  The linear Boltzmann equation for elastic and/or inelastic scattering is
applied to derive the distribution function of a spatially homogeneous system
of charged particles spreading in a host medium of two-level atoms and
subjected to external electric and/or magnetic fields. We construct a
Fokker-Planck approximation to the kinetic equations and derive the most
general class of distributions for the given problem by discussing in detail
some physically meaningful cases. The equivalence with the transport theory of
electrons in a phonon background is also discussed.
