Theory of the nuclear hyperfine structure of paramagnetic resonance spectra in crystals

A theory is given of the nuclear hyperfine structure of the paramagnetic resonance in hydrated crystals. Starting from the states of a free ion, neglecting spin-dependent terms in the energy, successive steps of perturbation calculation introduce the effect of the crystalline electric field, the spin-orbit and spin-spin interactions, the coupling with an external magnetic field, and the coupling between the nucleus of the ion and the electrons. The self-consistent field approximation, in which the state of the free ion is described by a single electronic configuration, is found to be inadequate, and it is found necessary to consider admixtures, through configurational coupling, of configurations in which there are unpaired s-electrons, as these contribute significantly to the hyperfine splitting. A review of the application of the theory to the ions of the iron transition group is given.