ELECTRON-SPIN RELAXATION OF PHOTOCHEMICALLY GENERATED RADICAL PAIRS DIFFUSING IN MICELLAR SUPERCAGES

A kinetic analysis of previous experimental data [T. Ulrich and U.E. Steiner, Chem. Phys. Letters 112 (1984) 365] on the magnetic-field dependent recombination kinetics of radical pairs photochemically produced with triplet spin correlation in water-in-oil microemulsion droplets of variable size (r(M)) provided the r(M)- and field-dependence of the rate constant k(r) of the T +/- half arrow right over half arrow left S, T0 spin relaxation process. It was found independent of r(M) at low fields, but strongly decreasing with increasing r(M) at fields greater than or similar to 100 G. The latter behaviour was attributed to the mechanism of electron-spin dipolar interaction. Quantitative treatments of this relaxation mechanism are provided (i) by adapting Torrey's analytical result for homogeneous solution to the situation of one radical pair in a micellar supercage and (ii) by Monte Carlo calculation of the autocorrelation function of the pertinent perturbation matrix element, considering diffusion in the volume or on the surface of the supercage. The electron-spin dipolar relaxation mechanism can quantitatively account for the experimental data on k(r) at fields greater than or similar to 100 G.