Photodissociation dynamics of HNCO at 248 nm

Using the laser photolysis/laser-induced fluorescence (LP/LIF) pump-probe technique, the gas-phase photodissociation dynamics of room temperature HNCO was studied at a photolysis wavelength of 248 nm. H atoms produced via HNCO + hν → H(2S) + NCO(X2Π) were detected by vacuum-UV laser-induced fluorescence (VUV-LIF) at the Lyman-α transition. By means of a calibration method — using H2S photolysis as a source of well defined H atom concentrations — the absolute cross section for direct photolytic H atom formation was determined to be σH = (1.2 ± 0.3) × 10−21 cm2 molecule−1. From the H atom Doppler profiles, measured under single-collision conditions, the fraction of the available energy released as product translational energy was determined to be fT(H + NCO) = (0.55 ± 0.02). In addition, the second energetically accessible ‘spin-forbidden’ dissociation channel, HNCO + hν → NH(X 3Σ−) + CO(X 1Σ+), was investigated. Our results show that at a wavelength of 248 nm, direct H atom formation is the dominant dissociation channel in the HNCO photolysis.