Surface scattering of NO from graphite: A statistical description of energy distributions

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Abstract
In the present theoretical study, inelastic scattering of NO from graphite surfaces is analyzed with a statistical model. The results are in good agreement with previous classical trajectory calculations by Pettersson et al. (1988). Angular distributions and the ‘‘rotational cooling’’ effect found in experiments published by Frenckel et al. (1982), Segner et al. (1983), and Häger and Walther (1984) are successfully reproduced. The model describes a small part of the graphite surface together with a scattering diatom as a collision complex, which decomposes in a unimolecular fashion. The surface is assumed to be flat, whereby the diatom angular momentum component along the surface normal and the linear momentum parallel to the surface are conserved. Otherwise the diatom translation and rotation are allowed to exchange energy with the surface, which is characterized by a set of harmonic oscillators. The experimentally observed ‘‘rotational cooling’’ effect is clearly demonstrated to be due to the conservation of the normal component of the angular momentum. The surface oscillator mass and the number of surface oscillators are treated as parameters. The results indicate that on the average one to three surface atoms are directly involved in each molecule-surface collision. ‘‘Rotational rainbow’’-like distributions are observed at high total energies, even though the simulations are purely statistical with no dynamic effect included.