Rotational excitation in scattering of hyperthermal NO from Pt(111)

Rotational excitation of NO scattered from Pt(111) has been measured for incoming energies from 0.3 to 1.6 eV. For an initial energy of 0.3 eV a clear rotational rainbow, which we assign to O-end collisions, is visible at superspecular exit angles. This is very surprising, since NO-Pt(111) is a chemisorption system with a binding energy of 1 eV. Sharp, pronounced rainbows are visible in the range of incoming kinetic energies where the initial sticking coefficient is high. For an initial energy of 0.3 eV the initial sticking coefficient is 0.9, and at this energy the clearest rotational rainbow is observed. In contrast, at an initial energy of 1.6 eV the sticking coefficient is 0.5 and no rotational rainbow is observed. At subspecular exit angles the distributions are indistinguishable from a Boltzmann distribution at all the energies investigated, and show a clear energy dependence. Boltzmann-type distributions at high incident energy indicate a thorough redistribution of the available energy, although they cannot be explained in terms of a simple statistical model. The scattering results are interpreted as indicating a competition between direct scattering from the repulsive wall and indirect scattering via the deep potential well. Most of the direct scattering can be assigned to O-end collisions with the surface, where the binding energy is expected to be much smaller. Indirect scattering becomes more important at higher energies as more molecules are able to escape the well promptly.