Mössbauer relaxation spectra of an Ising pseudo-doublet: Influence of the off-diagonal hyperfine coupling

The influence of direct relaxation between the states of an Ising pseudo-doublet on the paramagnetic Mössbauer spectra is examined. An analytic expression for the line shape is obtained, and this is used to show the importance of accounting for the off-diagonal hyperfine coupling, even when the crystal field splitting of the electronic states is very much greater than the hyperfine interaction. The fast relaxation limit is found to be independent of the crystal field splitting, and shows the full magnetic hyperfine structure associated with an unsplit Ising doublet. The influence of a small applied magnetic field is examined and the conditions under which the off-diagonal hyperfine coupling may be ignored are delineated.