Scattering and trapping dynamics of gas-surface interactions: Vibrational excitation of CF3Br on graphite

Abstract

We present results from molecular beam experiments and classical trajectory calculations of CF3Br scattering from graphite. Direct inelastic scattering dominates for initial translational energies Etr=0.6–3.5 eV and surface temperatures Ts=500–1170 K. An increase in the CF3Br vibrational temperature is observed in the scattered flux using the method of electron impact-induced fragmentation. The vibrational excitation depends on Etr and Ts, and a maximum vibrational temperature increase of 254±15 K is reached for Etr=3.5 eV and Ts=830 K. The vibrational excitation, angular distributions, and average translational energies are semi-quantitatively reproduced by classical trajectory calculations, indicating that the vibrational excitation can be explained by an electronically adiabatic “mechanical” process. The calculations suggest that a large fraction of the incident molecules experience multiple collisions with the surface. These transiently trapped molecules are slowly vibrationally excited while moving long distances, and are not thermalized even after 100 ps on the surface.