Alpha-Induced Decomposition of Ammonia. II. Effects of Variations in Intensity and Pressure

Abstract

The alpha‐induced decomposition of ammonia was studied as a function of pressure (70–740 mm) and intensity. The ion yield was found to increase both with increasing intensity and decreasing pressure. The variation with intensity was more pronounced below 200 mm than at higher pressures. Several models were examined to account for the experimental results. The decrease in the yield with increasing pressure was due to the deactivation of NH₃^* generated by the reaction NH₂+H→NH₃^*. At all intensities used in this work, gas phase ion recombination was negligible. The positive ions were assumed to react with NH₃ to form NH₄⁺ which was neutralized at the wall. At sufficiently high pressure and intensity, the NH₂ radicals reacted homogeneously in the gas phase, but at low pressure and intensity, wall reactions became important. This ``intensity'' effect was caused by the ions and does not appear in photolysis. The ion‐molecule reactions generate the NH₄⁺, the pressure controls the diffusion process, and the intensity determines the relative importance of the gas phase and the wall reactions. The intensity effect is a function of all four variables. On the basis of the reaction scheme and the present knowledge of the mass spectrum, it was possible to calculate a lower limit for the ion yield.