Photodissociation dynamics of acetylacetone: The OH product state distribution

Acetylacetone in the supersonic jet, which exists predominantly as an enolic form, is found to give rise to the OH fragment after the π–π* transition induced by the UV absorption. The nascent OH product state distributions are determined using a laser-induced fluorescence technique at the excitation wavelengths of 291 and 266 nm. The OH fragment is vibrationally cold, and its rotational state distribution is peaked at N=3 or 4 at the pump wavelength of 291 or 266 nm, respectively. No fluorescence from the excited acetylacetone has been observed even in the energy region near the origin, suggesting the ultrafast nonradiative processes of the excited state. From the measured OH product state distributions, the upper bound for the dissociation energy of the acetylacetone is estimated to be 90.3 kcal/mol. The ratios of Λ-doublets and spin–orbit states of the OH fragment are also measured. A slight preference of the OH fragment in the Π3/22 state over the Π1/22 state is observed. The Π−/Π+ ratios, determined by the relative intensity ratios of Q and P (or R) lines, are found to be less than unity, suggesting the preferential cleavage of the C–OH bond on the molecular plane probably due to a relatively strong intramolecular hydrogen bonding of the enolic acetylacetone. The prior calculation reproduces the experiment quite well for the OH rotational state distribution at 291 nm, while it does not for that at 266 nm. This suggests that the transition state in the acetylacetone dissociation, at the low energy near threshold, may be completely loosely defined on the potential energy surface which does not have a barrier to recombination, and it becomes tightened as the energy increases above the reaction threshold.