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 radical pairs’ absorption properties. We succeeded in demonstrating such quantum beats in the photo-induced 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.

Uv/vis absorption and emission spectroscopy, (spectro)electrochemistry, ns-transient absorption spectroscopy, pump push spectroscopy, classical and quantum dynamical simulations, NMR spectroscopy, mass spectrometry