Vibrational coupling of CO adsorbed on monodispersed Ni11 clusters supported on magnesia

We present infrared and thermal desorption spectroscopic studies of CO adsorbed on size-selected nickel clusters composed of 11 atoms and supported on ultrathin magnesium oxide films. Thermal desorption spectroscopy of CO shows that on the average 4 molecules are weakly adsorbed per cluster at saturation coverage. Using an isotopic mixture of 12CO and 13CO, infrared spectroscopy reveals the existence of a vibrational coupling interaction between the 4 COs. A semi-classical model of interacting dipoles is applied to correlate the observed vibrational frequency shifts with the arrangement of the COs on the cluster. This simple analysis favors a three-dimensional structure for the deposited clusters.