Quantum beats in the S1 dynamics of acetaldehyde

We have investigated jet-cooled acetaldehyde, CH3CHO and CD3CDO, excited to the S1(nπ∗) singlet state. For excess energies Δ E(S1) = 1500–2000 cm−1, we observed fluorescence decays with superimposed quantum beats due to coherently excited S1–T1 eigenstates and an increase in the decay rates parallelled by a decrease in the fluorescence, most dramatic at Δ E(S1) ≈ 2000 cm−1. These results show that S1–T1 intersystem crossing is not a dissipative process up to the fluorescence breakdown. It is proposed that at this energy the continuum for α-cleavage in the T1 state is reached which causes the S ⇝ T1 intersystem crossing to become the dominant deactivation pathway.