Pulsed endor experiments

Abstract

A new technique for performing electron nuclear double resonance experiments, based on the observation of electron spin echoes, has been developed. Radio-frequency resonance transitions are induced during the time interval between pulses II and III of a stimulated echo sequence, and cause a fall in the echo signal amplitude. Echo signals are integrated by means of a boxcar circuit and made to operate a pen recorder which plots the endor spectrum as the radio frequency is varied. The method does not depend on achieving any particular balance between relaxation rates, and has been applied in studying the Ce-W system in (Ca, Ce) WO 4 down to very low radio frequencies. Three microwave pulses are required in order to generate a stimulated electron echo (figure 17 ( a )). According to the classical description, in terms of a co-ordinate system rotating at the microwave frequency, the first pulse, which is a 90° pulse, turns the spins into the equatorial plane where they precess freely for a time ז , some advancing and some lagging the microwave field in phase. The second pulse, like the first a 90° pulse, has an effect which varies according to the phase divergence of the spins. Those which precess exactly at the microwave frequency, or which have diverged by a multiple of 2 π , are turned along the polar axis in a direction opposite to that of their original alinement; the two 90° pulses are, for these spins, equivalent to a single 180° pulse. Spins which have diverged by an odd multiple of π are turned back into their original alinement as if no pulses had been applied. Other spins are turned by varying amounts, the overall effects being to leave a sinusoidal spectral pattern of periodicity 2 π/ז in place of the original smooth line shape (figure 17 ( b )). This pattern will persist for a relatively long time T , limited by lattice relaxation, or by spectral diffusion, but not by the phase memory time observed in normal spin echoes. The third microwave pulse of the stimulated echo sequence, also a 90° pulse, turns the M z pattern into the equatorial plane where a phase convergence takes place, leading to the emission of a stimulated echo signal a time ז later. If this simple picture is to be applicable, the microwave field strength H 1 should be several times greater than the resonance line width. Such large fields, and the corresponding short pulse times, are not always convenient in electron echo experiments, nor are they usually necessary. Echoes can be obtained with lower fields, although the behaviour of the spins becomes more complicated (Bloom 1955). In practice one can visualize the situation sufficiently well by imagining a spectrum of spins ~ 2 H 1 wide to be selected and to constitute the effective resonance line.