SHAPE RESONANCE ENHANCEMENT OF VIBRATIONAL EXCITATIONS FOR CO CHEMISORBED ON NI(111) PROBED VIA INELASTIC ELECTRON-SCATTERING

This paper examines how the formation of a transient negative ion during the scattering of an electron from CO chemisorbed on Ni(111) can lead to significant enhancement in the probability for vibrationally inelastic scattering. We specifically report on the incident energy dependence for transient negative ion formation for a c (4 X 2) overlayer. The signature for this resonance is the energy dependence of the probability for vibrationally inelastic scattering from the CO intramolecular stretch and a CO frustrated rotation, both broadly peaking in the vicinity of 18 eV. Additional support for this scattering mechanism comes from the monotonic rise in scattering intensity for both of these vibrational modes as the final (detector) angle moves towards the surface normal under otherwise fixed kinematic conditions. We also observe the presence of weak first overtone scattering for the CO intramolecular stretch under resonant scattering conditions. All of the aforementioned observations are consistent with the formation of a SIGMA shape resonance which is slightly lower in energy, and has a shortened lifetime, than in the gas phase. We also report dispersion measurements along the [112BAR] direction for the c(4X2) structure, as well as for a saturated (square-root 7/2X square-root 7/2) R19.1-degrees CO/Ni(111) overlayer which give information about intermolecular couplings in these compressed structures. Discussions are presented, including wave packet arguments, which emphasize that the presence (or absence) of vibrational excitation in a given vibrational coordinate following negative ion formation can be used to infer important details about femtosecond nuclear coordinate evolution for the system in the excited state.