The effect of co-adsorption of on-top CO on the sum-frequency generation signal of bridge CO on the Ni(111) surface

Anomalous coverage dependence was observed on the sum-frequency generation signal of bridge CO on the Ni(111) surface when co-adsorbed by on-top CO. The bridge CO peak was distinct and normal from 0.04 to ∼0.40 monolayer (ML) but weakened after the appearance of the on-top CO peak at 0.47 ML and disappeared at 0.60 ML, although both peaks were observed in the infrared reflection absorption spectra up to 1.0 ML. The spectra of mixtures of C16O and C18O eliminate the possibility that the weakening of the bridge CO band is caused by its coupling with the on-top band.