A study of the scattering of Ar from a well-ordered standing-up phase of 1-decanethiol adsorbed on Au(111) at surface temperatures from 110 to 185 K is presented. The final energies and intensities were measured as a function of incident polar and azimuthal angles using incident energies from 65 to 600 meV. These experimental results are compared to classical trajectory calculations. Scattering shows two distinct exit channels. The higher energies are due to direct inelastic scattering and have the greatest intensities at glancing incident and final angles. The lower energy channel is due to trapping-desorption; it has a Maxwell-Boltzmann energy distribution at the surface temperature and a cosine angular intensity profile. The simulations show that the timescale for normal momentum accommodation is very fast. The parallel momentum accommodation takes slightly longer, dependent on the initial conditions, but is still complete within only a few picoseconds. The result is that much of the Ar undergoes trapping-desorption, and the promptly scattered direct inelastic component, which interacts with the surface for similar to1 picosecond, retains more of its parallel than perpendicular momentum, leaving the surface preferentially at glancing polar angles. Another interesting observation is that the energy exchange between the surface and the directly scattered Ar has a dependence on the incident azimuthal angle. This is, in a sense, another type of structure scattering, where it is the anisotropic elastic response of the surface rather than the corrugation that leads to the angular dependence of the atom scattering. (C) 2003 American Institute of Physics.