Modulation effects in the electron spin echo of biradical systems

The results of a theoretical and experimental investigation of the modulation effects in the electron spin echo of biradical systems are presented. SO₄^–radicals stabilized as pairs in UV-irradiated potassium persulfate and frozen solutions of the stable N,N-[bis-(, -diphenylpicrylhydrazyl)--picryl-3]-piperazine biradical (A) were chosen as models. The modulation effects in the ESE of SO₄^–radicals in potassium persulfate are shown to be caused by a dipole-dipole interaction between the radicals. The results of the study of the ESE of the A-biradicals leads to the assumption that the observed modulation effects are caused by an exchange interaction between unpaired electrons.The authors are grateful to V. E. Khmelinskii, M. D. Shchirov, A. M. Raitsimring, and R. I. Zusman for their help in carrying out the experiments. The authors are especially grateful to R. O. Matevosyan for kindly supplying biradical A.