Fast and efficient VUV/UV emissions from (Ba,La)F2:Er crystals

We report observation of fast and efficient VUV/UV luminescence from the mixed (Ba,La)F2:Er crystals. The broad bands, peaking at 162.5, 181.9, 194.2, 202.8, 216.1, 233.5 and 281.5 nm and decaying, at 10 and 293 K, with time constants of 46 and 35 ns respectively, are due to spin-allowed transitions from the low-spin (LS) state of the 4f105d configuration. We also observed a weak and slow broad band emission peaking at 170 nm due to the spin-forbidden transition from the high-spin (HS) state of the 4f105d configuration. While at room temperature the excitation into any of the three identified LS bands (J=8, 7 and 6) dominating the excitation spectrum yields fast VUV and UV emissions, at 10 K the excitation into higher lying J=7 and 6 bands generates slow and sharp line emissions. The positions of these lines fit energies of transitions originating from the 2G7/2 multiplet at 66140 cm-1. The emission from the 2G7/2 multiplet has been never, to the best of our knowledge, observed before. The efficient and fast VUV and UV emissions from the higher (LS, J=8) with almost no contribution from the lower (HS, J=8) level of the 4f105d configuration are possible because the modified crystal field in (Ba,La)F2 shifts the level of the (LS, J=8) state below the 2F5/2 multiplet which, therefore, does not contribute to nonradiative relaxation between the LS and HS levels. We conclude that the 2G7/2 and 2F5/2 levels have major impact on VUV and UV emissions from the Er3+ ion in (Ba,La)F2 contributing to complex emission pattern described in this report Their key role, elucidated by the VUV and UV luminescence spectroscopy, is consistent with predictions from a simple configuration coordinate model based on experimental results and calculations of the 4f11 energy levels.