The frequencies and amplitudes of CO vibrations at a metal surface from model cluster calculations

Abstract

The general features of localised vibrations in adsorbed layers are discussed. Model calculations performed on small clusters using force constants and interatomic distances from Ni(CO) 4 and Ni metal give sets of frequencies for on-top and bridge-bonded CO on Ni(100) in reasonable agreement with experiment. The dependence of each frequency on the force constants is investigated, and gives “best fit” stretching force constants, F NiC and F CO, of 2.6 and 16.8 mdyn Å −1 for on-top CO and 1.1 and 14.9 mdyn Å −1 for bridging CO. The amplitudes of the bending modes in the same clusters are also calculated and the temperature dependence of the mean angle of inclination of the CO molecule to the surface normal is presented. Because the bending modes are soft and the occupation of higher quantum states becomes important, the mean square amplitudes are strongly temperature dependent: the mean angle of inclination increases from ∼5° at 77 K to ∼8° at room temperature for the on-top species.