The reaction NH2 + PH3 → NH3 + PH2: Absolute rate constant measurement and implication for NH3 and PH3 photochemistry in the atmosphere of Jupiter

It has been suggested that the photochemistries of NH3 and PH3 are linked in the upper atmosphere of Jupiter. Specifically, the reaction NH2 + PH3 → NH3 + PH2 would, if the rate were fast enough at Jovian stratospheric temperatures, regenerate ammonia in the upper atmosphere while at the same time accelerating the decomposition of phosphine. The absolute rate constant for this reaction has been measured over the temperature interval 218–456 K using the technique of flash photolysis-laser-induced fluorescence (FP-LIF). NH2 radicals were produced by flash photolysis of ammonia highly diluted in argon, and the decay of fluorescent NH2 photons was measured by multiscaling techniques. At each of the five temperatures employed in the study the results were shown to be independent of variations in [PH3], total pressure (Argon), and flash intensity (i.e., initial [NH2]). The rate constant results are best represented for 218 ≤ T ≤ 456 K by the expression k = (1.52 ± 0.16) × 10−12 exp (−928 ± 56/T) cm3 molecules−1 s−1, where the error quoted is 1 standard deviation. This represents the first determination of the rate constant for the reaction NH2 + PH3. The data are compared with an estimate made in order to account for results of the radiolysis of NH3-PH3 mixtures. The Arrhenius parameters determined here for NH2 + PH3 are contrasted with those for the corresponding reactions of η and OH with PH3. The results are also discussed theoretically and the implications considered for models of the photochemistry of the atmosphere of Jupiter. Since we now find that the rate constant for the reaction NH2 + PH3 → NH3 + PH2 is 2 orders of magnitude shower than required in the model, it appears that this reaction can make only a negligible contribution to both the recycling of NH3 and the decomposition of PH3 in the upper atmosphere of Jupiter.