Multiple spin echoes in multicomponent liquids

Abstract

Multiple spin echoes (MSE) can occur in pulsed NMR experiments carried out on liquid samples in high magnetic fields. The extra echoes are generated by the dipolar demagnetizing field, which introduces nonlinear terms into the Bloch equation. Previously it was found that the theory which was developed to describe similar MSE in solid 3He is in good agreement with data obtained for samples of water in a field of 11.7 T. In this paper the behavior of the MSE for samples containing different spin components each with their own chemical shift is described. In this situation, interference between the magnetization of the different components leads to “beating” in the amplitudes of the MSE. The theory is extended to obtain equations which describe the evolution of the separate components. When the degree of nonlinearity is small, these equations can be solved analytically; the resulting solutions give a good description of experimental data obtained in a field of 11.7 T from samples of methyl acetate and from a mixture of acetone and water. As the demagnetizing field increases in size, the nonlinearity becomes more severe and it becomes necessary to solve the equations numerically. Numerical solutions to simulate the behavior which can be expected in fields considerably greater than 11.7 T have been obtained.