VUV spectroscopy of pure and Tm3+-doped LicaAlF6 crystals

Nominally pure and Tm3+-doped LiCaAlF6 crystals were grown by the Czochralski technique in a reducing atmosphere. The optical properties of transparent single crystals were studied using absorption and time-resolved luminescence spectroscopy in the VUV spectral range (330-100 nm). The strongest VUV emission peaking at 60 800 cm−1 with a decay time of 5.6 μs (7 μs) at 300 K (7.4 K) was assigned to the spin-forbidden 4f115d-4f12 transition of Tm3+. The fine structure observed in the VUV emission and corresponding excitation spectra indicate intermediate strength of electron-phonon coupling in this system. The efficient excitation of f-f emissions above 72 000 cm−1, higher than the onset of f-d absorption at 63 000 cm−1, is mainly caused by the F− to Tm3+ charge transfer absorption. The nature of various host-related excitation processes in the energy transfer to the Tm3+ ions is discussed.