Pulsed ESR in Solids

The 2-pulse or Hahn-echo experiment, originally developed in NMR is extensively used in pulsed ESR in solids for measurements of magnetic relaxation and of anisotropic hfc. 

The experiment can be qualitatively understood by a vector model for the reorientation of the electron spins by the microwave pulses. The first pulse rotates the spins by 90 to the y-axis (Fig. 2.18(b)). During time x between the first and second pulse, the different spins precess in the xy-plane at different rates. The second pulse inverts the orientations of the spins causing them to realign after a time 2r and an echo is observed (Fig. 2.18(e)). 

The pioneering work by Mims [7] has been extended and summarized in recent monographs and reviews [6, 9, 38]. 

The methods most widely used by non-specialists are briefly described below. Surveys of advanced methods and applications have been described in several specialist reports [6, 9-11]. 

In the basic two-pulse or primary echo experiment, two pulses separated by a time T are applied. The second pulse is twice as long as the first. At time t after the last pulse a transient response appears from the sample, the so called spin echo. By monitoring the echo amplitude as a function of the time t, a spin echo envelope can be recorded. The hyperfine couplings are obtained either by trial-and-error simulations to reproduce the modulations superimposed on the decaying echo amplitude (the original procedure) or by a Fourier transform to obtain nuclear frequencies in modern instruments as in Fig. 2.20. The frequencies are the same as obtained in ENDOR. Contrary to ENDOR, combination peaks at the sum and difference frequencies may also occur. 

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