The influence of very small magnetic fields on radical recombination reactions in the limit of slow recombination

A magnetic field can modify the recombination probability of pairs of free radicals even though it may be weaker than the local magnetic (hyperfine) i nteractions of the unpaired electrons. For a radical pair born in an electr onic singlet state, there is a remarkably simple and general relation betwe en the (singlet) recombination yield Phi(S) in zero field and in the presen ce of a weak field, when the recombination is very slow. The magnitude of P hi(S) at zero field in excess of its statistical value of one quarter is si mply three times that in a weak field. The origin of this relation and the conditions under which it is obeyed are explored. (C) 1998 Elsevier Science B.V. All rights reserved.