Laser photofragmentation dynamics of an acrolein supersonic molecular beam at 193 nm

A pulsed supersonic molecular beam of acrolein has been photodissociated by an ArF excimer laser light at 193 nm in an ultrahigh vacuum chamber. Time-of-flight (TOF) spectra show that the molecule dissociates to give C2H3 and CHO radicals upon one-photon excitation to a (π, π*) state. The average translational energy of the fragments is 10.1 kcal/mol. Observed isotropic angular distribution implies that acrolein molecule lives a much longer period of time than the rotation period, suggesting that the photodissociation of acrolein at 193 nm is predissociative. The TOF spectra also suggest that the molecule undergoes extensive photofragmentation into smaller fragments (e.g., C2H2) via two-photon excitation of 193 nm laser light. Moreover, emission resulting from the photofragmentation of acrolein has been investigated by incorporating a pulsed supersonic free jet as a sample source. Strong emission from the CH(A 2Δ, B 2Σ−, C 2Σ+→X 2Π) systems and weak emission from the C2 (d 3Πg→a 3Πu) Swan system have been observed. From the laser fluence dependence on the emission intensity, the generation of the CH(A 2Δ) and C2(d 3Πg) states require the absorption of two and three photons, respectively. The molecules are considered to be pumped into superexcited states from which they rapidly undergo fragmentation.