EVIDENCE FOR THERMIONIC EMISSION FROM SMALL AGGREGATES

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