SPIN EFFECTS AN SPUR KINETICS - REENCOUNTERS AND THE INDEPENDENT PAIRS APPROXIMATION

Recent developments in the theory of reaction kinetics in multi-pair s purs have incorporated the possibility that spin correlations may affe ct the chemistry in a complex manner. The analyses published so far ha ve employed two distinct approximations: that pairs evolve independent ly, except on encounter, and that a non-reactive (e.g. triplet) encoun ter does not affect the subsequent encounter rates. This paper present s tests of these two approximations, both separately and in combinatio n, using two Monte Carte simulation techniques: a random-flights metho d, in which spin effects can be incorporated without either approximat ion, and a modification of the independent reaction times method, whic h is subject to the independent pairs approximation, but treats reenco unters properly. The accuracy of both approximations is assessed, sepa rately and in combination; both result in small errors in opposite dir ections. As a result of this cancellation the predictions of random-fl ights simulations and the master equation model, where both approximat ions are used in combination, agree to within 2%. This error is consid erably smaller than the spin effects under investigation, and is also much smaller than errors incurred by using either (i) the prescribed d iffusion approximation for the encounter rate coefficient (rather than the independent pairs approximation) or (ii) a deterministic diffusio n-kinetic analysis of the spur chemistry employing a macroscopic react ion rate coefficient without explicit consideration of spin correlatio n effects.