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.