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

Abstract

The Cassini–Huygens probe has uncovered the existence of a profusion of negatively charged molecular species in the upper atmosphere of Titan (∼950km). 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 49–294K temperature range in uniform supersonic flows using the CRESU technique. We find that the kinetics of this reaction is fast (k≳4×10−9cm3molecule−1s−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.