Luminescence of cation excitons in and crystals in a wide excitation VUV region

Host luminescence excitation spectra and reflection spectra of and crystals at low temperatures have been measured in the energy region of 4-30 eV using synchrotron radiation. High-efficiency host luminescence is connected to radiative decay of self-trapped cation excitons. It is shown that also in the region of excitation multiplication, e.g. if the excitation energy is larger than twice the band gap energy , and in the relaxation process of core excitons the radiative self-trapped cation excitons are formed. However, the recombination of hole and self-trapped electrons (band-band excitation) does not give rise to host luminescence.