Observation of Translation-to-Vibration Excitation in Acetylene Scattering from Au(111): A REMPI Based Approach

We present quantum-state and velocity resolved experiments for molecular beam scattering of acetylene (C₂H₂) from a single-crystal Au(111) surface, observations that reveal translational, rotational and vibrational inelasticity. The experiments are made possible by a novel (1 + 2) REMPI scheme for acetylene. The scattered molecules' velocity distributions as well as their ro-vibrational quantum-state distributions depend on the translational energy of incidence, E_I, providing unambiguous evidence that the scattered molecules were not trapped and equilibrated on the surface. We report the E_I-dependence of the collisional excitation of one and two quanta of the trans-bending vibrational mode, ν₄ = 0 → 1, 2, which is consistent with a mechanism involving conversion of incidence translational energy to acetylene vibration. Rotationally resolved velocity measurements on scattered acetylene in its ground vibrational state are interpreted in terms of orientation-dependent rotational and vibrational excitation probabilities.