SECONDARY-EMISSION OF BOUND EXCITONS WITH LARGE AND SMALL RADII IN LITHIUM HYDRIDE SINGLE-CRYSTALS

Relaxation of free excitons into the states of bound excitons with lar ge and small radii for the single crystals of lithium hydride with iso valent and heterovalent impurities has been studied on the basis of th e low-temperature exciton-phonon spectra of luminescence and resonant combinational scattering. Bound excitons with large radii have resonan t levels at E < E-ex (E-ex is the energy of the basic excitonic state n = 1 s) and are characterized with a lifetime not less than 10(-10) s . The excitons with small radii have a lifetime of 10(-5)-10(-6) s at 78 K. They may be obtained either by the recombination or by formation of the near-impurity excitations or by the relaxation of bound excito ns with large radii into small radii states. Such relaxation takes pla ce due to the generation of LO(Gamma) phonons.