UPS spectrum

Figure 5-3. UPS spectrum of Au illustrating the procedure to obtain vacuum level referenced spectra.

Figure 5-18. UPS spectrum of the emcraidinc base form of polyanilinc is compared with the VEH calculated band structure (bottom) and the VEH DOVS.

Figure 14 The left hand side shows the band structures of poly(pyridine) calculated using a DFT-LMTO method for helical polymers. The right hand side shows its calculated density of states spectrum (solid line) and the experimental UPS spectrum (dashed line). The UPS spectrum was taken from Miyamae et al. [104]. Reproduced with permission from Vaschetto et al. [103], Figure 6. Copyright 1997 the American Chemical Society.

Figure 3.17 Schematic UPS spectrum of a d-metal and the corresponding density of states. 

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."... 

Another important application of UPS is the study of adsorbates. Figure 3.19 shows schematically what we observe in the UPS spectrum of an adsorbed gas ... 

Occupied molecular orbitals of the adsorbate with ionization potentials between 0 and hv- (p become visible. If one compares their binding energies with those in a UPS spectrum of a physisorbed multilayer of the same gas, one readily recognizes which of the molecular orbitals are involved in the chemisorption bond. For example, the adsorbate level in Fig. 3.19 has shifted a few eV with respect to its position indicated in the density of states picture (taken as the position in a physisorbed gas), indicating that the level is involved in the chemisorption bond. 

Photoemission of adsorbed xenon, abbreviated as PAX, is a site-selective titration technique, in which the UPS spectrum of physisorbed Xe reveals the nature of the Xe adsorption site [52, 53]. Since we are dealing with a weakly physisorbing atom, these experiments have to be done at cryogenic temperatures on the order of 50-60 K. We first explain the theory behind the PAX method and then illustrate the technique with an example. 

Figure 3.21 UPS spectrum of Xe physisorbed on Ru(OOl) showing the superposition of the Xe 5p levels with the d-band of Ruthenium. The position of the Xe 5pm peak with respect to the Fermi level of Ru is a measure of the work function of the adsorption site, as the potential diagram indicates (from Wandelt et al. L54J).

Figure 4.23. Photoemission process and UPS spectrum of a semiconducting material. F vacCs) and vac(d) represent the sample and detector vacuum levels, respectively. The Fermi edge of the metal is represented by the discontinuous line.

Figure 2.2. Comparison between atom-specific and symmetry-resolved XE spectra [6,10] with an UPS spectrum of N2 adsorbed on Ni(100) measured at a photon energy of 35 eV [9]. From Ref. [3].

Finally, the width of the total UPS spectrum may be used to obtain a reasonable estimate of the work function of the solid sample under consideration49. The energy of the intensity cut-off of the secondary electron background in the UPS spectrum is... 

Fig. 3.5 The determination of the work function, (ps, from a UPS spectrum of gold.

Fig. 7.29 A UPS spectrum of the surface of DHPPV doped -with calcium, showing the doping-induced bipokron peaks.

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