The effect of spin relaxation on ENDOR spectra recorded at high magnetic fields and low temperatures

A simple theoretical model that describes the pulsed Davies electron-nuclea r double resonance (ENDOR) experiment for an electron spin S = 1/2 coupled to a nuclear spin I = 1/2 was developed to account for unusual W-band (95 G Hz) ENDOR effects observed at low temperatures. This model takes into accou nt the thermal polarization along with all internal relaxation processes in a four-level system represented by the electron- and nuclear-spin relaxati on times T-1e and T-1n, respectively, and the cross-relaxation time, T-1x. It is shown that under conditions of sufficiently high thermal spin polariz ation, nuclei can exhibit asymmetric ENDOR spectra in two cases: the first when t(mix) much greater than T-1e and T-1n, T-1x much greater than T-1e, w here ENDOR signals from the a manifold are negative and those of the beta m anifold positive, and the second when the cross- and/or nuclear-relaxation times are longer than the repetition time (t(mix) much less than T-1e much less than t(R) and T-1n, T-1x > t(R)). In that case the polarization of the ENDOR signals becomes opposite to the previous case, the lines in the a ma nifolds are positive, and those of the beta manifold are negative. This cas e is more likely to be encountered experimentally because it does not requi re a very long mixing time and is a consequence of the saturation of the nu clear transitions. Using this model the experimental t(mix) and t(R) depend encies of the W-band H-1 ENDOR amplitudes of [Cu(imidazole)(4)]Cl-2 were re produced and the values of T-1e and T-1x much greater than T-1e were determ ined. The presence of asymmetry in the ENDOR spectrum is useful as it direc tly provides the sign of the hyperfine coupling. The presented model allows the experimentalist to adjust experimental parameters, such as t(mix) and t(R), in order to optimize the desired appearance of the spectrum. (C) 2001 Academic Press.