SECONDARY EXCITONS IN ALKALI-HALIDE CRYSTALS

The peculiarities of multiplication of electronic excitations have bee n investigated in wide-gap alkali halide crystals where, in contrast t o semiconductors, free and self-trapped excitons as well as free and s elf-trapped holes coexist. In particular, the luminescent methods for separation and quantitative investigation of the formation processes o f secondary excitons have been elaborated. These methods are based on the investigation of the dependence of the intensity ratio for two dif ferent emissions from the same crystal as a function of the energy of exciting photons. The excitation spectra of triplet (pi) and singlet ( sigma) emissions of self-trapped excitons as well as of impurity lumin escence have been measured, using synchrotron radiation of 12 to 32 eV , for high-purity KI, KBr, and doped KI:Na, KI:TI, KBr:I, KBr:TI cryst als at 8 K. The analysis of the intensity ratio spectra for two emissi ons allowed us to separate the regions of secondary triplet exciton fo rmation in KI (13-15 and 26.5-29 eV) and KBr (15-17 and 28-30 eV). The threshold photon energy for the formation of a secondary exciton has been experimentally determined to be equal to 13.1+/-0.2 eV in KI and 15.2+/-0.2 eV in KBr. The probability of secondary exciton formation b y hot photoelectrons reaches the main maximum at 0.5-0.7 eV above the value of threshold energy. The energy transport to impurity centers by free triplet secondary excitons has been revealed in KI:Na and KBr:I crystals. The decay of 3p(5)4s cation excitons in KI and KBr, generate d by 20-eV photons, leads to the formation of a double amount of elect ron-hole pairs.