Effects of theNa+impurity on the relaxation of an exciton in KBr at low temperatures

Studies on defect formation in ${\mathrm{Na}}^{+}$ -doped KBr at low temperatures, where primary interstitials are stable, have been performed by means of optical-absorption and luminescence measurements. It is found that the ${\mathrm{Na}}^{+}$ impurity suppresses the colorability, enhances the 2.90-eV emission band, but does not affect the intensity of intrinsic luminescence. The origin of the 2.90-eV emission band is identified to be radiative decay of the ${(V_{\mathrm{KA}}+e^{-})}^{*}$. The suppression of the defect formation is ascribed to the trapping of the exciton state which is responsible for the creation of the pair of $F$ and $H$ centers by the ${\mathrm{Na}}^{+}$ impurity: The trapping gives rise to the formation of the ${(V_{\mathrm{KA}}+e^{-})}^{*}$. A model for the relaxation of an exciton is discussed based on these results.