Stationary and time-resolved dynamic nuclear polarization in weak magnetic fields in the presence of degenerate electron exchange

The stationary and time-resolved dynamic nuclear Polarization (DNP) of diamagnetic reaction products in weak magnetic fields in the presence of degenerate electron exchange (DEE) has been investigated both experimentally and theoretically. The theoretical description of this effect, based on the solution of the Liouville equation for a spin density matrix of radical ions, has been suggested and compared with the description based on the solution of phenomenological Bloch equations which had been proposed previously. An analytical expression for the dependence of DNP on the delay time between the laser flash and the radiofrequency field pulse has been obtained within the Bloch model. The kinetics of DNP in the photolysis of perdeuteronaphthalene in the presence of fumaronitrile (trans-1,2-dicyanoethylene) at different rates of DEE has been investigated. The analysis and comparison of the theoretical calculations and experimental data have shown that the decay of the kinetics is determined by the time of electron spin relaxation caused by DEE and the lifetime of radical ions. It has been shown that time-resolved DNP can be applied to measure DEE rates and to investigate chemically induced dynamic electron polarization in weak magnetic fields.