Surface scattering of NO from Ag[111]: A statistical description of rotational energy distributions

Abstract

A statistical model is applied to inelastic scattering of NO molecules from the Ag[111] surface. Calculated final rotational energy distributions are found to be in good agreement with experimental distributions including pronounced ‘‘rotational rainbows’’ [Phys. Rev. Lett. 47, 1169 (1981)]. The model has previously been applied to NO scattering from graphite at lower collision energies [J. Chem. Phys. 93, 845 (1990)]. In the model, a scattering molecule and a small part of the surface form a collision complex which decomposes in a unimolecular fashion. The molecule is treated as a rigid rotor, and the simulated part of the surface as a few harmonic oscillators. The calculations indicate that the experimental results to a first approximation are statistical, and that no detailed dynamics have to be taken into consideration to explain them. The shape of the rotational energy distributions is due to conservation of the angular momentum component in the surface normal direction, introduced since the surface is treated as flat. Rotational rainbows are thus reproduced without introducing any detailed information about the molecule–surface interaction potential. The number of surface oscillators used in the model is varied, and in general one to four oscillators best reproduce the experimental results. The calculations indicate that the angular acceptance of the laser-induced fluorescence experiments is of large importance for the obtained final rotational energy distributions. An analytical solution to the statistical problem is derived for the case of fixed initial energy terms, and it is shown to describe well the experimental distributions here discussed.