Screening of SFG signals from bridged CO on Ni(111) by the coexistence of linear CO

A spectroscopic study of adsorbed CO on Ni(111) was carried out using infrared-visible sum-frequency generation (SFG) and infrared reflection absorption spectroscopy (IRAS). An anomalous coverage-dependence of the SFG signal intensities of bridged and linear CO was found. While the SFG signal of bridged CO at 1920 cm^-1 gained intensity with increasing CO coverage in the region between 0 and 0.4 ML (1 monolayer, ML: saturation coverage at 130 K), it weakened and disappeared when linear CO (2074 cm^-1) was present on the surface above 0.47 ML. The IRA peak of bridged CO, however, did not disappear at the increased coverage suggesting that the weakening of the SFG signal of bridged CO is not caused by the decrease in the coverage but by the suppression of the SFG process of bridged CO by the coexistence of linear CO. The spectra of isotope mixtures of C¹⁶O and C¹⁸O clearly indicated that the weakening of SFG signals is not caused by the dipole-dipole coupling between the vibrational modes of bridged and linear CO. The anomalous coverage-dependence of the SFG signal was ascribed to the change of the Raman tensor by the coadsorption of linear CO.