The branching ratios into the O H and H 2 0 producing channels in the reaction of NH2 with N O have been measured at four temperatures ranging from room temperature to 900 O C . The reaction was initiated by production of NH2 by excimer laser photolysis of ammonia at 193 nm in the presence of nitric oxide and was probed using a color center laser in conjunction with fast IR detectors. Upon photolysis, the NH3 infrared absorption decreases with consequent appearance of NH2 infrared absorption lines. The magnitude of the decrease in the NH3 infrared absorption is compared with infrared absorbance of O H and H 2 0 produced by the reaction. Measurement of room temperature peak absorption cross sections for NH3, NH2, and H 2 0 combined with the literature value of the OH cross section permits the determination of the branching ratios.For elevated temperatures, the appropriate Boltzmann factors and partition functions were used to calculate the required cross sections from the room temperature values. The branching ratio into the O H channel was observed to increase from 10% a t room temperature to 17% at 900 OC. However, the total contribution of the two channels decreased from -94% a t 26 OC to -70% at 900 OC, possibly indicating the onset of an additional reaction channel. The possibility that the O H signal might arise from an artifact source was investigated.
The rates of reaction of C2H with H2 and C2H2 have been measured over the temperature range 295-854 K.The C2H radical was produced by excimer laser photolysis of C2H2 at 193 nm, and its transient absorption was monitored throughout the reaction by using a tunable infrared color-center laser. The temperature dependence of the rate constant for the reaction with H2 exhibited a non-Arrhenius form that could be well represented by the following expression: k = (9.44 f 0.24) X 10-14P9 exp(-1003 f 20/T) cm3 molecule-' s-l. The rate constant for the reaction with C2H2 disDlaved no definite temperature dependence. It was measured as (1.6 --k 0.3) X 10-lo cm3 molecule-' s-l.
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