Broadband femtosecond sum-frequency spectroscopy of CO on Ru{101̄0} in the frequency and time domains

CO on Ru{101̄0} was investigated by broadband femtosecond sum-frequency spectroscopy at 200 K. Approximately half of the frequency shift of 71 cm−1 over the coverage range from 0.15 to 1.22 monolayers is shown to originate from dipole–dipole coupling, with the remainder due to a chemical shift. Despite low adlayer-surface registration at the highest coverages, the linewidth of the C–O stretch is comparatively low, and is described by homogeneous broadening according to sum-frequency free-induction decay measurements in the time domain. This can be explained by the dominance of the CO dipole coupling strength over the static disorder present in a coincidence structure. As the coverage decreases below 0.3 monolayer, the linewidth increases considerably, indicative of inhomogeneous broadening. Supported by a concomitant frequency change we suggest that at low coverages CO molecules form chains of irregular length in the [0001] direction, as has been shown for other surfaces with similar symmetry.