Photochemical Study ofFandMCenters in Additively Colored Alkali Halides

A study of absorption changes in the visible and near infrared spectral region, produced by $F$-band irradiation at 77°K which recover spontaneously when the $F$ light is removed was made in additively colored KCl, KBr, and KI crystals containing $M$ bands. These temporary changes are caused by a superposition of two processes. In crystals containing small $M$-center concentrations the tunneling of excited $F$-center electrons to nearby $F$ centers could account for the temporary process. In crystals containing large $M$-center concentrations, the dominant process involves the transformation of $M$ centers to related centers. In KCl, the principal $M$-center transitions at 801 and 546 mμ are changed to 685 and 505 mμ, respectively. Studies in crystals containing polarized $M$ bands in which the temporary changes were produced with polarized light, show that this irradiation is not absorbed by $M$ centers, but probably by $F$ centers. The $F$-center excitation energy is then transferred locally to nearby $M$ centers. The consistent interpretation of both processes requires the occurrence of $F$-center clustering during the early stages of room temperature irradiation and the formation of $F$ and $M$ centers in close proximity during later stages. The possible nature of the transformed $M$ center is discussed and may involve $F_2^{}{}_{}{}^{+}$-center formation or the excitation of the $M$ center into a metastable state. The recovery involves two channels: a temperature-independent process dominant below 100°K and one involving a thermal activation energy of about 0.035 eV dominant at higher temperatures.