Secondary excitons in alkali halide crystals

The peculiarities of multiplication of electronic excitations have been investigated in wide-gap alkali halide crystals where, in contrast to semiconductors, free and self-trapped excitons as well as free and self-trapped holes coexist. In particular, the luminescent methods for separation and quantitative investigation of the formation processes of secondary excitons have been elaborated. These methods are based on the investigation of the dependence of the intensity ratio for two different 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 luminescence 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 crystals at 8 K. The analysis of the intensity ratio spectra for two emissions allowed us to separate the regions of secondary triplet exciton formation 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 by 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, generated by 20-eV photons, leads to the formation of a double amount of electron-hole pairs.