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).
If the population of radicals is in thermodynamic equilibrium, its statistical distribution is described by the Maxwell–Boltzmann equation: ≈ 0.998, meaning that the upper energy level has a slightly smaller population than the lower one.
Therefore, transitions from the lower to the higher level are more probable than the reverse, which is why there is a net absorption of energy.
Electron paramagnetic resonance (EPR) or electron spin resonance (ESR) spectroscopy is a method for studying materials with unpaired electrons.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.By using phase sensitive detection only signals with the same modulation (100 k Hz) are detected. Note field modulation is unique to continuous wave EPR measurements and spectra resulting from pulsed experiments are presented as absorption profiles.In practice, EPR samples consist of collections of many paramagnetic species, and not single isolated paramagnetic centers.Clock of elapsed time - the place of ESR dating; introduction - what is ESR?; ESR dating and dosimetry - principles and procedures; assessment of radiation dose rate; carbonate (Ca CO3) - cave deposits; aragonite (Ca CO3) - shells and corals in marine geology; sulphates and other carbonates - evaporates; phosphates - bioapatite for anthropology; Si O2 - faults and sediments; Si O2 and silicates - geothermal and volcanic materials; space and environmental sciences; chemical ESR dating - valency changes; ESR dosimetry - accident and monitoring; ESR imaging (microscopy); a portable ESR - automatic sample changer.The larger the difference the larger the intensity detected by the detector tuned to 100 k Hz (note this can be negative or even 0).As the difference between the two intensities is detected the first derivative of the absorption is detected.Because the mechanisms of spin–orbit coupling are well understood, the magnitude of the change gives information about the nature of the atomic or molecular orbital containing the unpaired electron.In general, the g factor is not a number but a second-rank tensor represented by 9 numbers arranged in a 3×3 matrix.