Faraday Discussions of the Chemical Society

Rotationally mediated selective adsorption of HD on W(110): Physisorption without chemisorption

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Rotationally mediated selective-adsorption resonances have been observed in studies of the angular-scattering distributions of HD from W(110). Efforts have been made to determine the ultimate fate of these selectively adsorbed molecules, with emphasis on the extent to which this process may provide a route to dissociative chemisorption. Although the initial chemisorption probability for HD on this surface is found to be a few percent, no correlation is found between chemisorption and selective-adsorption resonances, nor do these molecules contribute significantly to incoherent zero-phonon scattering, insofar as they do not appear in the wings of the coherent-scattering peaks. However, it does appear that molecules which undergo selective adsorption trap into a physisorption state before leaving the surface. Initial chemisorption probabilities have been measured using flash desorption to determine surface coverages. For a 0.109 eV HD beam on a 370 K sample these values are found to vary from 0.06 ± 0.01 at normal incidence to 0.020 ± 0.002 at incidence angles ≳60°. The selective-adsorption measurements yield five energy levels for molecules bound in the laterally averaged HD-W(110) potential, corresponding to distinct selective-adsorption resonances. In the specular, J = 0 → J = 0, channel, resonances are observed at 80.0°, 76.0° and 73.5° from the surface normal, giving J = 1 energy levels of -7.8, -4.7 and -2.3 meV. The rotationally inelastic, J = 0 → J = 1, channel displays shallow resonances at 68.0° and 64.0°, indicating J = 2 energy levels of -17.8 and -12.2 meV. Anomalously weak J = 0 → J = 2 scattering is observed, possibly caused by coupling between inelastic and reactive channels.