The effect of Heisenberg spin exchange on the avoided level crossing (ALC) μSR spectra of organic free radicals is discussed within the framework of a two-state jump model. Numerical expressions are derived which also apply to chemical exchange processes. A simple expression relating ALC amplitude and linewidth to the exchange and chemical reaction rates is inferred from numerical calculations. The theory is applied to the kinetics of the radical-radical reaction between the muonated cyclohexadienyl radical and the diphenylpicrylhydrazyl radical (DPPH•) which is investigated by means of transverse field (TF) and ALC μSR. A total rate constant of 4.7 (5) × 109M-1s-1is found. ALC experiments allow its separation into a spin exchange contribution of ≈ 75% and a chemical contribution of ≈ 25%. The efficiency of the chemical reaction amounts to ≈ 1 3 per encounter of electron singlet radical pairs. The strong exchange-coupling limit holds true, and 10-10s is obtained as an upper limit for the radical pair lifetime at room temperature. © 1989.
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