Dissociation dynamics of HNCO and DNCO after laser photoexcitation in the vacuum ultraviolet

Abstract

Using the laser photolysis–laser-induced fluorescence “pump–probe” technique, the gas-phase dissociation dynamics of HNCO(X̃ 1A)+hν→H+NCO and DNCO(X̃ 1A)+hν→D+NCO after photoexcitation at the Lyman-α wavelength were studied under collision-free conditions at room temperature. In the vacuum ultraviolet photodissociation experiments narrow band tunable Lyman-α laser radiation (λ≈121.4–121.6 nm) was used both to photodissociate the parent molecules and to detect the produced nascent H and D atom products via (2p 2P←1s 2S) laser induced fluorescence. The following quantum yields ΦH–D for H–D atom formation were determined by a photolytic calibration method: ΦH=(0.62±0.15) and ΦD=(0.51±0.17). For HNCO and DNCO the measured H–D atom Doppler line shapes can be well described by a single Gaussian function, which corresponds to a statistical Maxwell–Boltzmann-like distribution of the translational energy. From the measured H and D atom Doppler profiles the average H and D atom kinetic energy was determined to be ET(H)=(137±10) kJ/mol and ET(D)=(115±4) kJ/mol, respectively. The average kinetic energies were found to be in reasonable agreement with results from simple statistical calculations in which it is assumed that H–D atoms are produced in combination with NCO in the ground electronic state (X̃ 2Π). A dissociation mechanism is suggested in which H–D atom formation proceeds via a statistical unimolecular decay of a hot H–DNCO intermediate formed by a radiationless transition of the optically excited bound H–DNCO state to a lower-lying dissociative state.