THE REVERSE SHIFT OF THE EPR LINE OF PARAMAGNETIC CENTERS COUPLED TO SPECIES WITH A FAST PARAMAGNETIC RELAXATION

The EPR spectrum of the spin 1/2 paramagnetic centers with a relativel y slow relaxation is considered in the case when they are coupled via the Heisenberg exchange interaction to partners which have short times of the longitudinal and transverse paramagnetic relaxation. Under the se conditions only the EPR line of paramagnetic centers with ii relati vely slow relaxation is detectable in experiment. The share of this li ne is analyzed by solving numerically kinetic equations for the spin d ensity matrix for simple model systems. Depending on a ratio between t he exchange integral and the paramagnetic relaxation rates of partner spins, the EPR line shifts in opposite directions. For moderate relaxa tion rates, as the relaxation rates decrease, the EPR line shifts towa rd the gravity center of the total EPR spectrum. In the case of extrem ely fast relaxation, as the relaxation rates decrease, the reverse shi ft of the EPR line is expected, the line shifts away from the gravity center of the total EPR spectrum. This type of the non-monotonous line shift was experimentally observed for the monocrystal of [CuNd2(C4O4) (4)(H2O)(16)]. 2H(2)O when relaxation rates were changed by temperatur e variation.