ZERO-PHONON LINE BROADENING IN ION-BOMBARDED LIF

LiF crystals have been bombarded with H+ and He+ ions of energy 1.0 Me V at ambient temperature. The concentration of F and F aggregate centr es as a function of ion fluence displays irregular growth behaviour. T he width and lineshape of the F4(1) zero-phonon line (ZPL) has been us ed as a probe to determine the nature of dominant internal strains in the crystal. At relatively low ion fluences, the growth of major colou r centre concentrations F and F2 is well correlated with the increase in width of the ZPL; here the lineshape is largely Lorentzian in accor d with point defects being the dominant cause of internal strain. At h igher 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 i n point defect behaviour. Rutherford backscattering experiments have a lso been carried out in an effort to provide additional information re garding the halogen and lithium disorder. The results are discussed in relation to the current ideas on defect production in alkali halides.