Vibrational excitation and relaxation of NO molecules scattered from a Au(111) surface

We present results of recent and ongoing experiments on molecular-beam surface scattering of NO molecules from a Au(lll) surface. Vibrational excitation of NO(ν=0) into ν=l,2 was studied in great detail over a wide range of incidence energies (0.10-1.05 eV) and surface temperatures (300-1100 K). We find behavior characteristic of electronically nonadiabatic coupling of molecular vibration to electron-hole pair excitation in the gold crystal. A state-to-state kinetic model shows that for ν=2 excitation both the sequential (0→1→2) and direct (0→2) excitation pathways are important. The absolute excitation probabilities are also compared to the results of a first principles independent-electron surface hopping calculation, and good agreement is obtained. In addition to ν=l,2 excitation, we present the first evidence for second-overtone ν=3 vibrational excitation. Preliminary data for vibrational relaxation of laser-prepared NO(ν=3) show strong relaxation; the intrinsic coupling strengths are consistent with those obtained from vibrational excitation. Measurements of the translational inelasticity of NO(ν=3) show that the molecules lose a large fraction of translational energy in the collision. The translation-vibration coupling is significant but may depend on incidence energy, and a clear anticorrelation is observed between the final translational and rotational energies.