Adsorption Fermi resonance

FIG. 10. Theoretical calculations reveal that in the case of adsorption of Xe on Ni the resonance associated with Xe(6s) state is broadened significantly with a long tail that extends to the Ni Fermi level. STM images are determined by the LDOS at the Fermi level. Although the contribution of Xe to the LDOS is small, it significantly extends the spatial distribution of the electronic wave function further away from the surface thereby acting as the central channel for quantum transmission to the probe tip. (From Ref. 71.)... 

For atomic H adsorption on surfaces the electronic structure as obtained by UPS studies and DFT calculations on Ni, Pd, and Pt shows a similar picture. There is a strong bonding H-induced feature around 7-9 eV below the Fermi level observed both in UPS and band structure calculations [43]. This has been related to that the H Is level interacts with both the metal -and 7-bands. Since the H Is level is much lower in energy in comparison with the previously discussed adsorbates, for which the outer level was of p character, it is anticipated that the metal s-electrons will be more strongly mixed into the adsorbate bonding resonance. Since no X-ray spectroscopy measurements can be conducted on H it is difficult to derive how much H Is character there is in the 7-band region, respectively, above the Fermi... 

Previous work by our and other groups has shown that the Pt L3 XANES is sensitive to the adsorption of H. This sensitivity was interpreted by our group as being caused by the creation of a chemical bond between Pt and H30. The Pt-H anti-bonding state (AS) above the Fermi level was thought to produce a shaperesonance arising from the interference between the resonantly and nonresonantly scattered photoelectron waves. 

The observations illustrate that inelastic and thermally activated tunnel channels may apply to metalloproteins and large transition metal complexes. The channels hold perspectives for mapping protein structure, adsorption and electronic function at metallic surfaces. One observation regarding the latter is, for example that the two electrode potentials can be varied in parallel, relative to a common reference electrode potential, at fixed bias potential. This is equivalent to taking the local redox level up or down relative to the Fermi levels (Fig. 5.6a). If both electrode potentials are shifted negatively, and the redox level is empty (oxidized), then the current at first rises. It reaches a maximum, convoluted with the bias potential between the two Fermi levels, and then drops as further potential variation takes the redox level below the Fermi level of the positively biased electrode. The relation between such current-voltage patterns and other three-level processes, such as molecular resonance Raman scattering [76], has been discussed [38]. 

Recent studies of the photochemistry of Mo(CO)6 on Ag(l 11) 05 graphite 05 Cu(l 1 1 )70.71,106 and Si( 11 1 )70,71,106 have also shown clear evidence for direct adsorbate excitation. The main techniques employed were MS, EELS and TPD. That the electronic structure was essentially unperturbed on adsorption was confirmed by EELS.105 The photochemical action spectrum followed the gas phase spectrum rather closely, even to the extent of reproducing the charge transfer bands. This is very clear evidence for the direct excitation mechanism. There was an additional enhancement at 325 nm on Ag (111) due to the field enhancement associated with the resonance between the d bands and the Fermi level. Several interesting results were obtained with regard to the excited state quenching. Since the electronic transitions are... 

Ultraviolet photoelectron spectroscopy (UPS) results have provided detailed information about CO adsorption on many surfaces. Figure A3.10.24 shows UPS results for CO adsorption on Pd(l 10) [M ] that are representative of molecular CO adsorption on platinum surfaces. The difference result in (c) between the clean surface and the CO-covered surface shows a strong negative feature just below the Fermi level (Ep), and two positive features at 8 and 11 eV below E-. The negative feature is due to suppression of emission from the metal d states as a result of an anti-resonance phenomenon. The positive features can be attributed to the 4a molecular orbital of CO and the overlap of the 5a and Iti molecular orbitals. The observation of features due to CO molecular orbitals clearly indicates that CO molecularly adsorbs. The overlap of the 5a and Iti levels is caused by a stabilization of the 5a molecular orbital as a consequence of forming the surface-CO chemisorption bond. 

Although valence band information could be acquired by conventional X-ray sources, analysis of the valence band region is not as simple as the core region, since all the components in the sample contribute in this narrow region (with E of 30 eV or less). Due to the broad line width of conventional X-ray sources and the low ionization cross section. X-ray-excited valence band spectroscopy is less commonly used for surface analysis. Instead, ultraviolet sources (e.g.. He I and He II) are adopted to acquire the valence band spectra, a surface technique called ultraviolet photoelectron spectroscopy (UPS). He I and He n resonance lines have inherently narrow widths of only a few meVs and high ionization cross sections in the valence band. This technique is widely used in the study of adsorption phenomena and valence band structure of metals, alloys, and semiconductors. Work functions can be derived from the Fermi level and the secondary electron (SE) cutoff of the UPS spectrum. 

The numerical value of Fermi energy Ep may be used for approximate estimation (according to the Koopmans theorem) of the surface ionization potential that defines the adsorption energies. Due to the correlation effects the DFT Fermi energy Ep differs essentially from that in HF calculations. Table 11.4 shows that Ep is inside the valence band of a perfect crystal, i. e. the resonance surface states are predicted both in HF and DFT single-slab calculations. 

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