Readout of spin quantum beats in a charge-separated radical pair by pump-push spectroscopy

Quantum oscillations in radical pairs The spin dynamics of photoinduced radical pairs, involving an interconversion between singlet and triplet spin states, plays an important role in nature, for example, in avian magnetoreception. The spin interconversion is a truly quantum process with characteristic coherent oscillations (quantum beats) that should be reflected in the reaction kinetics. However, their experimental observation has remained challenging. Mims et al . developed an optical readout technique that can directly monitor the singlet-triplet interconversion quantum beats, as demonstrated for a photoinduced, charge-separated state of an electron donor–acceptor dyad (see the Perspective by Hore). The present work opens a new way to monitor the spin evolution in radical pairs, which will be important not only in biological physics but also in organic solar cells and other practical applications. —YS