LUMINESCENCE OF CATION EXCITONS IN PBCL2 AND PBBR2 CRYSTALS IN A WIDEEXCITATION VUV REGION

Host luminescence excitation spectra and reflection spectra of PbCl2 a nd PbBr2 crystals at low temperatures have been measured in the energy region of 4-30 eV using synchrotron radiation. High-efficiency host l uminescence is connected to radiative decay of self-trapped cation exc itons. It is shown that also in the region of excitation multiplicatio n, e.g. if the excitation energy is larger than twice the band gap ene rgy (E > 2E(g)), and in the relaxation process of core excitons the ra diative self-trapped cation excitons are formed. However, the recombin ation of hole and self-trapped electrons (band-band excitation) does n ot give rise to host luminescence.