Electron Paramagnetic Resonance Spectroscopy

AbstractElectron paramagnetic resonance (EPR) spectroscopy, also called electron spin resonance (ESR) or electron magnetic resonance (EMR), measures the absorption of electromagnetic energy by a paramagnetic center with one or more unpaired electrons. In the presence of a magnetic field, the degeneracy of the electron spin energy levels is removed and transitions between the energy levels can be caused to occur by supplying energy. When the energy of the microwave photons equals the separation between the energy levels of the unpaired electrons, there is absorption of energy by the sample and the system is said to be at “resonance”. Typically the experiment is performed with magnetic fields such that the energies are in the microwave region. Hyperfine splitting of the signal occurs due to interaction with nuclear spins and can therefore be used to identify the number and types of nuclear spins in proximity to the paramagnetic center.The information content of EPR arises from the ability to detect a signal and from characteristics of the signal, including integrated intensity, hyperfine splitting by nuclear spins,gvalue, line shape, and electron spin relaxation time. The characteristics of the signal may depend on environmental factors, including temperature, pressure, solvent, and other chemical species. Various types of EPR experiments can optimize information concerning these observables. Some commonly asked materials science questions that one might seek to answer based on these observables and some corresponding experimental design considerations are listed. Some general information and practical considerations are given in this article.Whenever unpaired electrons are involved, EPR is potentially the best physical technique for studying the system. Historically, the majority of applications of EPR have been to the study of organic free radicals and transition metal complexes. Today, these applications continue, but in the context of biological systems, where the organic radicals are naturally occurring radicals, spin labels, and spin‐trapped radicals, and the transition metals are in metalloproteins. Applications to the study of materials are extensive, but deserve more attention than they have had in the past. Recent studies include the use of EPR to monitor the age of archeological artifacts, characterize semiconductors and superconductors, measure the spatial distribution of radicals in processed polymers, monitor photochemical degradation of paints, and characterize the molecular structure of glasses. In this article enough information is provided so that a potential user can determine whether EPR is likely to be informative for a particular type of sample, and which type of EPR experiment would most likely be useful.