Photodissociation Dynamics of Propiolic Acid at 193 nm: The State Distribution of the Nascent OH Product

Abstract

Photolysis of propiolic acid (HC⋮C−COOH) upon π−π* excitation at 193 nm leads to the C−O bond fission generating the primary product OH in a good yield. The partitioning of the available energy into the internal states and the translation of OH(v,J) is evaluated by measuring the relative intensities of ro-vibronic lines employing laser-induced fluorescence and their Doppler profiles, respectively. The vibrational distribution of nascent OH(v‘ ‘=0 and 1) corresponds to the vibrational temperature of 1030 ± 80 K. The rotational population of OH(v‘ ‘=0 and 1) is characterized by rotational temperatures of 800 ± 30 and 620 ± 30 K, respectively. Using widths of the Doppler-broadened lines, the OH(v‘ ‘=0) average translational energy was measured to be 24.4 ± 3.0 kcal/mol, which implies about 75% of the total available energy goes to the translation of the fragments. The observed high translational energy is due to the presence of a barrier in the exit channel, suggesting the C−O bond cleavage occurs on an electronically excited potential energy surface. The measured partitioning of the available energy between the fragments OH and HCCCO is explained using the hybrid model with 37.5 kcal/mol of barrier in the exit channel. We employed ab initio molecular orbital theory to calculate structures and energetics of the ground and the excited electronic states of propiolic acid.