Mixing Time ENDOR

JEFFREY HARMER, GEORGE MITRIKAS, and ARTHUR SCHWEIGER 

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Figure 22. Pulse sequence for the variable mixing-time ENDOR experiments based

ENDOR and FT ESEEM spectra differ mainly in the intensities of the lines, which in ESEEM are given by a factor related to the ESR transition probabilities. A necessary prerequisite for modulations in the time domain spectrum is that the allowed Ami = 0 and forbidden Ami = 1 hyperfine lines have appreciable intensities in ESR. The zero ESEEM amplitude thus predicted with the field along the principal axes of the hyperfine coupling tensor is of relevance for the analysis of powder spectra. Analytical expressions describing the modulations have been obtained for nuclear spins I = V2 and / = 1 [54, 57] by quantum mechanical treatments that take into account the mixing of nuclear states under those conditions. Formulae are reproduced in Appendix A3.4. 

Mims ENDOR is based on the stimulated echo sequence with three nonselec-tive m.w. n/2 pulses (Fig. 14b). The preparation part, nil - r - nil, creates a r-dependent grated polarization pattern. During the mixing period, the polarization is changed by a selective r.f. pulse if it is on-resonance with a nuclear frequency. The electron polarization is then detected via a stimulated echo created at time r after the last nil m.w. pulse. The ENDOR efficiency is given by [64]... 

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