). The Ir 4f and Ru 3d core level curve fitting analysis reveals that the contribution of metallic state is higher for iridium, which indicates its much weaker chemical interaction at the interface with perylene red. Umeacted mthenium was not even present at low coverage up to 1 nm (Fig. 22.7). 

In addition to the valence-band photoemission, useful information can be obtained from 4f core-level spectra, because their line shape is almost exclusively influenced by the various types of screening of a 4f hole by conduction-band states. The presence of the 7 eV satellites is taken as an indication for a decreasing hybridization and consequent increasing localization of the 5f states in the uranium compounds (Schneider and Laubschat 1981a). Various aspects of the core-hole screening are discussed by Naegele et al. (1985). 

Prom X-ray photoemission spectroscopy study, Biswas et al. (2003) found that the argon core levels exhibit a systematic shift in binding energy and change in the line shape as function of Ar-ion incidence energy on Al(lll). This is related to the increase in size of the subsurface bubbles created by argon ion bombardment where the Ar atoms are trapped. 

In the intrinsic part of the photoemission spectrum, that is, the elastic lines, there are basically three observables associated with each core-level or valence band peak line positions, line intensities, and line widths or line shapes. From these, different pieces of information can be gained. In core-level emission, the rough line positions reveal the elemental composition of the sample surface, whereas the exact positions are characteristic of the specific chemical environment of the atoms [6j. The intensities are determined not only by the atomic concentrations but also by the photoelectric cross sections and instrumental effects such as the photon flux and the transmission of the spectrometer. Finally, information on the many-body dynamics of the solid after the sudden creation of a photohole (the missing electron that has been ejected) is contained in the shape and width of the peak. In the simplest case, the line shape is Lorentzian and its width is a measure of... 

On X-ray excitation, electrons can be excited from core levels. Their atom-specific binding energies make up much of the appeal of XPS. In this section, the application to quantitative surface chemical analysis is discussed, and also the finer art of interpreting exact line positions and line shapes. Finally, diffraction effects of the photoelectron waves within the crystalline surface are discussed, which lead to substantial intensity modulations in photoemission from single-crystalline samples. These directional signals can be exploited for structural analysis of the surface layers. Before that, AES is briefly introduced. These electrons result from nonradiative relaxation processes of the core hole left behind after a primary photoemission process. 

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