"Dynamical" versus "statistical" rotational distributions in hyperthermal NO-Pt(111) scattering

Rotational distributions from NO-Pt(111) scattering have been reported [see next article, no. 2739]. At lower incoming energies (<1 eV) clear rotational rainbows are seen, but the distributions for higher energies approach Boltzmann distributions with apparent temperatures far exceeding the surface temperature. We compare here the No-Pt(111) scattering distributions to the predictions of a simple statistical model. The model assumes randomization of the available energy, subject to (partial) conservation of parallel linear momentum and angular momentum about the surface normal. Some characteristics of the rotational and angular distributions which arise from such a statistical energy repartitioning are discussed and compared to experimental results. It is seen that a combination of peaked angular distributions and Boltzmann-type rotational distributions independent of the scattering angle are reproduced by a simple statistical calculation with partial conservation of parallel linear momentum. For the NO-Pt(111) system, it is shown that a complete description of the high-energy scattering requires specifically dynamical assumptions. The transition from "dynamics" to "statistics" most likely arises from a combination of increased averaging, resulting from a competition between scattering via different regions of the potential energy surface, and a weakening of the rainbow features, perhaps due to the onset of chattering collisions.