Quantum beats in the S-1 dynamics of acetaldehyde

We have investigated jet-cooled acetaldehyde, CH3CHO and CD3CDO, excited to the S-1(n pi*) singlet state: For excess energies Delta E(S-1) = 1500-2000 cm(-1), we observed fluorescence decays with superimposed quantum beats due to coherently excited S-1-T-1 eigenstates and an increase in the decay rates parallelled by a decrease in the fluorescence, most dramatic at Delta E(S-1) approximate to 2000 cm(-1). These results show that S-1-T-1 intersystem crossing is not a dissipative process up to the fluorescence breakdown. It is proposed that at this energy the continuum for alpha-cleavage in the T-1 slate is reached which causes the S --> T-1 intersystem crossing to become the dominant deactivation pathway.