Interstitial Stabilization and Color-Center Production in KBr

This paper deals with the color-center production in KBr containing alkali impurities (Li, Na, and Rb). It turns out that alkali impurities enhance the color-center production at 80°K only if their size is smaller than that of the substituted ion. Typically, in sodium- or lithiumdoped samples (1% in the melt) the average $\alpha$- and $F$-center production efficiencies are increased by more than an order of magnitude and by a factor of 2-3, respectively. In KBr:Na, the enhancement of the $\alpha$- and $F$-center production is related to the growth of the well-known $V_1$ and $I$ (interstitial negative ion) bands (not observed at 80°K in sufficiently pure samples). Similarly, in KBr:Li, the increase of the $\alpha$- and F-center production is related to the growth of an $L$-type band (peaking at 223 mμ) and of two $V_1$-type bands (peaking at 390 and 438 mμ). The analysis of the growth- and thermal-decay properties of the $I$- and $V_1$-type bands, with respect to those of the $\alpha$ and $F$ bands, leads to the conclusion that the enhancement of the $\alpha$- and $F$-center production is due to the trapping and the stabilization of interstitial halogens by sodium or lithium impurities. Finally, the fact that lithium-trapped interstitials are more stable than those trapped by sodium, along with the fact that rubidium impurities (1% in the melt) do not affect the color-center production, supports an interstitial-stabilization mechanism based on the reduction of the repulsive forces acting on the interstitials when they are trapped near suitable impurities.