Ultrafast Dynamics of Acetylacetone (2,4-Pentanedione) in the S_2 State

摘要

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(ππ~*). The resulting dynamics was probed by multiphoton ionization at 800 nm. It was investigated for 80 ps on the S_2(ππ~*) and S_1(nπ~*) 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 β-diketones.