Electron Paramagnetic Resonance and Electron Nuclear Double Resonance Spectroscopy

An unpaired electron can be aligned by an applied magnetic field so that its spin is precessing about the field. The direction of spin precession can be either clock- or anticlockwise corresponding to the two energy states of the electron (Fig. 1). Irradiation with an electromagnetic beam of the correct frequency can induce transitions of the unpaired electron from one spin direction to the other. The transition is induced by the oscillating magnetic field component of the electromagnetic radiation. Thus the transition is a magnetic dipole process and consequently the selection rule for the transition is that the spin component along the direction of the applied field, given by M_s, must change by ± 1 unit. The energy equation for resonancee is the well-known expression ${\rm h}v_0=g\mu_{\rm B}{\rm B}_0$ where g is the splitting factor, μ_B the Bohr magneton and B₀ the applied field. For a magnetic induction field of 0.33 T and a g-factor of 2.0 the microwave energy, hv₀, is 0.3 cm−1. A resonance spectrum is obtained by placing the sample in a microwave cavity tuned to the frequency of the microwave radiation. An external magnetic field is applied and swept until the resonance condition is found.