Threshold laws in delayed emission: an experimental approach

Delayed emission is a common decay process for very excited complex systems where the excitation energy is, to a large extent, statistically distributed over all accessible degrees of freedom. A rich variety of delayed decay processes is found in complex molecules and clusters such as thermionic emission, evaporation of heavy fragments or blackbody radiation. In this article, we present the general threshold laws that govern the kinetic energy spectrum of matter particles (electrons or fragments) ejected from spherically symmetric species in such processes, and we illustrate these laws by experimental results obtained in photoelectron and photoion spectroscopy. Deviations from simple laws for non spherical species are also evidenced.