Carbon monoxide adsorbed on Cu(100) Studied by infrared spectroscopy

Abstract

The stretch vibration of CO adsorbed on Cu(100) has been studied by infrared spectroscopy. Measurements on isotopic mixtures show that the molecules constitute a strongly coupled vibrational system. The results are successfully interpreted in terms of a dipole-dipole interaction between the adsorbed molecules. The vibrational polarizability deduced from the measurements on the isotopic mixtures is in fair agreement with what is obtained from the total absorptance. The frequency shift for one isotope from zero to full coverage is 9cm −1, whereas the dipole-dipole interaction would give a shift of about 40 cm −1. It is suggested that the negative shift of ca. 30 cm −1 is due both to changes in the chemisorption bond and to the fact that the molecules are tilted for low coverages but forced to orient their axes normal to the surface as the coverages increase. For the compressed phase the results are consistent with a model with 75% of the molecules in ontop position and 25% bridge-bonded. The intrinsic peak width, the vibrational frequency dependence of the damping, and additional peak broadening caused by different sorts of disorder are also studied and compared with existing theories.