Kinetic energy losses and vibrational excitation in hyperthermal C 60-surface scattering: dependence on vibrational energy of the incident molecule

Abstract

Kinetic energy losses and vibrational excitation in scattering of a polyatomic molecule from a surface at hyperthermal energies were studied for the first time as a function of the average vibrational energy of the incident molecule. C 60 molecules with variable amounts of vibrational energy in the range of < E v = 7–15 eV and fixed kinetic energy were prepared in a helium seeded beam and scattered from a carbonized nickel surface. The primary beam kinetic energies and kinetic energy losses were measured with an accuracy of ± 0.1 eV while vibrational energies before and after the scattering event were measured with an accuracy of ± 0.25 eV (most probable values of the corresponding energy distributions). The vibrational thermometry method employed is shown to be practically independent of the specific model relating vibrational temperature to average vibrational energy (canonical vs. finite heat bath). Experiments were done at two different scattering angles corresponding to near normal and near grazing conditions, and for both angles it was found that the kinetic energy loss is independent of the initial vibrational energy to within 0.2 eV. The collisional vibrational excitation showed a weak inverse dependence on initial vibrational energy for both scattering angles. For the lowest incident C 60 vibrational energy (< E v = 9.8 eV) and near normal scattering (30.7 eV in the normal impact energy component) this excitation was found to be about 5% of the impact kinetic energy. We discuss these results in relation to possible mechanisms for normal and tangential energy transfer in a single C 60-surface collision at the hyperthermal (10–50 eV) energy range.