Exciton-InducedF-Center Growth in KI and KBr Crystals

A study has been made at room temperature of the growth in $F$-center concentration resulting from the absorption of photons in the energy range of the first fundamental band (exciton band) of KI and KBr crystals. The growth in $F$-center concentration was followed by measuring the fractional change in transmission at the maximum of the $F$ band by an ac method capable of detecting a change in $F$-center concentration of ${10}^{11}$ ${\mathrm{cm}}^{-2\mathrm{}}$. The crystals used in the study of the dependence of $F$-center production on irradiating wavelength and crystal history were grown both by the Kyropoulos method (seed-pulled) and the Bridgman method (crucible-grown). For seed-pulled KI crystals the $F$-center growth showed a consistent behavior for irradiation throughout the exciton band. The growth was found to be describable as a volume process for which the $F$-center density as a function of the number of photons absorbed per unit volume is given by a saturating curve whose shape and initial slope (quantum efficiency) are approximately independent of irradiating wavelength but whose saturation level increases with decreasing wavelength. The $F$-center saturation density was found to increase from 5×${10}^{15}$ ${\mathrm{cm}}^{-3\mathrm{}}$ for irradiation in the tail of the band to about 5×${10}^{17}$ ${\mathrm{cm}}^{-3\mathrm{}}$ at the peak of the band, with the initial quantum efficiency remaining between 0.1 to 0.2 for this wavelength range. While the behavior for seed-pulled KI samples was relatively unaffected by either plastic deformation or previous irradiation in the exciton band, the crucible-grown samples showed large changes due to either of these treatments. Before these treatments the $F$-center density induced in the crucible-grown samples had predominantly a square-root dependence on the number of absorbed photons; afterwards the behavior was very much like that of the seed-pulled samples. The KBr crystals were found to behave like the seed-pulled KI samples. The results are discussed in terms of the properties of the exciton and its interaction with negative-ion vacancies to form $F$ centers.