Cold chemistry and physics with supersonic uniform flows
Ludovic Biennier
Astrophysique de Laboratoire, Institut de Physique de Rennes, UMR CNRS 6251, Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France
Understanding the mechanisms of elementary reactions leading to the formation of molecules and clusters, in particular at low collision energies, is of fundamental interest and yields crucial information for modeling gaseous environments encountered in the fields of atmospheric chemistry and astrophysics. The kinetics and the products of many key reactions remain poorly known at temperatures relevant for cold environments. In the past thirty years, the development of supersonic uniform expansions greatly contributed to the experimental determination of the rate of neutral-molecule reactions at low temperatures. Many of the chemical reactions have been found extremely rapid at low temperature; hence stimulating theoretical calculations and leading to the revision of photochemical models of planetary atmospheres and dark molecular clouds.
Recently, our efforts have been extended to reactions with anions, which have been recently discovered in the interstellar medium and in the upper atmosphere of Titan. Their presence was unexpected and these discoveries strongly encouraged the search for new anions and spurred interest in the formation and destruction mechanisms of detected anions. In the laboratory, we have conducted a series of experiments using the Cresu (French acronym standing for Kinetics of Reactions in Uniform Supersonic Flows) combined with quadrupole mass spectrometry to explore the reaction of CxN (x=1,3) with cyanoacetylene HC3N – both abundant in astrophysical environments - over the 50 300 K temperature range [1, 2]. Astrophysical implications of this work and perspectives will be discussed.
The Cresu can also be employed to study the mechanisms of nucleation and molecular growth. This investigationstarted with the condensation of Polycyclic Aromatic Hydrocarbons of interest for the formation of haze in the atmospheres of Titan and Jupiter. More recently we focused our efforts on the study of the first steps of cluster formation of water[3], and small hydrocarbons (acetylene, ethylene, ethane, propane, butane, butene).First kinetics results were obtained with a dedicated Cresu chamber combined to a time of flight mass spectrometer equipped with an electron gun. This work opens the way to the systematic exploration of the onset of nucleation of a great diversity of molecular species.