Electron spin resonance (ESR) is a form of spectroscopy used on paramagnetic materials — materials that become magnetic when exposed to an external magnetic field. ESR is also referred to as electron paramagnetic resonance, or EPR. Electron spin resonance has a variety of applications in chemistry and biology, and even has uses in fields such as quantum computing.
An electron carries a charge and spins. It therefore induces a magnetic moment. If placed in an external magnetic field, the electron’s magnetic moment will align with the direction of the magnetic field. It is also possible for the electron to align in the opposite direction of the magnetic field, but this requires more energy and is not the natural state of the electron. This is the scientific foundation for electron spin resonance.
With ESR, a substance with molecules that have extra, or unpaired, electrons is placed into a magnetic field and energy, usually in the form of microwaves, is applied to it. The unpaired electrons will absorb the electromagnetic energy and move to a higher energy state by re-aligning their magnetic moments to be opposite of the externally applied magnetic field. The frequency of energy absorbed by the electrons indicates the chemical structure of the molecule to which they are attached. This way, electron spin resonance can be used to determine the chemical composition of different materials.
It is critical that the substance have unpaired electrons. This is because paired electrons, per the Pauli Exclusion Principle, will have spins in opposite directions and, therefore, no net magnetic moment. These materials are known as diamagnetic and are not suitable for ESR.
As with other resonance spectroscopy techniques, the electrons used in electron spin resonance must be allowed to relax and return to their lower energy states. If not, all electrons will be excited and no further absorption will be possible. In this case, there will be nothing to measure and, consequentially, no signal will be produced. Spin-lattice relaxation, where an electron gives energy to its surroundings, and spin-spin relaxation, where an electron gives energy to another electron, are the two methods whereby relaxation can occur.
ESR is especially well suited to the detection of free radicals, which are a set of highly reactive molecules with unpaired electrons. Free radicals are known to be the cause of several diseases, poisonings, and even cancers. They also cause the decay of tooth enamel at a known rate, which means electron spin resonance can be used to date teeth and, by extension, humans. Excess free radicals are also present in beer and wine that are past their shelf life.
ESR is also a leading candidate in several cutting-edge technologies. These include artificial photosynthesis and quantum computing. In the latter, by fine-tuning ESR to work on a single electron instead of a group of electrons, a logic gate can be created that corresponds to the energy states of the electron’s magnetic moment.