PERTURBATION TREATMENT OF A SPIN-HAMILTONIAN INCLUDING THE M=0 ELECTRONIC STATES APPLICATION TO THE HYPERFINE-STRUCTURE OF TRIPLET-STATE EPR

The electron spin resonance spectra of paramagnetic species in crystals can be described by the spin-Hamiltonian: H = βBS·g·h+S·D·S+ ∑ i=1 N {Ij·Aj·S-ηnBIj ·gnj·h+Ij· Pj·Ij}, In the case of large electronic quadrupole interaction, the magnetic field created by the electrons at the nucleus is not equal to zero in the M = 0 electronic state. Thus, in this state, and if the nuclear Zeeman interaction is small, the direction of nuclear quantization cannot be taken along the external magnetic field. Usually, perturbation treatments use this external field as the direction of quantization, causing a failure to describe the EPR spectra correctly. A perturbation treatment is given here in which the nuclear part of the Hamiltonian (hyperfine, nuclear Zeeman, and quadrupole) is considered as a perturbation of the whole electronic one (electronic Zeeman and quadrupole). Two examples (pyrazine copper acetate and a crystalline nitroxide radical) illustrate the good agreement between the observed experimental spectra and the simulated EPR spectra computed by use of the present method. From these examples, the problem of absolute sign determination of fine or hyperfine splitting constants is discussed. © 1979.