Analytical model for spin dynamics in radical pairs under the spin-locking condition

Spin dynamics in triplet radical pairs are theoretically studied under the spin-locking condition, where singlet-triplet mixing is blocked by the resonant microwave field. A key assumption in the theory is simultaneous excitations of T+-T0 and T--T0 transitions in triplet radical pairs. This assumption allows for the application of a three-state model [Yago, J. Chem. Phys. 151, 214501 (2019)] to describe the spin dynamics of triplet radical pairs. The analysis based on the three-state model shows that the triplet states are quantized along the direction of a microwave-induced magnetic field (B1) in the rotating frame under the spin-locking condition. This gives rise to a new spin-locking phenomenon where T+-T0 and T--T0 mixing are most enhanced at magnetic fields that deviate from the resonance by ±B1. It is also shown that the quantum beats observed under the spin-locking condition originate from the spin dynamics in triplet radical pairs.