DISSOCIATION DYNAMICS OF HNCO AND DNCO AFTER LASER PHOTOEXCITATION INTHE VACUUM-ULTRAVIOLET

Using the laser photolysis-laser-induced fluorescence ''pump-probe'' t echnique, the gas-phase dissociation dynamics of HNCO((X) over tilde ( 1)A')+h nu-->H+NCO and DNCO((X) over tilde (1)A')+h nu-->D+NCO after p hotoexcitation at the Lyman-alpha wavelength were studied under collis ion-free conditions at room temperature. In the vacuum ultraviolet pho todissociation experiments narrow band tunable Lyman-alpha laser radia tion (lambda approximate to 121.4-121.6 nm) was used both to photodiss ociate the parent molecules and to detect the produced nascent H and D atom products via (2p P-2<--1s S-2) laser induced fluorescence. The f ollowing quantum yields Phi(H-D) for H-D atom formation were determine d by a photolytic calibration method: Phi(H)=(0.62+/-0.15) and Phi(D)= (0.51+/-0.17). For HNCO and DNCO the measured H-D atom Doppler line sh apes can be well described by a single Gaussian function, which corres ponds to a statistical Maxwell-Boltzmann-like distribution of the tran slational energy. From the measured H and D atom Doppler profiles the average H and D atom kinetic energy was determined to be E-T(H)=(137+/ -10) kJ/mol and E-T(D)=(115+/-4) kJ/mol, respectively. The average kin etic energies were found to be in reasonable agreement with results fr om simple statistical calculations in which it is assumed that H-D ato ms are produced in combination with NCO in the ground electronic state ((X) over tilde (2) Pi), A dissociation mechanism is suggested in whi ch 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 st ate. (C) 1997 American Institute of Physics.