Lateral Hopping of CO on Ag(110) by Multiple Overtone Excitation

A novel type of action spectrum representing multiple overtone excitations of the v(M-C) mode was observed for lateral hopping of a CO molecule on Ag(110) induced by inelastically tunneled electrons from the tip of a scanning tunneling microscope. The yield of CO hopping shows sharp increases at 261±4 mV, corresponding to the C-O internal stretching mode, and at 61±2, 90±2, and 148±7 mV, even in the absence of corresponding fundamental vibrational modes. The mechanism of lateral CO hopping on Ag(110) was explained by the multistep excitation of overtone modes of v(M-C) based on the numerical fitting of the action spectra, the nonlinear dependence of the hopping rate on the tunneling current, and the hopping barrier obtained from thermal diffusion experiments.