Forbidden hyperfine transitions in the electron spin resonance of Mn 2+ in CaO and SrO crystals∗

Abstract

Forbidden hyperfine transitions with selection rules Δ M = ±1, Δ m = ±1 were observed in the electron spin resonance spectrum of Mn 2+ in CaO and SrO single crystals, following similar work by Rubins and Drumheller in MgO. In CaO, the angular variation I(θ) of the forbidden lines, for H 0 in a plane normal to a cubic axis, follows a (sin 4θ) 2 law. This verifies previous results from analysis of the fine structure, which indicated that the Mn 2+ are in cubic sites. In SrO, the corresponding I(θ) seen in our samples follows roughly though not exactly a (sin 2θ) 2 law. The latter would be expected if a tetragonal crystal field term of [100]-type symmetry were present. The plotted I(θ) contains also a smaller constant part, explainable if random-axial (distortions in random directions) terms are present in the crystal field of some of the Mn 2+. Since the usual fine structure pentads are unresolved in our samples of Mn 2+ in SrO, the anisotropic forbidden line intensity provides the only key to the point symmetry of the Mn 2+ sites. We interpret the D term as a measure of the random strain fields in our particular samples. Our forbidden line intensity data yields a distributed D splitting of ~20 × 10 −4 cm −1. This would completely mask the fine structure splitting of ~4 × 10 −4 cm −1 which has been reported by Holroyd and Kolopus for their own SrO crystals.