ON THE VIBRATIONAL TEMPERATURE OF METAL CLUSTER BEAMS - A TIME-RESOLVED THERMIONIC EMISSION STUDY

Delayed ionization rates for small niobium clusters are measured as a function of the cluster size, the laser wavelength, the number of photons absorbed, and the initial internal energy content of the cluster. It is shown, for the first time, that vibrational excitation of the clusters modulates their rate of delayed ionization. An analysis of the rate of ionization in terms of the total energy content of the cluster establishes unequivocally that delayed ionization is a statistically determined, unimolecular, activated process. It is shown that the rate of delayed ionization can be used to gauge the initial vibrational energy content of the cluster. Quantitative analysis of this effect establishes, for the first time, a thermometry for metal cluster beams. Thermal rate parameters, activation energies, and ''Arrhenius factors'' are presented for delayed ionization of a series of niobium clusters Nb(n), n=5, 6, 7, 8, 9, 11, 12, and 13. The activation energies are lower than the corresponding ionization potentials by approximately 1 eV. The ''Arrhenius factors'' are all in the range 10(11) s-1. These parameters highlight the differences between delayed ionization and photoionization of clusters and draw attention to the need for an adequate theory of unimolecular processes in clusters taking into account both nonadiabatic effects and the important effects arising from the fluxionality of the cluster at high internal energies.