High and low spin energy states of the Tb 3+ 4f 75d configuration in BaF 2

Abstract

In this paper we present results of detailed spectroscopic studies of Tb 3+ luminescence from BaF 2:0.075 mol% Tb performed at Superlumi station of Hasylab, DESY, Hamburg and at Institute of Physics, N. Copernicus University, Torun, Poland (IF UMK Torun). We have measured UV/VUV excitation spectra of the dominant blue Tb 3+ luminescence and emission spectra under excitation into various bands found from the excitation spectra. The excitation spectra are dominated by the two well known broad bands due to the parity and spin-allowed (SA) 4f 8 → 4f 75d transitions. The higher energy triple t-band and unresolved single e-band at lower energies are widely separated (10 Dq about 16,500 cm −1) reflecting the single d-electron energy in the high symmetry cubic crystal field with a relatively low contribution from the low symmetry component. In addition to these bands we have also detected a number of weaker bands at lower energy sides of both dominant SA bands. While the structured band with components at 252.8, 252.3, 250.4 and 248.4 nm clearly corresponds to the SF (spin-forbidden) band found earlier in LiYF 4:Tb, the bands at 183, 178 and 172 nm have been never, to the best of our knowledge, reported before. We present arguments that these new bands are the SF counterparts of the triple SA t-band at about 158 nm. The d–f exchange energies for d(e) and d(t) electrons are different at 7779 and 8245 cm −1, respectively. Although the energy levels corresponding to the identified high and low spin (HS and LS) states of the 4f 75d configuration are, in BaF 2, well separated, there are no SA nor SF transitions generating d–f emission in BaF 2:Tb. This is because the numerous 4f 8 energy levels intercept the excitation energy from 4f 75d levels leading to the well known blue and green Tb 3+ 4f 8 emissions.