ADIABATIC VERSUS NONADIABATIC EFFECTS IN THE VIBRATIONAL-EXCITATION IN NO/AG SCATTERING

By modifying the Newns-Anderson model to incorporate the observed strong change of NO stretch frequency on adsorption we have re-investigated the relative importance of adiabatic versus non-adiabatic effects in the vibrational excitation in NO/Ag scattering. The change of stretch frequency is related linearly to the occupancy of the NO affinity level which is, in contrast to earlier investigations, assumed to be approximately one at the adsorption equilibrium position. This introduces drastic changes. The non-adiabatic coupling of the vibrations to thermal electron-hole pairs of the substrate leads only to a small excitation probability (almost-equal-to 0.5%) which is almost independent of incident energies. At high energies (greater-than-or-equal-to 0.4 eV) the adiabatic trans-vib coupling is the dominant mechanism for the vibrational excitation. The surface temperature dependence of the experimental results is explained by a reasonable temperature dependence of the potential parameters.