Paramagnetic species zero field splitting

Paramagnetic species trapped in solid materials usually possess anisotropic g- and hyperfine couplings. Zero-field splittings occur when 5 > V2. The spin Hamiltonian formalism described in Appendix A3.1 is a convenient means to summarise the different interactions. The following spin-Hamiltonian is adequate to illustrate most aspects of the analysis. 

Zero-field splittings occur in ESR spectra of paramagnetic species with electron spin S > 2. The energy for the transition (ms -<-> ms - 1) is AE = gHBB+1 F(ms-l/2) provided that the electron Zeeman term is the dominant one, gfXBB >> F. The ESR spectrum then contains 2 S lines, separated by 3 F. With the field in the xy-plane one has ... 

Spin quantitation is a key part of the EPR signal characterization. EPR spectroscopy is sufficiently sensitive that signals can sometimes be observed from species that constitute only a small fraction of the potentially paramagnetic centers. For an 5=1/2 metal ion the double integral of the first-derivative EPR signal is proportional to the number of spins in the sample. Comparison with a spin standard can then be used to determine the spin concentration for the species of interest. Quantitation is more difficult for metal ions with 5 >1/2 and zero-field splitting greater than the EPR quantum, because only some of the transitions may be observable for a particular microwave frequency. ... 

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