A unified approach to spin polarization (CIDEP) and relaxation by Heisenberg spin exchange. I. Generalized equations for reactive radicals
Abstract
A theory of Heisenberg spin exchange (HSE) has been formulated in this and the following paper to consider the processes of chemically induced dynamic electron polarization (CIDEP) and relaxation. The framework, which includes all the spin states of a radical pair (RP), relies on an exact transient solution of the encounter RP density matrix. To aid in interpreting timeresolved ESR studies of these effects, a nonphenomenological formulation of Blochtype equations is explicitly derived for the ṡRH and ṡRH_{2} radical spin systems as well as for general radical systems in terms of their nline ESR spectra. The longitudinal and transverse relaxation rates and the frequency shifts are calculated for conditions spanning the weak to the strong exchange limit under a wide range of viscosities. The normally slight frequency shifts are shown to be strongly enhanced by CIDEP although not to an extent that is easily observable. Aside from the usual degeneracy effects, a curious HSE relaxation behavior is predicted for ESR lines that exhibit multiplet CIDEP, wherein the observed longitudinal relaxation rate can appear faster than the transverse rate by an amount that depends on the degeneracy of the line vs the total number of transitions of the radical. The present theory is formulated to be descriptive of timeresolved ESR experiments. Implications to reactive radicals are discussed and the solutions of the modified Bloch equations are presented to consider (i) instantaneous, geminate, and diffusive RP CIDEP, (ii) coupled and time dependent relaxation by HSE, and (iii) secondorder chemical decay.
 Publication:

Journal of Chemical Physics
 Pub Date:
 July 1987
 DOI:
 10.1063/1.453335
 Bibcode:
 1987JChPh..87.1022S
 Keywords:

 Free Radicals;
 Polarization (Spin Alignment);
 Spin Exchange;
 SpinLattice Relaxation;
 Fine Structure;
 Line Spectra;
 Matrices (Mathematics);
 Atomic and Molecular Physics