Femtosecond surface vibrational spectroscopy of CO adsorbed on Ru(001) during desorption

Using time-resolved sum-frequency generation spectroscopy, the C-O stretch vibration of carbon monoxide adsorbed on a single-crystal Ru(001) surface i s investigated during femtosecond near-IR laser excitation leading to desor ption. A large transient redshift, a broadening of the resonance, and a str ong decrease in intensity are observed. These originate from coupling of th e C-O stretch to low-frequency modes, especially the frustrated rotation, t hat are highly excited in the desorption process.