The CO stretch frequency of fully CO covered Ag-clusters in dependence on cluster size

Silver clusters Ag_n prepared either by the gas aggregation technique (GAT) or by matrix aggregation of Ag were embedded in CO and CO/Ar-matrices. The vibrational CO stretch frequency at the monolayer coverage, which is found on bulk silver at 2110 cm^-1, preserves its value with decreasing particle size, Ag_n n≥300. For smaller clusters a decrease of this frequency with decreasing size was observed. For the smallest investigated clusters Ag_n, n～30, a value of 2080 cm^-1 was obtained. The weakening of the CO bond is an indication of a charge transfer from the cluster to the CO monolayer. This result is in accordance with the surface plasmon absorption measurements of Ag-clusters isolated in CO matrices.