Ultrafast dynamics of acetylacetone (2,4-pentanedione) in the S-2 state
Résumé
The dynamics of the enolic form of acetylacetone (E-AcAc) was investigated using a femtosecond pump-probe experiment. The pump at 266 nm excited E-AcAc in the first bright state, S-2(pi pi*). The resulting dynamics was probed by multiphoton ionization at 800 rim. It was investigated for 80 ps on the S-2(pi pi*) and S-1(n pi*) potential energy surfaces. An important step is the transfer from S-2 to S-1 that occurs with a time constant of 1.4 +/- 0.2 ps. Before, the system had left the excitation region in 70 +/- 10 fs. An intermediate step was identified when E-AcAc traveled on the S-2 surface. Likely, it corresponds to an accidental resonance in the detection scheme that is met along this path. More importantly, some clues are given that an intramolecular vibrational energy relaxation is observed, which transfers excess vibrational energy from the enolic group O-H to the other modes of the molecule. The present multistep evolution of excited E-AcAc probably also describes, at least qualitatively, the dynamics of other electronically excited beta-diketones.