A magnetic field can modify the recombination probability of pairs of free radicals even though it may be weaker than the local magnetic (hyperfine) interactions of the unpaired electrons. For a radical pair born in an electronic singlet state, there is a remarkably simple and general relation between the (singlet) recombination yield Phi(S) in zero field and in the presence of a weak field, when the recombination is very slow. The magnitude of Phi(S) at zero field in excess of its statistical value of one quarter is simply 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. [References: 17]
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