In photoelectron spectroscopy experiments it is generally assumed that the Coulomb force between charged particles is small compared with external fields, and that the free kinetic electrons will quickly leave the ions. This is the basis of the ZEKE photoelectron spectroscopy. However as the density of charged particles is increased, plasma physics effects begin to become important, and the kinetic electrons become trapped by the net positive charge and move so as to set up a self-field which can cancel any externally imposed electric fields. For high densities, fewer electrons than expected are able to escape the self-field. The production of self-consistent electric fields is studied by means of particle-in-cell plasma simulations and by N-body trajectory calculations, and simple expressions are derived for when plasma physics effects become significant. An experimental illustration of plasma effects in ZEKE is presented. (C) 1997 American Institute of Physics.