Photo-ionization efficiency curves of alkali nanoclusters in a beam and determination of metal work functions

Abstract.

We produced free beams of cold nanometer-sized particles of lithium, sodium and potassium and measured the three corresponding photo-ionization yield curves. Quadratic (Fowler) plots, originally developed for bulk surfaces, were found to provide a good fit to the threshold shape and were used to obtain the particle ionization potentials. The latter match precisely the bulk work functions cited in the literature, suggesting that photo-ionization of free nanoclusters may form a useful complement to traditional photoelectron studies of surfaces. Within 0.25–1 eV above the threshold, the ionization efficiency begins to drop. This effect, which has parallels in bulk-surface and small-cluster photoemission, is presently not well understood but may be related to an interplay between electron emission and collective surface plasma excitations.