Spin correlation effects in radiolysis

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Abstract
Radiolysis of liquid hydrocarbons produces radical cations and electrons. Their recombination releases sufficient energy in the solvent, or in the solute if a charge scavenger is added, to produce excited states, Because of the low dielectric constant, most of the recombination is geminate, i.e. the positive and negative charges do not separate as far as the Onsager escape distance (1∼30nm for an alkane at room temperature). For a single ion pair, this means that the two unpaired electrons were initially paired in a molecular singlet state. If recombination is very fast (< 1-10ns), the spin correlation is retained: only singlet products will result. Over longer periods, the correlation decays because of hyperfine interaction between the electrons and magnetic nuclei. This leads to time-dependent magnetic field and magnetic isotope effects on the product yields, easily detected by studying the fluorescence in suitable systems. Spin relaxation randomizes spin orientations but the hyperfine effect is a coherent process and oscillations can be detected. Theory and experiment are in good agreement in most respects. Electron spins can be reversed by absorption of microwaves: the changes in fluorescence can be used for spectroscopic studies of the ions. Similar arguments apply to pairs of neutral radicals though the dynamic processes are quite different: triplet molecular states are usually repulsive—only singlets react—and diffusive separation is not constrained by coulombic attraction. Recombination is spin-selective which leads to spin polarization, i.e. to CIDNP and CIDEP. These phenomena and described briefly, together with effects of magnetic field on product yields of thermal and photochemical reactions, notably in micelles and in photosynthetic centres, and other related phenomena. A peculiar feature of radiolysis is the production of spurs consisting of 1, 2, 3 or more radical and radical-ion pairs. Single ion pairs constitute only about 20% of the total. While the overall spin state of a spur should be singlet, recombination with a different partner will give triplet products. The possible consequences are described but this is a controversial area and no clear conclusions have yet been reached.