Low temperature reaction kinetics of CN− + HC3N and implications for the growth of anions in Titan’s atmosphere

The Cassini–Huygens probe has uncovered the existence of a profusion of negatively charged molecular species in the upper atmosphere of Titan (∼950 km). The presence of large amounts of anions was unexpected and the chemical pathways leading to their formation mostly unknown. The investigation of the negative ion chemistry appears therefore to be a key factor for modeling Titan’s upper atmosphere. We present here the first low temperature experimental kinetic study involving CN−, proposed by Vuitton et al. (2009) to be one of the negative ions detected by the CAPS-ELS instrument onboard the Cassini spacecraft. The temperature dependence of the rate coefficient of the reaction CN− + HC3N, was explored over the 49–294 K temperature range in uniform supersonic flows using the CRESU technique. We find that the kinetics of this reaction is fast (k ≳ 4 × 10−9 cm3 molecule−1 s−1) and presents a weak negative temperature dependence which, considering the experimental error bars, agrees with long-range based capture theory. We also observe that C3N− + HCN represents the main exit channel demonstrating that the studied reaction participates efficiently to the chemical growth of negative ions in the atmosphere of Titan.