Shape resonance scattering as a probe of excited-state adsorbate dynamics for CO chemisorbed on Ni(111)

This paper demonstrates that electronically resonant inelastic electron scattering can be used to probe the vibrational dynamics of chemisorbed species, and that the formation of a transient negative ion during scattering can lead to significant enhancement in the probability for vibrationally inelastic scattering. We specifically examine CO/Ni(111) chemisorbed in a c(4×2) structure. Our measurements consist of the incident energy dependencies for vibrationally inelastic scattering from different adsorbate modes, as well as the final angular distributions for electrons emerging from the transiently formed resonance. We also observe the presence of weak first overtone scattering for the CO intramolecular stretch under resonant scattering conditions. All of the observations are consistent with the formation of a Σ shape resonance in the vicinity of 18 eV which is slightly lower in energy, and has a shortened lifetime, than in the gas phase. Discussions are presented 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.