Two-step photoionization and photophysics of color centers in LiF crystals

Abstract

The results are presented of theoretical and experimental studies of two-step photoionization and photochemistry of color centers in LiF crystals. The exact analytic expressions were obtained for the population of optic centers energy levels in the process of two-step photoionization within a framework of kinetic equations in three-level approximation. A transfer from cw to pulsed-periodic excitation was analyzed. An analytic dependence was obtained of the ionization cross-section on the incident light flux intensity. For the first time the cross-section was estimated of the photoionization from the excited level of F<SUB>2</SUB> color center in LiF crystal being pumped by 532 nm wavelength radiation. The processes were detected of mutual phototransformations of F<SUB>3</SUB>, F<SUB>3</SUB><SUP>+</SUP> and N color centers in oxygen-free LiF crystals. It was demonstrated that the presence of F<SUB>3</SUB> and F<SUB>3</SUB><SUP>+</SUP> color centers in crystals is necessary for selective accumulation of N color centers at high temperature (300 K) action of Nd:YAG laser third harmonic radiation. In LiF- OH crystals the processes of mutual phototransformations were detected in a CC ensemble with absorption bands maxima at wavelengths 380 nm, 450 nm, and within the range 500 - 570 nm. It was demonstrated that an effective destruction of the CC absorption band (380 nm) along with the formation of absorption bands with maxima within 500 - 570 nm takes place under high-temperature (300 K) action of Nd:YAG laser third harmonic radiation ((lambda) equals 354 nm). Reverse phototransformation with the recovery of the band at 380 nm takes place along with the known phototransformation F<SUB>2</SUB> (450 nm) yields F<SUB>2</SUB><SUP>+</SUP> (654 nm) under the subsequent action of the same laser second harmonic radiation ((lambda) equals 532 nm).