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

Spin quantum beats prove the quantum nature of reactions involving radical pairs, the key species of spin chemistry. However, such quantum beats remain hidden to transient absorption–based optical observation because the spin hardly affects the absorption properties of the radical pairs. We succeed in demonstrating such quantum beats in the photoinduced charge-separated state (CSS) of an electron donor–acceptor dyad by using two laser pulses—one for pumping the sample and another one, with variable delay, for further exciting the CSS to a higher electronic state, wherein ultrafast recombination to distinct, optically detectable products of singlet or triplet multiplicity occurs. This represents a spin quantum measurement of the spin state of the CSS at the time instant of the second (push) pulse.