F-center formation in KI crystals under high-density optical excitation

Abstract

The $F$-center formation in KI crystals under high-density optical excitation has been investigated at low temperature. The irradiation of the ArF excimer laser with the power level above $\ensuremath{\sim}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{mJ}∕{\mathrm{cm}}^{2}$ creates the $F$-absorption band in KI crystals at $14\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, at which the $F$-center formation does not occur under the ordinary excitation. The $F$-center density under the high-density excitation has been analyzed by using the rate equations for the $F$-center formation. This analysis reveals that the generation rate of the $F$ center depends on the second power of laser power. This fact verifies that the $F$-center formation occurs through a two-photon process, which suggests the interaction between two precursor centers leading to paired self-trapped excitons (STEs). On the other hand, the efficiency of the STE luminescence decreases and the additional decay components in the STE luminescence are newly observed under the same power level. As a result, it is deduced that both the reduction of luminescence efficiency and the appearance of the new decay components relate to the $F$-center formation through the two-photon process. The microscopic mechanisms for these phenomena have been discussed by using a simple rate-equation model which involves the paired precursor state and the paired STE state generated by the two-photon process. The present model has succeeded to explain qualitatively that the reduction of the luminescence efficiency and the appearance of the additional decay components relate closely to the $F$-center formation.