Symmetry constraints on EPR spin-Hamiltonian parameters

Abstract

Constraints on EPR spin-Hamiltonian parameters for all crystallographic point symmetries possible for a paramagnetic center are presented. Particular attention is paid to ambiguities in parameter sets, and equivalent sets are tabulated for each symmetry for various conventional choices of coordinate frames. In symmetry classes where two or more conventions for coordinate frames are in common use, conversions between the different parameter sets are tabulated. The tables are particularly useful in the analysis of EPR spectra of high-spin ions and/or low-symmetry sites, since these cases provide the greatest number of possible ambiguities in spin-Hamiltonian parameters. A procedure which uses the tabulated results is described for finding the symmetry of a spin Hamiltonian whose parameters are determined with respect to an arbitrary coordinate frame, and with no initial assumptions about symmetry. The spin-Hamiltonian symmetry provides an upper limit to the paramagnetic site symmetry.