Microwave-induced quantum beats in micellized radical pairs under spin-locking conditions

The effect of short microwave pulse irradiation on the spin correlated radi cal pairs generated by the hydrogen abstraction reaction of triplet excited xanthone with tert-butyl- substituted phenols in a micellar sodium dodecyl sulfate solution is investigated by a time-resolved transient optical abso rption technique. An increase in the microwave pulse duration under the ele ctron spin resonant conditions leads to the alternation of the radical pair concentration. It is explained by the periodical change of the radical pai r spin state population with an angular frequency of omega (1) = gamma B-e( 1) due to the pumping with resonant microwave field B-1. The double frequen cy (2 omega (1)) quantum beats are observed under spin-locking conditions. The behavior of quantum beats in this case is rationalized by the effect of microwave pulse on the electron spins of both radicals in the radical pair and reflects the spin dynamics in the triplet manifold of radical pairs th at is isolated from that of the singlet during the microwave pulse by the s pin-locking effect. Theoretical analysis of the double frequency beats is p erformed and shows that the spin dephasing process between the triplet stat es of the radical pair (triplet-triplet dephasing) is the main cause of the double beats decay. Modulation of electron-electron interradical and elect ron-nuclear intraradical (anisotropy of hyperfine coupling) dipole-dipole i nteractions by molecular dynamics is considered as a possible source of the triplet-triplet dephasing.