Nonadiabatic coupling between C-O stretching and Pt substrate electrons enhanced by frustrated mode excitations

We report the transient enhancement of coupling between adsorbate internal stretching vibration and substrate electrons induced by femtosecond laser excitation. A Pt(111) surface covered with carbon monoxide (CO) is irradiated with a 150 fs pump pulse at 400 nm. Transient changes in the imaginary part of nonlinear susceptibility in the C-O stretching region are probed by using femtosecond time-resolved infrared-visible sum-frequency generation with phase-sensitive detection. The line shape of C-O stretching shows remarkable asymmetry due to Fano interference with a continuum of electron-hole pair excitation. Excitations of CO-Pt frustrated modes play a key role in the enhancement of the nonadiabatic coupling.