Excitonic and electron–hole mechanisms of the creation of Frenkel defect in alkali halides

Abstract

Excitonic and electron–hole (e–h) mechanisms of stable F centre creation by VUV radiation in alkali halide crystals are discussed. In KCl at 4.2 K, the efficiency of stable F–H pair creation is especially high at the direct optical formation of triplet excitons with n=1. At 200–400 K, the creation processes of stable F centres in KCl are especially efficient at the formation of one-halide exciton in the Urbach tail of an exciton absorption. In KCl and KBr, the decay of a cation exciton (∼20 eV) causes the formation of two e–h pairs, while in NaCl a cation exciton (33.5 eV) decays into two e–h and an anion exciton. An elastic uniaxial stress of a crystal excited by VUV radiation decreases the mean free path of excitons before their self-trapping (KI) and increases the mean free path of hot holes before self-trapping (NaCl).