Non-Resonant Measurement Techniques

Abstract

The first measurements of spin-lattice and spin-spin relaxation times were carried out by using non-resonant techniques. An excellent account of this early work is given by Gorter in his book¹ and a later summary is given by Cooke.² In much of this work the measurements consist either of dispersion or of absorption measurements at audio frequencies and the relaxation time is deduced from an analysis of the data in a manner which will be described below. There is one obvious and important difference between such measurements and those referred to elsewhere in this book. With the resonant techniques, a single pair of energy levels is studied and the influence of neighbouring and distant energy levels is apparent through the disturbance of the expected relaxation between the two levels under study. In the non-resonant method, magnetic susceptibility data, for example, are taken which clearly in some way represent an average behaviour over all populated energy levels. Any relaxation parameter thus determined must itself be an average one and it is not surprising that in certain cases the observed behaviour cannot be truly explained in terms of a single relaxation time. There is, moreover, a second difficulty.