The ionization potentials of clustered alkali metal atoms

Small clusters of alkali metal atoms produced in the expansion of near-saturated vapours through a small orifice have been ionized by ultra-violet light and detected with a mass spectrometer. Ionization potentials were determined by measuring the threshold wavelength for the formation of the corresponding ion species. The experimental technique is described in detail. Results are presented for clusters of up to four atoms in sodium, potassium, caesium, a sodium/potassium mixture and for the double molecule of lithium. The values for clusters of three and four potassium and caesium atoms are the lowest of any molecular species yet reported. There are no theoretical values with which to compare these results but comparison is made with previous experimental values for double molecules. The ionization potential for clusters of three and four atoms approximates to the expected work function of a corresponding metal drop. These clusters can have a dominant effect on the electrical conductivity of a metal vapour.