Paramagnetic Resonance Spectrum of Ammonium Chromium Alum

A detailed solution of the fine-structure splitting of the ground state of the chromium ion is given. This solution assumes an electric field of cubic symmetry with a small trigonal distortion and the residual spin-orbit coupling superposed. The effect of a magnetic field at an arbitrary orientation is calculated.Absorption positions and relative intensities are calculated and checked with experiment. Lines of character predominantly ΔSz=2, 3 are shown to have sufficient ΔSz=1 character to give the experimentally determined intensities. So-called "forbidden transitions" are accounted for quantitatively by this detailed calculation. Shapes of absorption peaks are Lorentzian for dilute specimens and Gaussian for full-strength alum. A small variation of trigonal Stark field from one paramagnetic ion to another is sufficient to account for the observed line width.