The flash photolysis—shock tube technique using atomic resonance absorption for kinetic studies at high temperatures

A flash photolysis‐shock tube technique is described for making kinetic measurements at high temperature. Coupled with sensitive atomic resonance absorption detection, this method allows bimolecular rate constants for atom‐molecule reactions to be measured directly under conditions free from kinetic complications. Experiments were performed in the reflected shock regime, and the temperature and density were calculated using ideal shock wave theory in this initial work. Results for the reaction of atomic hydrogen with ammonia are presented to illustrate the potential of the technique. The values of the Arrhenius rate parameters found in these experiments, 900 K ≤ T ≤ 1850 K, were A = (1.14 ± 0.12) × 10−10 cm3 molecule−1 s−1 and Ea = 13,216 ± 242 cal mol−1. This result gives rate constants that are about five times larger than those from previous studies. Although corrections for nonidealities in the reflected shock region are anticipated and under investigation, the expected changes will be relatively small and thus the large discrepancy noted here will remain.