Photoemission of adsorbed xenon

The 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 [71]. As we are dealing with a weakly physisorb-ing atom, these experiments must be conducted at cryogenic temperatures on the order of 50-60 K. First, we explain the theory behind the PAX method, and then illustrate the technique with an example. 

EXeF is the binding energy of Xe 5pj/2 electrons with respect to the Fermi level. 

It may not immediately be obvious that Eq. (3-13) is indeed correct. One expects that the Xe atom becomes slightly polarized upon adsorption, which affects the binding energy. As the extent of polarization changes from one substrate to another, the value of xevac is also expected to vary. Apparently, such effects are small and fall within the experimentally determined range for xevac of 12.3 + 0.15 eV. Care should be taken, however, with the application of Eq. (3-13) to Xe adsorption sites adjacent to adsorbed atoms or steps, where the polarization of the Xe may be quite different from that on a flat surface. 

Its capability to titrate sites on heterogeneous surfaces makes PAX (in principle) a particularly attractive technique for investigating the surfaces of catalysts. Unfortunately, the technique has its limitations, because the Xe 5p /2 peak has a finite linewidth of about 0.4 eV. Moreover, if a surface possesses more than three to four different adsorption sites, the spectra may become too complicated, rendering analysis by curve fitting unwarranted. In fortunate cases, however, where all adsorption sites are populated sequentially, one may be able to identify many sites. Further applications in this respect may be found in the literature [71-74]. 

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LEED, namely one with a, c(2x2) and one with a, p(2x2) superstructure. They are compatible with CusPt and CusPta layers. The first atomic layer was in both cases found by means of photoemission of adsorbed xenon to be pure copper. Details of the experimental work can be found in ref. 9 and 10. A schematic view of both structures can be seen in figure 1. Both consist of alternating layers of pure copper and of mixed composition. In the CuaPt case, the second and all other evenly numbered layers have equal numbers of copper and platinum atoms, whereas in the CusPta case the evenly numbered layers consist of thrice as many platinum as copper atoms. 

Fig. 3.23 Photoemission of adsorbed xenon (PAX) spectra at 60 K of 0.27 monolayer of Ag deposited on Ru(001) followed by annealing, showing that Xe first populates Ru, and then Ag. The top spectrum corresponds to a complete monolayer ofXe on the Ag/Ru(001) sample and can be used for quantitation. (From [73]).

Fermi level of the metal [19]. This is the value that one measures with scanning tunneling microscopy (see Chapter 7) and with photoemission of adsorbed xenon (see Chapter 3). Thus, on a heterogeneous surface we have local work functions for each type of site, and the macroscopic work function is an average over these values. 

After 30 years of continuing investigation, the adsorption properties of the noble ses on metal and semiconductor surfaces have recently attracted renewed interest. On the one hand, some fundamental aspects have come within the reach of modem experimental and theoretical techniques, sueh as the very nature of physisorption and the noble gas - substrate interaction, the possibility to study growth and surface kinetics at the atomic scale, and the recent interest in nanoscale surface friction and related tribological issues, where noble gas adlayers serve as model systems [99P]. On the other hand, noble gas adsorption is being used as a non-destmctive and quantitative surface analytical tool as, for instance, in photoemission of adsorbed xenon (PAX) [97W] and for titration analysis of heterogeneous surfaees based on the site specificity of the interaction strength [96S, 98W]. 

X-ray photoemission spectroscopy electron energy loss spectroscopy photoemission of adsorbed xenon... 

Because UPS delivers information from several top atomic layers, it is difldcult to characterize trace amounts of adsorbates. This limitation can be overcome e.g. by photoemission of adsorbed xenon (PAX) [90]. This technique is a site-selective titration technique, in which Xe adsorption sites are revealed by means of UPS it has been used effectively to characterize catalytic systems [91, 92]. An alternative surface sensitive technique capable of determining trace amounts of adsorbates is metastable impact electron spectroscopy (MIES). 

The photoemission of adsorbed xenon (PAX) method, developed by Wandelt to measure local surface potentials with atomic resolution, deserves particular attention. Here, the 5p core levels of adsorbed xenon atoms are used as probes for the local surface potential. The binding energy of these orbitals is measured by photoemission. The method is based on the idea that the Xe core potential floats with the local surface potential. In fact, the 5p binding energy depends on the initial state of the adsorbed neutral Xe atom and the final state of the adsorbed, ionized Xe atom. In a first approximation, both, the polarization of the Xe atom in the initial state and the interaction of the ionized Xe atom with the substrate in the final state... 

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