Evidence for thermionic emission from small aggregates

The delayed emission of electrons from small isolated aggregates has been reported now by several laboratories. When the emission is from a negative ion, it can plausibly be identified with the vibrational autoionization which has been known for many years from ions such as SF6-. Whether delayed emission from neutral species should be called ''thermionic'' is more problematic. Do these aggregates sample all of the available phase space prior to emission? We examine the experimental evidence which bears on this question. Particular reference is made to studies of ''metallic'' aggregates and of caged carbon clusters. Tantalizing evidence for the thermionic paradigm is noted, although recent experiments with niobium clusters can only be understood by postulating that they have unusual thermodynamic properties. Finally, Schottky's expression for the effect of an externally applied electric field on a work function is extended to aggregates of arbitrary size.