On a new effect in electron-nuclear double resonance in distant nuclei

Anomalously intense ENDOR signals due to polarization of the lattice nuclei ('Larmor' ENDOR) are observed in LiF crystals with relatively low F-center concentrations (n approximately 2x1017 cm-3) at the Larmor frequency of the lattice nuclei. The dependences of the ENDOR signal intensity on u.h.f. field strength and on the detuning (H - Hsub(0)) (Hsub(0) is the magnetic field strength corresponding to the center of the EPR line) are studied at various temperatures and concentrations of the paramagnetic centers. It is found that with respect to dynamic behaviour the Larmor ENDOR is the opposite of the distant' ENDOR described in the literature in which signals are also observed at the Larmor frequency of the lattice nuclei. 'Larmor' ENDOR is observed in samples with relatively low concentrations of paramagnetic centers and is maximal in the EPR center. The ENDOR mechanism for distant and near nuclei is not the same. It is shown that polarization of lattice nuclei required for observation of 'Larmor' ENDOR is not related to dipole pool effects and may be ascribed to relaxation processes or saturation of 'forbidden' u.h.f. transitions. It is important that Larmor ENDOR is observed in the presence of regions with nonequilibrium nuclear polarization in the sample even if there is no total polarization. It is found that the existence of 'Larmor'ENDOR may be regarded as an experimental proof that the lattice nuclei relax via a paramagnetic impurity