Divalent rare earth spectra selection rules and spectroscopy of SrCl2: Sm2+

Abstract

Selection rules for vibronic transitions appearing in 4f→4f fluorescence of divalent rare earth (RE2+) ions in alkaline-earth halide lattices are derived. These are applied to the cases of SrF2: Sm2+, SrCl2: Sm2+, and CaF2: Tm2+. On the basis of a simple theory which is outlined, selection rules for nonradiative (relaxation) transitions of RE2+ ions in m3m symmetry are also presented. From these rules one can predict instances where spectral broadening of fluorescent lines should occur due to spontaneous emission of phonons, with consequent lifetime broadening. Comparison with experiment seems to verify these predictions. The structure of the 4fn-1 5d bands of RE2+ ions is shown to be due to vibronic coupling of pure electronic states with states involving multiquantum excitation of even-parity vibrations. Simple coupling schemes for the 4fn-1 5d levels are examined. Results of measurements of SrCl2: Sm2+ absorption and fluorescence spectra are presented. Lifetimes and quantum efficiencies are discussed. © 1963 The American Physical Society.