LOWTEAM · Low temperature experiments for astrochemical models

Low temperature experiments for astrochemical models

Project context and objectives The interstellar medium is composed mainly of hydrogen and helium, with traces of oxygen, carbon, nitrogen and dust in a wide range of physical conditions. The coldest regions (T = 10-30 K) are called dark molecular clouds where neutral atoms, radicals and molecules are abundant and a rich neutral chemistry is now known to occur, thanks to the wealth of experimental results obtained with the CRESU technique (Cinétique de Réaction en Ecoulement Supersonique Uniforme: reaction kinetics in a uniform supersonic flow), which determined rate constants at very low temperatures. To date, CRESU studies have furnished data for more than 100 reactions of radical species with neutral molecules. However, there remain many reactions of importance in dark clouds for which there is no rate constant data at low temperature, and most notably between two unstable radical species. The main aim of this project was to study the gas-phase chemistry of atomic nitrogen at low temperatures using the CRESU technique. Work performed The ion-neutral chemistry of atoms in dark clouds is initiated by the X + H3+ -> XH2+ + H reaction. This process occurs readily when X is carbon or oxygen, but an energy barrier inhibits the equivalent process for nitrogen. As a result, neutral-neutral reactions play a significant role in interstellar nitrogen chemistry. Two of the most important processes involving atomic nitrogen, which were targeted for study in this project, are: N + OH -> NO + H (1) and N + NO -> N2 + O (2). These two reactions were thought to mediate the transformation of atomic to molecular nitrogen in dark clouds; chemical models of such regions predicted that most of the nitrogen should be present in the form of N2 if the estimated rate constants for these processes were correct. Until this project was initiated, only laser-based photolysis methods had been used to generate reactive species in CRESU apparatuses. We incorporated the well-known microwave discharge method into our CRESU apparatus to generate atomic nitrogen from precursor N2 molecules. We studied reaction (2) first and were able to generate low concentrations of atomic nitrogen; we followed the loss of these atoms through resonance fluorescence in the presence of a known excess of NO (one of the reactant species must be held in excess so that the kinetic analysis can be simplified to yield the rate constant for reaction). Rate constants were obtained down to temperatures as low as T = 48 K. The study of reaction (1) was much more complex as both reactants are transient species and need to be generated simultaneously. It was necessary to create an excess concentration of one of the reactants and measure its concentration in order to determine the rate constant. Large concentrations of atomic nitrogen were formed through the microwave discharge technique. We noticed that the NO product of reaction (1) would react again with excess atomic nitrogen according to reaction (2). By following both the OH and NO radical temporal profiles by laser-induced fluorescence, we were able to determine rate constants for reaction (1) down to 56 K using reaction (2) (and our previously determined rate constants for this reaction) as a reference. Main results We have been able to confirm that reactions (1) and (2) are much less efficient in dark clouds than previously thought.

Data: CORDIS, © European Union