If we consider a free electron and substitute the proper value of constants in the equation: g = 2.00, μ0 = 0.927X1020 erg/gauss & h = 6.625 X l0-27 erg sec, we get
ν0/H0 = 2.8MHz/gauss
That is ESR can be observed at radio frequencies in a magnetic field of a few gauss or in the microwave region in a magnetic field of a few kilogauss. The latter alternate has many advantages: improved signal-to-noise ratio, high resolution etc. and is always preferred for accurate work, though it is very sophisticated and expensive. However, if the basic understanding of the subject is the main criteria as is usually the requirement of class room experiments, the observation of ESR in low magnetic field and in a radio frequency region makes it a lot simple, inexpensive and within the reach of every post-graduate laboratory.
Description of the ESR Spectrometer
A block diagram of the ESR Spectrometer is given below in Fig. 1, and a brief description follows.
The first stage of the ESR circuit consists of a critically adjusted (marginal) radio frequency oscillator with 4-digit frequency display. This type of oscillator is required here, so that the slightest increase in its load decreases the amplitude of oscillation to an appreciable extent. The sample is kept inside the tank coil of the oscillator, which in turn, is placed in the 50Hz magnetic field, generated by the Helmholtz coils. At resonance, i.e. when the frequency of oscillation equal to the Larmour’s frequency of the sample, the oscillator amplitude registeres a dip due to the absorption of power by the sample. This obviously, occurs periodically four times in each complete cycle of the supply voltage. The result is an amplitude modulated carrier which is then detected using a FET demodulator and amplified by an op-amp circuit.