Spin correlation and magnetic field effects in radiolysis

When pairs of radicals meet at random the reaction probability is limited to one quarter by spin considerations, unless spin relaxation is exceptionally fast. Conversely, dissociation or ionisation from a singlet state can always be followed by geminate recombination, providing the initial spin correlation is retained. The spin correlation must decay eventually because of spin relaxation but usually a faster process intervenes - spin evolution due to hyperfine coupling. This gives rise to magnetic field effects: so too does electronic fine structure (zero-field splitting) in processes involving pairs of triplet excited states. The role of these processes in radiation chemistry is reviewed and the possible future applications of magnetic studies are discussed. (C) 1997 Elsevier Science Ltd.