Basic Principles of Paramagnetic Resonance

The science of electron paramagnetic resonance (EPR) spectroscopy is very similar in concept to the more familiar nuclear magnetic resonance (NMR) technique. Both deal with the interaction between electromagnetic radiation and magnetic moments; in the case of EPR, the magnetic moments arise from electrons rather than nuclei. Whether or not the reader has an immediate interest in the multitude of systems to which EPR is applicable, the insights that it provides cannot be ignored. Furthermore, there is hardly another technique from which one can gain a clearer insight into many of the fundamental concepts of quantum mechanics. Much of our knowledge of the structure of molecules has been obtained from the analysis of molecular absorption spectra. Such spectra are obtained by measuring the attenuation versus frequency (or wavelength) of a beam of electromagnetic radiation as it passes through a sample of matter. Lines or bands in a spectrum represent transitions between energy levels of the absorbing species. The frequency of each line or band measures the energy separation of two levels. Given enough data and some guidance from theory, one may construct an energy-level diagram from a spectrum. Comparison of an energy-level diagram and an observed spectrum shows clearly that, of the many transitions that may occur between the various levels, only a relatively few ‘allowed’ transitions are observed. Hence the prediction of transition intensities requires a knowledge of selection rules. Electromagnetic radiation may be regarded classically as coupled electric (E1) and magnetic (B1) fields perpendicular to the direction of propagation (Fig. 1.1).