Isotope effects in gas-surface interactions: quantum-state resolved studies of D₂ scattering from Cu(100) and Pd(111)
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State resolved experiments are presented for the interaction of D2 and HD with Cu(100) and Pd(111). For this work, D2(v=1, J=2) molecules were scattered off of single crystal surfaces at near normal incidence. The reflected molecules were probed using quantum state specific spectroscopy. For D2 scattered from Cu(100) the survival probability and some transition probabilities were measured over a range of incident energies. The survival probability was found to be larger then that found previously for H2(v=1) scattered from the same surface. For H2 some of the incident flux was unaccounted for and could possibly have been lost by dissociative adsorption. In contrast, D2 molecules which do not reflect elastically from the surface are accounted for in other transition channels for most energies. The differences found for D2 compared to previous work with H2 are discussed in terms of the lower zero point energy and smaller vibrational energy spacings of D2. D2 translational energy exchange was studied for several different scattering channels and interpreted using simple classical calculations. These calculations agreed well with both the elastic scattering channel as well as the rotational relaxation channel. For rotational excitation some energy was gained by the molecule from the surface. The survival probability was also measured for D2(v=1) scattered from Pd(111) at one incident energy. Pd is very reactive for D2 dissociation and this survival probability was measured to be much smaller than that for H2(v=1) under similar conditions. Vibrational relaxation channels were studied for D2 scattering from both Cu(100) and Pd(111). The vibrational relaxation was also found to be smaller than that measured for comparable channels for H2. The smaller survival probability and vibrational relaxation probability for D2 on Pd(111) cannot be easily accounted for by the difference in zero point energy and vibrational energy spacings. Measurements were also done to study the rotational excitation of HD molecules scattered from one reactive surface, Pd(111), and two inert surfaces, Cu(100) and Pd(111):H(D). These measurements showed similar amounts of rotational excitation for HD molecules after scattering from these different surfaces.