Infrared intensity enhancement of the CN stretch of HCN by coadsorbed CO on the Cu(100) surface

Reflection absorption infrared spectra reveal a strong enhancement in the intensity of the CN stretch in a mixed ordered overlayer of HCN and CO on the Cu(100) surface. Various combinations of HCN and CO isotopomers show that the intensity enhancement decreases with increasing frequency difference between nu(CN) and nu(CO). The intensity of the 2092 cm(-1) band of (HCN)-C-12-N-14 is enhanced by a factor of 155 +/- 20 through coupling to the 2077 cm(-1) band of (CO)-C-12-O-16. A simple two-state coupling model explains the isotopomer dependence of the degree of enhancement.