Valence band, imaging

In addition to studying core levels, XPS can also be used to image the valence band. Figure 3.6 shows valence band spectra of Rh and Ag. The step at Eb=0 corresponds to the Fermi level, the highest occupied electron level. Figure 3.6 illustrates that the Fermi level of rhodium lies in the d-band where the density of states is high, whereas the Fermi level of silver, with its completely filled d-band, falls in the s-band, where the density of states is low (see also the Appendix). 

However, UPS and XPS do not both image the density of states in entirely the same way. In XPS, the photoelectrons originating from the valence band leave the sample with kinetic energies over 1 keV. In UPS, the exciting energy is on the order of 21 eV, and the kinetic energy of the electrons is low, say between 5 and 16 eV. This means that the final state of the photoelectron is within the unoccupied part of the density of states of the metal. As a result, the UPS spectrum represents a convolution of the densities of occupied and unoccupied states, which is sometimes called the "Joint Density of States."... 

Figure 7.9 Potential energy diagram for electrons in and near a metal to which a high negative potential is applied. Electrons in the valence band of the metal see an attractive potential equal to -eFr (F is the applied field in V/cm) outside the metal behind a barrier formed by the applied field and the image potential.

The assumption that some dyes can spectrally sensitize latent image formation in silver halides by direct electron transfer from the excited dye to the conduction band and other dyes by indirect electron transfer from the dye radical formed by photoinjection of a hole into the valence band is in good accord with experiment. The locations of the highest filled and lowest vacant energy levels of the dye relative to the valence and conduction bands of the silver halide determine which mode of sensitization will occur, or whether both can occur. 

Fig. 2. Comparison of photo electron- and X-ray absorption spectrum 1. Both types of spectra are a direct image of the orbital diagram of the element under study, however, the resolution in the X-ray absorption is much lower because of the numerous possible final states in the conduction or valence band...

Fig. 3. Energy levels in AgX. Ey = valence band Ec = conduction band. Exposure process A light absorption B charge motion to traps C trapped charges D latent image...

It must be borne in mind that if the situations above extend the potential range for imaging they are associated with an electrochemical current, which may be critical. At n-type electrodes illumination or electron emission from the valence band into the conduction band causes corrosion by generating holes at the surface (Fig. 7, top) at the surface. With p-type semiconductors the situation is conversely... 

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