A. LUSHCHIK zyxwvutsrq et al.: Formation and Decay of Electronic Excitations 413 phys. stat. sol. (b) zyxwvu 168, 413 (1991) Subject classification: 61.70 and 71.55; 78.55; S9.11 zyxwv Institute of Physics, Estonian Academy zyxwv of Sciences, Tartu zyxw ’) Formation and Decay of Electronic Excitations Localized near Divacancies in Alkali Halides BY A. LUSHCHIK, CH. LUSHCHIK, N. LUSHCHIK, A. FRORIP, and 0. NIKIFOROVA The thermal quenching or plastic strain of RbCI, KC1, and KBr crystals leads to an increase in the number of associations of anion and cation vacancies. Using the methods of VUV spectroscopy, electronic excitations localized near divacancies are detected at 5 K. The process of radiative and nonradiative decay of these excitations is studied as well as the process of radiational creation of divacancies. 1. Introduction The radiational instability of alkali halides (AH) built up of monovalent cations Mf and anions X- is caused by the process of nonradiative decay of self-trapped excitons e: (STE) or nonradiative recombination of electrons e- with self-trapped holes (X;),a (V,-centres) with the creation of anion Frenkel defect pairs (FD): anion vacancies v, (a-centres) and interstitial halogen ions X; (I-centres) as well as F-centres (vae-) and interstitial halogen atoms (X;), (a dihalide quasimolecule situated at one anion site, the so-called H-centre) [ 1, 21. According to electron microscope investigations [3, 41, the low-temperature irradiation of a crystals leads to the formation of FD in regular crystal lattice sites without the participation of pre-irradiation defects. On the other hand, the interaction of highly mobile H-centres with each other or with intrinsic and impurity pre-irradiation defects plays an important role in the primary stages of radiation action at higher temperatures (200 to 300 K). The use of AH with a content of impurity ions on the level of 1 to 10-’ppm [5,6] allowed one to investigate pre-irradiation intrinsic point defects in RbC1, KC1, and KBr crystals by measuring their optical characteristics in the UV and VUV spectral regions. The aim of the present study is to investigate the optical characteristics of the elementary associations of anion and cation vacancies (divacancies and quadrivacancies) in some AH, and to investigate, by means of highly sensitive luminescent methods, the possibility of radiational creation of divacancies as one of the manifestations of cation defect creation by radiation (cf. [7]). I) Riia 142, 202400 Tartu, Estonia. 27 physica (b) 168/2