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.