Strong-Radio-Frequency-Field Effects in Nuclear Magnetic Resonance and Electron Paramagnetic Resonance

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Abstract
A change in the rotating-coordinate transformations that are used in making calculations of magnetic-resonance phenomena with large rf fields permits a definition for the quantization axis of the spin system which is continuous in both the driving rf field strength and the departure of the rf frequency from resonance. This modified procedure is used to compute the line narrowing, which is expected in spin-stirring experiments with inhomogeneously broadened systems, as a function of stirring rf field strength. The increase in transverse relaxation time in correlation-time-narrowed inhomogeneously broadened spin systems is calculated. The behavior of the magnetization in the rotating frame in adiabatic fast-passage experiments is predicted. The increase in transverse relaxation time in multiple-pulse experiments in homogeneously broadened systems is calculated. These predictions agree with observations or with exact calculations made with trace-sum methods. It is shown that previous experiments which were predictable and in accord with the usual procedure are also in accord with the modified procedure proposed here.