Units, Constants and Symbols in ESR

One reason for the sensitivity increase at higher frequencies comes from the corresponding increased population difference between the upper and lower states of an ESR transition. In a sample with S = Vi containing No unpaired electron spins the excess population of the lower state is approximately AN No hv/kT, when the temperature is not too low, i.e. when hv/kT 1. The first ESR experiments were accordingly reported soon after microwave equipment became available around 1945. 

In the first experiments by Zavoisky [2] a wavelength of 25 m was used, corresponding to a resonance field of only 0.4 mT. As the line-width was about 5 mT, the observation was difficult. In the second experiment on a Cu salt the frequency was 133 MHz. In 1946 microwaves were employed giving a resonance at about 100 mT. 

Soon after the first experiments, studies on Mn salts were performed by employing 10 cm microwaves, fed into the cavity containing the sample. The resonance was detected by readings on a d.c. instrument connected to the detector. In some following papers around 1948, single crystal studies were first reported. Systematic ESR studies of transition metal ions were initiated in a paper titled Paramagnetic Resonance in Salts of the Iron Group [3]. Many of these ions had 

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