ENDOR amplitudes of triplet state molecules: I. Electric-circuit analogy treatment

ENDOR spectra of triplet state molecules have a chacteristic line from degenerate NMR transitions within the zero level (ZL) electron spin manifoldM _S=0. The ZL line, observed at the free nuclear Larmor frequency, dominates spectra when the number of nuclei is large. This line was found to be substantially reduced in intensity at low temperature. The strong variation of the ZL line intensity is analyzed within the frame of an electric-circuit analogy modeling. The result is as follows: At low temperature the electron and nuclear spin-lattice relaxation rates become small, and the nuclear-nuclear spin flip-flop transitions between degenerate substates,M _I=const within the ZL manifold, become relatively strong to compete for population redistribution. This reduces the population differences between nuclear sublevels. Additional NMR irradiation can thus do very little to reduce these differences even more, and the ENDOR effect becomes suppressed. A certain enhancement of the relaxationally suppressed ZL line occurring at increased EPR saturation is explained by the coherent action of the microwave field on the selected substate within the ZL manifold that shifts its energy out of the other degenerate states thereby closing the flip-flop relaxation channel.