Dynamics of OH formation in the dissociation of acrylic acid in its (n,π*) and (π,π*) transitions excited at 248 and 193 nm

Abstract

The (n,π*) and (π,π*) transitions in acrylic acid (H2C=CHCOOH) are excited by KrF (248 nm) and ArF (193 nm) laser pulses, respectively, and the dynamics of its photodissociation to give OH fragments is studied using laser induced fluorescence technique. At both the photolysis wavelengths, the OH fragments produced are vibrationally cold, but have different rotational state distributions. To get an insight into the potential energy surface involved in the dissociation process, spin–orbit and Λ-doublets ratios are also measured. Average relative translational energy partitioned into the photofragments is determined using linewidth of the Doppler profiles to be 13.2±3.1 and 10.2±2.8 kcal/mol at 193 and 248 nm excitations, respectively. High percentage of translational energy released into the photofragments suggests the presence of an exit barrier for the dissociation. On 248 nm excitation, the OH radicals are formed instantaneously during the laser pulse, while on 193 nm excitation, a risetime of ∼2 μs is seen. Another difference between the photodissociation at 193 nm and 248 nm is the observation of an intense fluorescence in UV–visible region at the former, and no fluorescence at the later wavelength. Our experimental results are compared with those obtained by recent ab initio calculations by Fang and Liu. It is concluded that when (π,π*) transition of acrylic acid is excited at 193 nm, the initially prepared S2 state undergoes nonradiative transitions to S1 and T2 states, and from where the molecule subsequently dissociates, while excitation to (n,π*) transition at 248 nm leads to dissociation solely from the initially prepared S1 state.