THE C(³P) + NH₃ REACTION IN INTERSTELLAR CHEMISTRY. II. LOW TEMPERATURE RATE CONSTANTS AND MODELING OF NH, NH₂, AND NH₃ ABUNDANCES IN DENSE INTERSTELLAR CLOUDS

A continuous supersonic flow reactor has been used to measure rate constants for the C(³P) + NH₃ reaction over the temperature range 50–296 K. C(³P) atoms were created by the pulsed laser photolysis of CBr₄. The kinetics of the title reaction were followed directly by vacuum ultra-violet laser induced fluorescence of C(³P) loss and through H(²S) formation. The experiments show unambiguously that the reaction is rapid at 296 K, becoming faster at lower temperatures, reaching a value of (1.8 ± 0.2) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ at 50 K. As this reaction is not currently included in astrochemical networks, its influence on interstellar nitrogen hydride abundances is tested through a dense cloud model including gas–grain interactions. In particular, the effect of the ortho-to-para ratio of H₂, which plays a crucial role in interstellar NH₃ synthesis, is examined