Observation of a change of vibrational excitation mechanism with surface temperature: HCl collisions with Au(111)

We have measured the vibrational excitation probability (P-v) of HCl incident on a Au(111) surface at kinetic energies (E-i) of 0.59 eV to 1.37 eV and surface temperatures (T-s) of 273 K to 1073 K. For all energies, the slope of the P-v as a function of T-s exhibits a sharp increase above T-s approximate to 800 K. We show this change in slope and the threshold behavior of P-v to be consistent with a change in excitation mechanism from an electronically adiabatic mechanical mechanism to an electronically nonadiabatic mechanism involving excited electron-hole pairs.