Experimental Investigation of Neutral-Neutral Reactions and Energy Transfer at Low Temperatures

Abstract

Abstract. Over the last few years, we have applied the CRESU (Reaction Kinetics in Uniform Supersonic Flow) technique to the study of neutral-neutral reactions and energy transfer processes in the gas phase. This has enabled the rates of a wide range of reactions between electrically neutral species to be measured down to temperatures as low as ∼10 K. These results have generated significant interest amongst theoretical chemists, and especially amongst astrochemists. Our measurements of low-temperature rate coefficients have had a significant impact on the models used to simulate the chemistry of dense interstellar clouds. In this article the motivations for the experimental study of reaction kinetics at low temperatures are described, and a detailed description of the CRESU technique is given, followed by a brief overview of the results obtained in Rennes and Birmingham. Four recent low-temperature kinetic studies are then described: the reaction O + OH and the interstellar oxygen problem, reactions of the C 2 radical, energy transfer in collisions of C( 3 PJ ) with He and H2, and preliminary results on the reaction kinetics of the C4H radical.

The Chemistry of Cold Interstellar Cloud Cores

“…Indeed, the radicals CN and CCH have the same ability. Such reactions have been studied with the CRESU apparatus down to temperatures as low as ≈10 K. 25,26 Often the rate coefficients show a weak inverse temperature dependence below 300 K so that at low temperatures the rate coefficients can be almost as large as Langevin values. Progress has recently been made in understanding this temperature dependence in terms of variational transition state theory 27 (see also Georgievskii and Klippenstein, this volume).…”

“…If the elemental carbon is initially in the form of C + , as is detected in diffuse clouds, then the initial reaction undergone by this ion is with H 2 but it is not the ion-molecule reaction 25) since this process is endothermic by 0.4 eV; rather it is an unusual, lowdensity process known as radiative association, 11 in which the collision complex is stabilised by emission of a photon:…”