Laser Studies of Metal Cluster Beams

Abstract

Recent developments in laser and molecular beam technology have now made it possible to produce supersonic beams of virtually any element in the periodic table. Using laser vaporization of the appropriate target, the beam source conditions may be adjusted to produce either the cold free atoms alone, or clusters of these atoms with each other—or with another element. Since the vaporization laser heats only a small spot on the target, extremely high local temperatures can be obtained without heating any other part of the apparatus; and beams of even the highest boiling element (tungsten) are readily obtained both in atomic and cluster form. The physics and chemistry of these exotic cluster species is almost completely unknown on the fundamental level. Even for clusters containing 100 atoms, most of these atoms lie on the surface, and the chemical and physical properties will be predominately surface phenomena. Initial studies of these clusters have entailed the use of one- and two- photon laser ionization with time-of-flight mass selective detection. Using a variety of fixed frequency lasers, the work function of copper clusters has been examined as a function of cluster size in the range from 2 to 29 atoms per cluster. Considerable detailed information has also been obtained for the electronic structure and bond lengths of a number of transition metal dimers and trimers (including Cu2, Cr2, V2, Mo2, and Cu3) through the use of high resolution laser spectroscopy with mass-selective photoionization detection.