CHEMICAL PROBES OF METAL CLUSTER IONIZATION-POTENTIALS

A procedure is described for the determination of metal cluster ionization potentials (IPs) using available excimer laser lines that gives error limits substantially smaller than traditional bracketing experiments. It is based on the observation that the adsorption of ammonia on cluster surfaces lowers cluster IPs, and that the IP lowering is linear in the number of adsorbed NH3 molecules. By determining the minimum number of NH3 molecules needed for ionization by the various excimer lasers, an approximation to the dependence of IP on coverage can be deduced. Extrapolation of this dependence to zero coverage gives the bare cluster IPs. Results are presented for clusters of iron, cobalt, and nickel having from 4 to 100 atoms. The effect of molecular adsorption on cluster IPs is analyzed theoretically, and the comparison with experimental results used to estimate the effective dipole moment of NH3 molecules adsorbed on these clusters. Comparison of the bare cluster IPs with the simple spherical drop model suggests that for transition metal clusters the Fermi level can be a significant function of cluster size.