Multiphoton excitation, ionization, and dissociation decay dynamics of small clusters of niobium, tantalum, and tungsten: Time-resolved thermionic emission

Abstract

The ionization dynamics of transition metal clusters have been investigated using time-of-flight mass and electron spectroscopy following single-photon (220 nm) and two-photon (351, 308, and 248 nm) excitation by pulsed laser light. At 220 nm, the ionization is direct and only prompt photoelectrons are produced. At 308 nm, delayed photoelectrons are produced. In consequence of this delayed ionization process, the time-of-flight mass spectrum peaks show exponential tails (decay time 0.67, 0.40, and 1.54 μs for Nb7+, Ta7+, and W7+, respectively). The decay time is shown to have an explicit dependence upon the cluster nuclearity and the laser wavelength. Experiments, in which the acceleration voltage of the time-of-flight spectrometer is pulsed on after the photoionization laser pulse, reveal that the precursor to the delayed ion signals is a neutral molecule, further evidence for a delayed ionization process. Similar effects are also seen for transition metal carbide clusters. Clusters of the same nuclearity have approximately equal decay times independent of the number of carbon atoms in the cluster. Transition metal oxide clusters do not give a two-photon ionization signal. These observations are explained using a model for the two-photon excitation, dissociation, and ionization dynamics. The central feature of this model is that following single photon excitation of an electronic transition below the ionization potential, there is rapid internal conversion among all vibronic states. The absorption of a second photon then creates a vibrationally excited cluster which contains internal energy greater than the ionization potential, but which can only ionize by a nonadiabatic process. This delayed ionization process occurs in competition with dissociation. As clusters of niobium, tantalum, and tungsten and their carbides are very strongly bound, the dissociation rate is slow and the delayed ionization may be observed. Oxidized clusters are expected to be less strongly bound as the diatomic transition metal oxide provides an excellent leaving group; in consequence, no delayed ionization is observed for partially oxidized clusters. The rates for dissociation and ionization of the bare metal clusters have been calculated within the framework of a generalized statistical theory for cluster processes. These rates are in general agreement with the measured decay times. In addition, the rates have been estimated by a procedure which uses tabulated thermodynamic parameters for the bulk elemental materials and makes an explicit correction for the size dependence. Once again, a reasonable agreement is obtained. These results provide the first experimental observation of a delayed ionization process for a neutral polyatomic molecular system. In analogy with materials properties, they also represent the first experimental observation of time-resolved thermionic emission. © 1991 American Institute of Physics.