Scattering hydrogen from metal surfaces

The translational and internal energy dependences of the dissociation probability can yield a great deal of information regarding the PES, but the final state is not fully specified (only given as dissociated or not dissociated) and this leads to some loss of information. Much more detail can be obtained by examining the scattered fraction instead. Diffraction intensities tell us about the surface site dependence of the PES, while comparison of the internal state populations before and after scattering tells us about the changes of vibrational and rotational state, and hence about the curvature of elbow PESs and the molecular orientation dependence of the PES. 

Theory quite naturally gives us the initial and final state resolved probabilities, but in experiment this is not always so. The internal state populations in a molecular beam are determined by the temperature of the nozzle used to produce the supersonic expansion. More than one state is present in such a beam. This has been partially overcome in recent years by Raman pumping of the incident molecular beam [66-69]. Laser beams intercept the molecular beam moving a fraction of the molecules into a particular ro-vibrational state (determined by the laser properties). With careful timing of the firing of the probe lasers, it is possible to measure changes in this fraction of molecules and measure some of the final states populated by the scattering process. 

Theoretical work produces an almost embarrassing wealth of information. In addition to diffraction intensities, and the probabilities of vibrational and rotational transitions, we can obtain combinations of these, e.g. vibrational de-excitation accompanying rotational excitation. These coupled changes probe the PES very precisely in particular regions. If we consider combined rotational-vibrational changes, 

This does not, however, resolve the problem fully. We still have to account for the remainder of the missing flux and for the overestimation of rotational excitation. Rotational inelasticity is a sensitive probe of the molecular orientational corrugation of the PES (i.e. the corrugation in 0 and cj ) - overestimation 

PES is strongly dependent on the molecular bond orientation, there can even be an ntj dependence of diffraction. Miura et al. [74] have shown that this leads to a difference in the rotational alignment of molecules scattered on or off-specular. However, as for rotational inelasticity, there are problems in the comparison of theoretical and experimental diffraction probabilities. 

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Adsorbed hydrogen on metal surfaces is of particular interest from both theoretical and experimental points of view. Vibrational spectroscopy data on hydrogen adsorbed from the gas phase have been obtained from IR reflection-absorption experiments as well as from electron energy loss spectroscopy and inelastic neutron scattering techniques [39-41]. In UHV, absorption bands for the M-H bond have been reported in the mid- and far-infrared regions [41, 42],... 

Diffusion of atoms from the point at which they dissociate on a metal surface to the edge of the metal crystallite is one of the component steps of hydrogen spillover. Quasielastic neutron scattering experiments have produced direct evidence for the diffusion coefficients of hydrogen on the surface of catalysts. The mean time between diffusional jumps for hydrogen on a Raney Ni surface has been found to be 2.7 0.5 x 10 9s at 150°C.72 For H on the surface of Pt crystals dispersed within a Y type zeolite the mean time between surface jumps was found73 to lie between 3.0 and 8 x 10-9s at 100 °C. 

Many of the by-products of microbial metaboHsm, including organic acids and hydrogen sulfide, are corrosive. These materials can concentrate in the biofilm, causing accelerated metal attack. Corrosion tends to be self-limiting due to the buildup of corrosion reaction products. However, microbes can absorb some of these materials in their metaboHsm, thereby removing them from the anodic or cathodic site. The removal of reaction products, termed depolari tion stimulates further corrosion. Figure 10 shows a typical result of microbial corrosion. The surface exhibits scattered areas of localized corrosion, unrelated to flow pattern. The corrosion appears to spread in a somewhat circular pattern from the site of initial colonization. 

The admixture of lead to platinum has a similar effect (Fig. 5). At the same time, the aromatizing activity increases up to about 1 1 Pt Pt atomic ratio 24). With even more lead it scatters aroimd somewhat lower values 66). Electron donation from lead to platinum has been proved by infrared spectroscopy, so one may wonder whether lead is present as metal in the catalyst (75). The additive effect can also be interpreted by its creating hydrogen-deficient surface sites favorable for aromatization. When more lead is present than platinum (i.e., where no more continuous platinum surface is probable), the inverse correlation between hydrogen adsorptivity and activity ceases to exist. 

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