The physical mechanisms involved in electron irradiation of
insulating specimens are investigated by combining some simple
considerations of solid-state physics (trapping mechanisms of electrons
and secondary electron emission) with basic equations of
electrostatics. To facilitate the understanding of the involved
mechanisms only widely irradiated samples having a uniform distribution
of trapping sites are considered. This starting hypothesis allows
development of simple models for the trapped charge distributions in
ground-coated specimens as investigated in electron probe microanalysis
(EPMA) as well as for the bare specimens investigated in scanning
electron microscopy (SEM) and environmental SEM (ESEM). Governed by
self-regulation processes, the evolution of the electric parameters
during the irradiation are also considered for the first time and
practical consequences in EPMA, SEM, and ESEM are deduced. In
particular, the widespread idea that the noncharging condition of SEM
is obtained at a critical energy E2 (where δ +
η = 1 with δ and η yields obtained in noncharging
experiments) is critically discussed.