Photoionization studies of niobium clusters: Ionization potentials for Nb2–Nb76.

The photoionization efficiency (PIE) spectra for niobium clusters up to Nb76 are measured using laser ionization time-of-flight mass spectrometry. Ionization potentials (IPs) assigned from the PIE spectra evolve with the same general pattern observed for other transition metal clusters (Mn) studied to date: a rapid yet nonmonotonic decrease in IP to n[bar_over_tilde:_approx._equal_to]15 followed thereafter by a relatively slow and smooth evolution. The measured IPs evolve with cluster radius R according to the predictions of the conducting spherical droplet model if the limiting (R→∞) IP is chosen to be 0.5 eV lower than the bulk polycrystalline work function. Kinetics experiments using D2 as the coreactant indicate the existence of two structural forms, one highly reactive, one relatively unreactive for clusters in the size range Nb9–Nb12, with the reactive forms predominant in each case. PIE spectra for Nb9–Nb12 were recorded using sufficient D2 added to the cluster source to selectively titrate the reactive forms of these clusters, leaving the unreactive forms as the predominant isomers. It is found that the unreactive forms of Nb9 and Nb12 possess higher IPs than those of the reactive forms, while both forms of Nb10 possess approximately the same IP. [ABSTRACT FROM AUTHOR]