Electron spin resonance (ESR) is a spectroscopy used on paramagnetic materials with unpaired electrons. ESR has applications in chemistry, biology, and quantum computing. ESR detects free radicals and can be used to date teeth. ESR is also used in artificial photosynthesis and quantum computing.
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 called Electron Paramagnetic Resonance or EPR. Electron spin resonance has a variety of applications in chemistry and biology and also has uses in fields such as quantum computing.
An electron carries a charge and rotates. It therefore induces a magnetic moment. When 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 line up in the opposite direction of the magnetic field, but this requires more energy and is not the electron’s natural state. This is the scientific basis for electron spin resonance.
With ESR, a substance with molecules that have extra, or unpaired, electrons is placed in 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 realigning their magnetic moments so that they are opposite the externally applied magnetic field. The frequency of energy absorbed by electrons indicates the chemical structure of the molecule to which they are attached. In this way, electron spin resonance can be used to determine the chemical composition of different materials.
It is essential that the substance has unpaired electrons. This is because the paired electrons, according to 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. Otherwise, all electrons will be excited and further absorption will not be possible. In this case there will be nothing to measure and, consequently, no signal will be produced. Spin-lattice relaxation, in which an electron energizes its surroundings, and spin-spin relaxation, in which one electron energizes another electron, are the two methods by which relaxation can occur.
ESR is particularly suited to the detection of free radicals, which are a collection of highly reactive molecules with unpaired electrons. Free radicals are known to cause various diseases, poisoning and even cancer. They also cause tooth enamel to decay at a known rate, meaning that 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 tuning the ESR to work on a single electron rather than a group of electrons, a logic gate can be created that matches the energy states of the electron’s magnetic moment.
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