Photoionization Yields, Appearance Energies, and Densities of States of Copper Clusters

Abstract

We present the results and analysis of photoionization measurements of neutral Cun=24–98 clusters, thermalized to T = 60 and 215 K. The photoion yield curves were used (1) to determine the cluster ionization appearance energies via a fit to the Fowler law of surface photoemission and (2) to explore the suitability of these curves for accessing information about electron states lying below the ionization threshold. The appearance energies follow a standard shell pattern up to n ≈ 40, but a subsequent structure suggests that the electronic shells become perturbed and split by crystal field effects. Nevertheless, it is apparent that the spectra are determined essentially by the number of delocalized electrons, i.e., by the sequential filling of electronic shell sublevels. The average shift of the appearance energies with cluster temperature was found to be in good agreement with a theoretical prediction for bulk copper. An important finding is that the derivatives of the ion yield curves overlap well with th...