Direct translation-to-vibrational energy transfer of HCl on gold: Measurement of absolute vibrational excitation probabilities

Vibrational excitation of HCl molecules (v=0 -> 1) has been observed when HCl molecular beams at energies of 0.59-1.37 eV are scattered from a Au(111) surface at low surface temperature (T-s=273 K). The incident and scattered HCl is probed state-selectively using 2+1 REMPI. The vibrational excitation probability depends strongly on incidence kinetic energy, E-i, exhibiting a threshold near similar to E-i=0.57 eV. We measured the absolute vibrational excitation probability which varies from 10(-6) to 10(-5) over this energy range, 1-2 orders of magnitude higher than the thermal equilibrium expectation value. The magnitude of the excitation probability, the near specular angular distributions of the scattered v=1 molecules and the dramatic narrowing of the angular distribution near threshold are all consistent with a direct translation to vibration (T-V) mechanical energy transfer mechanism. (C) 2006 Elsevier B.V. All rights reserved.