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Abstract

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, EI, providing unambiguous evidence that the scattered molecules were not trapped and equilibrated on the surface. We report the EI-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.