The origin of optical absorption at 414 nm in the F2 region of ion-implanted LiF crystals

We report a comparative study of optical absorption in LiF crystals following implantation with Al+, Mg+ and Ar+ ions of 100 keV energy. These crystals have been examined by slow positron annihilation spectroscopy, the results we report separately. Here we focus on a band at 414 nm which is significantly enhanced in aluminium implanted samples. The dependence of the 414 nm band on ion species (Al+, Mg+, Ar+) is investigated at fluences of 10(16) cm(-2). Annealing studies are used to further clarify the nature of the band. These observations raise questions about the origin of the band at 414 nm. We suggest it may be linked to conditions associated with ion tracks in the crystal.