SURFACE HETEROGENEITY OF C-60 AS STUDIED BY INFRARED-SPECTROSCOPY OF ADSORBED CO AND ADSORPTION POTENTIAL CALCULATIONS

In our recent investigation of the IR spectrum of CO physically adsorb ed on C-60 films, two well-resolved absorption bands at 2135 and 2128 cm(-1) were found, suggesting that the molecule is adsorbed on two dif ferent sites. To determine the nature of these adsorption sites, calcu lations of adsorption potentials and spectral shifts for the CO/C-60 s ystem were performed. The calculations were done for the fee (100), fe e (111) hcp (001), and hcp (111) surface planes. In the calculations t he 6-exponential and the Lennard-Jones potentials were used. A number of adsorption sites were chosen. These included the void space between four, three, and two neighboring C-60 molecules and the center of the hexagon and the pentagon on the C-60 surface. The calculated potentia ls and spectral shifts clearly indicate that adsorption sites in the v oids between the C-60 molecules are energetically preferred over sites on top of single C-60 molecules. Comparison is made between results o btained with the two potentials and with results obtained previously w ith the two other carbon allotropes: graphite and diamond.