An Experimental and Theoretical Investigation of the Gas-Phase C(³P) + N₂O Reaction. Low Temperature Rate Constants...

The reaction between atomic carbon in its ground electronic state, C(³P), and nitrous oxide, N₂O, has been studied below room temperature due to its potential importance for astrochemistry, with both species considered to be present at high abundance levels in a range of interstellar environments. On the experimental side, we measured rate constants for this reaction over the 50-296 K range using a continuous supersonic flow reactor. C(³P) atoms were generated by the pulsed photolysis of carbon tetrabromide at 266 nm and were detected by pulsed laser-induced fluorescence at 115.8 nm. Additional measurements allowing the major product channels to be elucidated were also performed. On the theoretical side, statistical rate theory was used to calculate low temperature rate constants. These calculations employed the results of new electronic structure calculations of the ³A″ potential energy surface of CNNO and provided a basis to extrapolate the measured rate constants to lower temperatures and pressures. The rate constant was found to increase monotonically as the temperature falls (k_C( (296 K) = (3.4 ± 0.3) × 10^-11 cm³ s^-1), reaching a value of k_C( (50 K) = (7.9 ± 0.8) × 10^-11 cm³ s^-1 at 50 K. As current astrochemical models do not include the C + N₂O reaction, we tested the influence of this process on interstellar N₂O and other related species using a gas-grain model of dense interstellar clouds. These simulations predict that N₂O abundances decrease significantly at intermediate times (10³ - 10⁵ years) when gas-phase C(³P) abundances are high.