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The upper spectrum below is the simulated absorption for a system of free electrons in a varying magnetic field.

The lower spectrum is the first derivative of the absorption spectrum.

A collection of paramagnetic centers, such as free radicals, is exposed to microwaves at a fixed frequency.

By increasing an external magnetic field, the gap between the energy states is widened until it matches the energy of the microwaves, as represented by the double arrow in the diagram above.

The basic concepts of EPR are analogous to those of nuclear magnetic resonance (NMR), but it is electron spins that are excited instead of the spins of atomic nuclei.

EPR spectroscopy is particularly useful for studying metal complexes or organic radicals.

EPR was first observed in Kazan State University by Soviet physicist Yevgeny Zavoisky in 1944,.

Experimentally, this equation permits a large combination of frequency and magnetic field values, but the great majority of EPR measurements are made with microwaves in the 9000–10000 MHz (9–10 GHz) region, with fields corresponding to about 3500 G (0.35 T).

Electron paramagnetic resonance (EPR) or electron spin resonance (ESR) spectroscopy is a method for studying materials with unpaired electrons.At this point the unpaired electrons can move between their two spin states.Since there typically are more electrons in the lower state, due to the Maxwell–Boltzmann distribution (see below), there is a net absorption of energy, and it is this absorption that is monitored and converted into a spectrum.As the difference between the two intensities is detected the first derivative of the absorption is detected.As previously mentioned an EPR spectrum is usually directly measured as the first derivative of the absorption. A small additional oscillating magnetic field is applied to the external magnetic field at a typical frequency of 100 k Hz.

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