The interaction of highly vibrationally excited molecules with surfaces: vibrational relaxation and reaction of NO(v) at Cu(111) and O/Cu(111)

Abstract

We have studied the reaction and inelastic scattering of ground and vibrationally excited NO on Cu(111). We employed laser-based techniques to prepare NO in vibrationally excited states, stimulated emission pumping (SEP) to prepare v=13 and v=15 and infrared overtone pumping to prepare v=2. Laser ionization detection schemes were developed for probing the state distribution of highly vibrationally excited NO molecules. Ground-state NO(v=0) dissociates at Cu(111) with a probability of ≈2×10^-4, with little dependence on the translational energy in the range between 29 and 65 kJ mol^-1. The dissociation probability is strongly enhanced by vibrational excitation to v=13 and 15. The dissociation continues until the oxygen coverage on Cu(111) reaches saturation. For highly excited NO(v=13, 15) scattering from O/Cu(111), we have seen efficient multi-quantum relaxation (up to Δv=-5). For NO(v=2), in contrast, the survival probability is nearly 90%. Measurements of the translational and rotational state distributions after scattering support a direct-inelastic mechanism for vibrational relaxation, with strong flow of energy into the surface. The branching ratios for vibrational relaxation are independent of the kinetic energy in our experiments.