Optical Properties and Mechanism of Formation of SomeF-Aggregate Centers in LiF

Abstract

A study of positively charged, neutral, and negatively charged centers in LiF has been made. Effects observed include the induced transformations of ionized centers to $F$-aggregate centers, and reformation of the ionized centers in the dark, when samples reirradiated at low temperatures are warmed up. Polarization properties of the luminescence from various centers have also been studied. From the temperature dependence of the emission bands and the associated excitation bands for the $M^{+}$ and $R^{+}$ centers, vibrational frequencies were determined and were found to be close to a theoretical normal-lattice phonon frequency. From detailed studies of transformations among the various centers observed under varying conditions, it was possible to propose a mechanism of formation of the $M$ and $R$ centers and other complex centers. $M$- and $R$-center formation seems to occur through diffusion of $\alpha$ and $M^{+}$ centers, respectively, towards $F$ centers ($\alpha +F\to M^{+}$ and $M^{+}+F\to R^{+}$) followed by capture of a single electron. Experiments indicated that $M^{\prime }$ and $R^{\prime }$ centers are produced from the corresponding $M^{+}$ and $R^{+}$ centers apparently by a process involving two-electron capture.