Electron-surface interaction mechanisms HREELS

A third mechanism, first observed in gas-phase electron-impact scattering, has been referred to as negative-ion resonance. In this process, an electron is trapped, within 10 s, inside the molecule in a negative-ion state. For chemisorbed molecules, however, the adsorbate-substrate chemical bond and the electron-surface interactions can dramatically alter the resonance properties. Hence, for HREELS at metal surfaces, this mechanism is quite rare it will not be treated further in this article. 

The other salient point of the interaction of 02 with stepped Ag surfaces is the direct observation of the subsurface migration mechanism, witnessed by the disappearence of the loss at 56meV in the HREEL spectra of 0/Ag(2 1 0) [97]. This vibration is ascribed to the metastable occupation by oxygen atoms of the tetrahedral interstitials, where the O/Ag mode can still be efficiently excited by HREELS. When heating the crystal to 300 K the loss disappears because the oxygen atoms move to the octahedral interstitials, which are too deep and well screened to be monitored by low energy electrons. 

Several recent overviews of principles and applications of Raman, FTIR, and HREELS spectroscopies are available in the literature [35-37, 124]. The use of all major surface and interface vibrational spectroscopies in adhesion studies has recently been reviewed [38]. Infrared spectroscopy is undoubtedly the most widely applied spectroscopic technique of all methods described in this chapter because so many different forms of the technique have been developed, each with its own specific applicability. Common to all vibrational techniques is the capability to detect functional groups, in contrast to the techniques discussed in Sec. III.A, which detect primarily elements. The techniques discussed here all are based in principle on the same mechanism, namely, when infrared radiation (or low-energy electrons as in HREELS) interacts with a sample, groups of atoms, not single elements, absorb energy at characteristic vibrations (frequencies). These absorptions are mainly used for qualitative identification of functional groups in the sample, but quantitative determinations are possible in many cases. 

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