  We survey recent comprehensive studies of exciton relaxation in the crystals
of lead halides. The luminescence and electron-spin-resonance studies have
revealed that excitons in lead bromide spontaneously dissociate and both
electrons and holes get self-trapped individually. Similar relaxation has been
also clarified in lead chloride. The electron-hole separation is ascribed to
repulsive correlation via acoustic phonons. Besides, on the basis of the
temperature profiles of self-trapped states, we discuss the origin of
luminescence components which are mainly induced under one-photon excitation
into the exciton band in lead fluoride, lead chloride, and lead bromide.
