Photodissociation dynamics of enolic-acetylacetone at 266, 248, and 193 nm: Mechanism and nascent state product distribution of OH

The photodissociation dynamics of acetylacetone (H3C–CO–CH2–CO–CH3), which exists predominantly as an enolic form [H3C–COCH=C(OH)–CH3] in gas phase, is studied using pulsed laser photolysis laser induced fluorescence (LIF) “pump-and-probe” technique at room temperature. Although two pathways for OH formation have been observed, we have focused on the nascent state of the primary OH radical, formed after photo-excitation of the molecule to its (π,π*) and Rydberg states. The (π,π*) and Rydberg transitions are prepared by excitation with fourth harmonic of Nd:YAG (266 nm)/KrF (248 nm) and ArF (193 nm) lasers, respectively. The ro-vibrational distribution of the nascent OH photofragment is measured in collision-free conditions using LIF. The OH fragments are formed in vibrationally cold state at all the above wavelengths of excitation, but differ in rotational state distributions. The rotational distribution is Boltzmann-like, and characterized by rotational temperatures of 950±50, 1130±60, and 1010±80 K at 266, 248, and 193 nm photodissociation, respectively. The spin–orbit and Λ-doublets ratios of OH fragments formed in the dissociation process are also measured. The average translational energy partitioned into the photofragment pairs in the center-of-mass co-ordinate is found to be 16.0−4.0+1.0, 17.3±4.2, and 19.2±4.7 kcal/mol at 266, 248, and 193 nm excitation, respectively. The energy partitioning into various degrees of freedom of products is interpreted with the help of different models, namely, statistical, impulsive, and hybrid models. To understand the nature of the dissociative potential energy surface involved in the OH production channel, detailed ab initio calculations are performed using configuration interaction-singles method. Although acetylacetone is initially prepared in the (1ππ*) state at 266 and 248 nm excitation, it is concluded that the OH fragment is formed from the lowest (3ππ*) state. However, upon excitation at 193 nm, the initially prepared Rydberg state of acetylacetone crosses over fast to the nearby σ* repulsive state along the C–OH bond, and dissociates to give the OH radical.