Ionization threshold energies for metal clusters

We have measured the ionization threshold energies as a function of cluster size for V, Nb, and Fe clusters. The metal clusters are produced by laser vaporization of a metal substrate inside the throat of a pulsed nozzle. The clusters are detected by photoionization TOF mass spectrometry. Using tunable UV lasers, the ionization thresholds are measured as a function of cluster size. In addition, ionization thresholds for clusters with dissociatively chemisorbed hydrogen are found to increase significantly over that of the corresponding bare cluster.