Zero-phonon line broadening in ion bombarded LiF

Abstract

LiF crystals have been bombarded with H+ and He+ ions of energy 1.0 MeV at ambient temperature. The concentration of F and F aggregate centres as a function of ion fluence displays irregular growth behaviour. The width and lineshape of the F4(1) zero-phonon line (ZPL) has been used as a probe to determine the nature of dominant internal strains in the crystal. At relatively low ion fluences, the growth of major colour centre concentrations F and F2 is well correlated with the increase in width of the ZPL; here the lineshape is largely Lorentzian in accord with point defects being the dominant cause of internal strain. At higher fluences the ZPL shows an increasingly Gaussian shape indicating the possible presence of dislocation structures. At high fluences the linewidth and shape of the ZPL is largely decoupled from variations in point defect behaviour. Rutherford backscattering experiments have also been carried out in an effort to provide additional information regarding the halogen and lithium disorder. The results are discussed in relation to the current ideas on defect production in alkali halides.