MECHANISMS OF CREATION AND DESTRUCTION OF POLYHALIDE V CENTERS IN IRRADIATED AHC

The mechanisms of creation and destruction of polyhalide V centers hz AHC were modeled by the MNDO semiempirical quantum-chemical method. It is shown that X ~ molecules dissociate from the ground state into X~ ~ and X- and from the triplet state into X ~ and X o. whereas X ~ and X 7 molecules dissociate into X ~ and X, ~ and X~ and X~ , respectively. At room temperatures. X; or larger aggregates, which consist only of X; centers, form stable polyhalide V centers. It is shown that the thermal stability decreases when going along the series X f, X ~, and X 7 . The radiation-induced instability of stable F centers in AHC at room temperatures is associated with the desmuction of X ~ centers and the thermal instability of X ~ and X 7 centers. The nature of creation of radiation-induced effects in AHC was clarified in detail when experiments on AHC irradiated by electron (or photon) pulses were performed, in which the absorption spectra of radiation-induced defects were registered practically immediately alter their creation [ 1-3]. An analysis of this transient absorption under irradiation at liquid-helium temperatures has unambiguously shown which radiation-induced defects are primary and which are formed by subsequent secondary conversions. Based on the data obtained by the method Of measuring the nonsteady-state absorption, the authors of [4, 5] have shown that 'the creation of unstable defects and the accumulation of stable detects occur according to different laws. By the time the electron pulse ends, absorption bands corresponding to F and H centers are detected in one group of crystals (KCI, RbCI, etc.), whereas bands characteristic of sell-trapped excitons (STE) appear in the other group of crystals (NaC1, KI, etc.). Moreover, it has been shown for many AHC that there are stable and unstable modifications of originally created F, H pairs and STE, and that the unstable component accounts for 85-90% of their number. With increase in temperature, the induced absorption of F, H pairs and STE changes and indicates the decrease of short-lived pairs and excitons and the increase of long-lived ones. Except for vacancy-type detects (co and F centers), interstitial detects having the structure of molecular ions of the X~ family (H, Vk, and V~ centers) and X3 (Vz and V7 centers) have been created by the time the pulse ends. It has been shown that at temperatures 160-250 K tbr RbC1 and 10(O150 K/or KI, the yield of stable F centers significantly increases. With further increase in the temperature up to 500 K, the F-center concentration varies only insignificantly. Under stationary irradiation, the radiation-induced defects are accumuhued as a result of a number of consecutive independent processes: the formation of the primary Frenkel pair, the spatial separation of the components of the pair, and the conversion of separate components of the pair to stable defects. Because dach process depends specifically on the irradiation temperature, the dependence of the total yield of radiation-induced defects on this parameter also appears to be complex. The dependence of the efficiency of F-center accumulation in KC1 on the irradiation temperature, generalized in [6] based on the data of different authors, has the following lorm: a significant decrease at 25-50 K followed by a plateau with the low efficiency of defect survival and a significant increase between 100 and 200 K: above 200 K, the efficiency of Fcenter accumulation decreases. The temperature dependence is due to the change of the tbrm of survival of defects that are halides by their nature and additional to F centers. F centers remain the main radiation-induced electron detects for the entire range of the examined temperatures. Thus, from the experimental data obtained on exposure to pulsed and continuous radiation it can be seen that complex processes, associated with the interaction of radiation-induced defects with each other and with electron excitations that are continuously created iu a crystal during irradiation, proceed in AHC. These processes have opposite effects in the sense that some of them head to the accumulation of defects, whereas others - to mutual annihilation of defects, thereby decreasing the rate of their accumulation.