All previous experimental and theoretical studies of molecular interactions at metal surfaces show that electronically nonadiabatic influences increase with molecular velocity. We report the observation of a nonadiabatic electronic effect that follows the opposite trend: The probability of electron emission from a low–work function surface—Au(111) capped by half a monolayer of Cs—increases as the velocity of the incident NO molecule decreases during collisions with highly vibrationally excited NO(X2π½, V = 18; V is the vibrational quantum number of NO), reaching 0.1 at the lowest velocity studied. We show that these results are consistent with a vibrational autodetachment mechanism, whereby electron emission is possible only beyond a certain critical distance from the surface. This outcome implies that important energy-dissipation pathways involving nonadiabatic electronic excitations and, furthermore, not captured by present theoretical methods may influence reaction rates at surfaces.
N. H. Nahler, J. D. White, J. LaRue, D. J. Auerbach, A. M. Wodtke, Inverse Velocity Dependence of Vibrationally Promoted Electron Emission from a Metal Surface, Science 2008, 321, 1191-1194, DOI: 10.1126/science.1160040