Multifrequency electron spin echo envelope modulation in S=1/2, I=1/2 systems: Analysis of the spectral amplitudes, line shapes, and linewidths

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Abstract
The nuclear modulation effect in S=1/2, I=1/2 systems is analyzed with particular emphasis on the impact of variation of the external field strength. We show that the modulation depth parameter k reaches its maximum possible value when the Zeeman and hyperfine interactions are made equal in magnitude, or ‘‘matched,’’ and that this matching plays the dominant role in determining the ESEEM (electron spin echo envelope modulation) line shapes; the distinctive ESEEM spectral features are readily understood on this basis. Through multifrequency ESEEM, it is possible to interpret the features and to utilize them to measure hyperfine interactions. We discuss several experimental strategies, involving the field dependences of ESEEM amplitudes, line shapes, and linewidths, for determining hyperfine coupling constants.