Fluorescence due to magnetic-dipole transitions in LiYF(4):Pr(3+).
The authors have previously performed a systematic spectroscopic investigation of Pr in LiYF4 (or YLF), and level assignments were made on the basis of electric-dipole transitions that were in agreement with theoretical predic­ tions. This Letter presents new experimental evidence that demonstrates the occurrence of magnetic-dipole transitions in Pr:YLF. Up to now magnetic-dipole transitions were assumed to be negligible in this crystal. LiYF4 has the scheelite structure with Pr going in sub­ stitutionally for Y. Assuming S4 site symmetry, the Pr ion will have energy levels corresponding to Γ1, Γ2, Γ3, and Γ4 representations. The latter two are degenerate and are des­ ignated as Γ3,4. The classification of experimental energy levels according to these representations depends on the na­ ture of the transitions involved. Although most transitions observed in the rare-earth ions are electric-dipole, mag­ netic-dipole transitions can also occur. The selection rules for electric-dipole and magnetic-dipole transitions for S4 site symmetry are shown in Table I. According to the group and transformational properties of electric-dipole and mag­ netic-dipole operators, identical axial and σ-polarized spectra indicate electric-dipole transitions, while identical axial and π-polarized spectra indicate magnetic-dipole transitions. It has been shown that the Pr ion is in a site of S4 pointgroup symmetry which is nearly D2d (S4 is a subgroup of D2d). Therefore D2d selection rules are also considered in analyzing the data and are listed in Table I. In S4 notation Γ1 levels correspond to Γ1 or Γ2 levels in D2d notation, Γ2 S4 levels correspond to Γ3 or Γ4 D2d levels, and Γ3 or Γ4 S4 levels cor­ respond to Γ5 D2d levels. Polarized D2 fluorescence spectra of 0.2% Pr:YLF were recorded at temperatures between 10 K and 300 K using res­ onant excitation of the D2 level by a cw dye laser. The σ, π, and axial spectra were obtained for the H4,H6,F2, and F3 manifolds. Polarization selection was accomplished by means of a Glan-Thompson polarizer and was better than 1%. Fluorescence measurements were obtained using a SPEX 1407 0.85-m double monochromator that has a maximum resolution better than 0.2 cm. Previous studies of the optical spectra in Pr:YLF showed that the lowest level of the D2 manifold as a Γ2 state. The second lowest level in this manifold is a Γ1 state ap­ proximately 70 cm above the Γ2 level. Fluorescence orig­ inating from the D2 manifold at 10 K comes from the Γ2 level at 16,740 cm (in vacuum). At higher temperatures emission from the 16,810-cm Γ1 level is also observed. The crystalline field splits the F3 manifold into one Γ1, two Γ2, and two Γ3,4 levels. Observed transitions in the D2 F3