Spin correlation effects in radiolysis

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. 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 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. The subject is reviewed under the headings: introduction; basic concept; theory; experiments with magnetic fields; spectroscopic studies of radical ions; implications for radiation chemistry; spurs; radiolysis of hexane; L.E.T. effects; electron exchange in spurs; coherent decay and spin relaxation in spurs; related phenomena - field effects on chemical reactions; spin correlation and spin resonance; muonium chemistry; triplet-triplet annihilation and triplet quenching; concluding remarks. (U.K.)