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.