A mechanism for vacancy generation in perfect crystals is proposed in which it is assumed that the progenitor for color-center formation is a dissociating $X_2^{}{}_{}{}^{=*}$ (excited) molecule: if this molecule is initially created, or subsequently placed, in an excited vibronic, predissociative, or repulsive state, it may dissociate into a normal halide ion and an uncaged atomic halogen fragment $X$ moving into an interstitial position and leaving behind a vacancy, according to the reaction $X_2^{}{}_{}{}^{=**}\to X_{\mathrm{int}}+\square +X^{-}+e$. The relative probabilities of dissociation and radiation depend on the openness and other parameters of the lattice. Capture of the electron and motion of the interstitial halogen atom results in the formation of $F$ centers and $H$ and other $V$ (halogen) centers, depending on the temperature. A general photochemical interpretation which can account for the formation and nature of the halogen centers as well as the $F$ centers is presented. The key part of the suggested mechanism is the molecular dissociation of molecules which can (1) provide the momentum needed for the generation of the Schottky and Frenkel defects noted after irradiation, and (2) account for the $X_2^{}{}_{}{}^{=*}$ luminescence (major process) noted during irradiation. On the basis of the proposed mechanism the uv colorability of KI has been predicted and made plausible, as well as such divergent color-center phenomena as the uv-induced formation of $F$ and $V$ centers in KCl:H and KCl:I, and the UV-induced generation of $V_k$ centers in Kl:Tl and strained KI.

• Received 14 March 1966

DOI:https://doi.org/10.1103/PhysRev.148.928