Electron Spin Resonance

When the molecules of a solid exhibit paramagnetism as a result of unpaired electron spins, transitions can be induced between spin states by applying a magnetic field and then supplying electromagnetic energy, usually in the microwave range of frequencies. The resulting absorption spectra are described as electron spin resonance (ESR) or electron paramagnetic resonance (EPR). Electron spin resonance has been used as an investigative tool for the study of radicals formed in solid materials, since the radicals typically produce an unpaired spin on the molecule from which an electron is removed. Particularly fruitful has been the study of the ESR spectra of radicals produced as radiation damage from ionizing radiation. Study of the radicals produced by such radiation gives information about the locations and mechanisms of radiation damage.

The interaction of an external magnetic field with an electron spin depends upon the magnetic moment associated with the spin, and the nature of an isolated electron spin is such that two and only two orientations are possible. The application of the magnetic field then provides a magnetic potential energy which splits the spin states by an amount proportional to the magnetic field (Zeeman effect), and then radio frequency radiation of the appropriate frequency can cause a transition from one spin state to the other. The energy associated with the transition is expressed in terms of the applied magnetic field B, the electron spin g-factor g, and the constant m_B which is called the Bohr magneton.

If the radio frequency excitation was supplied by a klystron at 20 GHz, the magnetic field required for resonance would be 0.71 Tesla, a sizable magnetic field typically supplied by a large laboratory magnet.

If you were always dealing with systems with a single spin like this example, then ESR would always consist of just one line, and would have little value as an investigative tool, but several factors influence the effective value of g in different settings. Much of the information obtainable from ESR comes from the splittings caused by interactions with nuclear spins in the vicinity of the unpaired spin, splittings called nuclear hyperfine structure.

Nuclear Hyperfine Structure in ESR Spectra