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

Vibrational excitation of HCl molecules (ν = 0 → 1) has been observed when HCl molecular beams at energies of 0.59–1.37 eV are scattered from a Au(1 1 1) surface at low surface temperature (Ts = 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, Ei, exhibiting a threshold near ∼Ei = 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 ν = 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.