Broadband femtosecond sum-frequency spectroscopy of CO on Ru{1010}in the frequency and time domains

摘要

CO on Ru{1010}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.