On the mechanism of multiplet CIDEP in radical ion pairs

It is shown that multiplet chemically induced dynamic electron polarization (CIDEP) in radical ion pairs is unlikely to be caused by electron spin exchange (J), unless the rate of electron transfer is slowed down either by exciplex formation or by a low Franck-Condon factor. In radical ion pairs for which the electron transfer is efficient, the electron tunnels prior to the spin exchange, and the multiplet CIDEP is due to dipole-dipole interactions of electron spins. The corresponding polarization factor is of the order of several Boltzmann units. In appearance, such a polarization is similar to the usual exchange-induced multiplet CIDEP at J > 0. It is concluded that the long-range dipole-dipole interaction caused a 'J > 0' multiplet CIDEP in all the pairs that involved a solvated electron as a partner.