  Metal cluster ionization potentials are important characteristics of these
"artificial atoms," but extracting these quantities from cluster
photoabsorption spectra, especially in the presence of thermal smearing,
remains a big challenge. Here we demonstrate that the classic Fowler theory of
surface photoemission does an excellent job of fitting the photoabsorption
profile shapes of neutral In_{n=3-34} clusters [Wucher et al., New J. Phys. 10,
103007 (2008)]. The deduced ionization potentials extrapolate precisely to the
bulk work function, and the internal cluster temperatures are in close
agreement with values expected for an ensemble of freely evaporating clusters.
Supplementing an earlier application to potassium clusters, these results
suggest that the Fowler formalism, which is straightforward and physical, may
be of significant utility in metal cluster spectroscopy. It is hoped also that
the results will encourage a comprehensive theoretical analysis of the
applicability of bulk-derived models to cluster photoionization behavior, and
of the transition from atomic and molecular-type to surface-type photoemission.
