VUV spectroscopy of complex fluoride systems Na0.4(Y1−xREx)0.6F2.2 (RE3+ = Nd3+, Tm3+)

• Intense VUV 5d-4f luminescence is revealed from Nd3+ and Tm3+ doped in Na0.4Y0.6F2.2.
• Nd3+ 5d-4f luminescence shows concentration quenching even for small doping levels.
• Spin-allowed 5d-4f luminescence of Tm3+ possesses very weak concentration quenching.
• Spin-forbidden 5d-4f luminescence of Tm3+dominates in VUV emission spectrum.
• Crystals Na0.4(Y,RE)0.6F2.2 (RE = Nd3+, Tm3+) are potential active media for VUV lasers.

Emission and excitation spectra as well as luminescence decay kinetics of complex non-stoichiometric fluoride crystals Na0.4(Y1−xNdx)0.6F2.2 (x = 0.005, 0.05, 0.2, 1) and Na0.4(Y1−xTmx)0.6F2.2 (x = 0.0005, 0.01, 0.05, 0.1) have been studied in the VUV spectral range at liquid-helium (T ∼ 10 K) temperatures. It has been shown that these crystals show intense broad-band VUV luminescence due to the interconfiguration 5d-4f transitions in Nd3+ and Tm3+ ions. Remarkable concentration quenching is observed for Nd3+ 5d-4f luminescence whereas fast (spin-allowed) 5d-4f luminescence of Tm3+ shows no concentration quenching for the studied doping level up to 10%. The spin-allowed 5d-4f luminescence of Tm3+ in these crystals was found to be rather weak compared to spin-forbidden 5d-4f luminescence because of efficient nonradiative relaxation from higher-energy 5d states of Tm3+ to the lowest-energy 5d level responsible for spin-forbidden 5d-4f luminescence. The studied fluoride systems can be considered as promising active media for the development of VUV solid state lasers with optical pumping.