Microscopy of ion-beam generated fluorescent color-center patterns in LiF

Color centers have been generated on [100] surfaces of LiF crystals by irradiation with a focussed beam of 35 keV Ga+ ions at room temperature and at 240 K. The luminescence of two centers at 655 and 540 nm, ascribed to F2 and F3+ centers, respectively, has been detected and spatially resolved, with spatial resolution in the range of 1 µm, by imaging microscopy, as well as by laser scanning confocal microscopy. Diffusion and pinning of color centers have been observed directly in this way. Due to the high intensity in the focal spot of the confocal microscope nonlinear photophysical and photochemical processes are observed. For example an additional emission at 775 nm could be detected at high excitation density. The luminescence band is tentatively ascribed to radiative triplet-triplet emissions of F2 centers after recombination of F2+ centers with an electron.