Simulation of magnetic field effects:: resonances due to g value differences in radical pairs

Yields of radical pair processes usually change smoothly with magnetic field. This paper is concerned with exceptions to this, due to crossings of the energy levels of the pair. The evolution of the spin wavefunction of an initially singlet pair of radicals is treated analytically in some special cases. Resonance effects are predicted to occur when differences in the electronic Zeeman interaction match the hyperfine interaction. There are two distinct effects: first-order differences will lead to resonances at high fields in almost all cases; in addition, if Sigma m = 0 levels cross as a result of higher-order hyperfine coupling there will be resonances at intermediate fields. Numerical methods are necessary when there are more than one type of magnetic nucleus on one of the radicals; they are used to illustrate the effects in several simple cases and in some real cases involving radical ion pairs produced by radiolysis.