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

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, w ere found to provide a good fit to the threshold shape and were used to obt ain the particle ionization potentials. The latter match precisely the bulk work functions cited in the literature, suggesting that photo-ionization o f free nanoclusters may form a useful complement to traditional photoelectr on studies of surfaces. Within 0.25-1 eV above the threshold, the ionizatio n efficiency begins to drop. This effect, which has parallels in bulk-surfa ce and small-cluster photoemission, is presently not well understood but ma y be related to an interplay between electron emission and collective surfa ce plasma excitations.