CN radical hydrogenation from solid H₂ reactions, an alternative way of HCN formation in the interstellar medium

¹ Aix-Marseille Université et CNRS, Laboratoire de Physique des Interactions Ioniques et Moléculaires, UMR 7345, Centre de St-Jérôme, case 252, Avenue Escadrille Normandie-Niémen, 13397 Marseille Cedex 20, France
e-mail: Fabien.Borget@univ-amu.fr
² Lehrstuhl für Organische Chemie II, Ruhr-Universität Bochum, 44780 Bochum, Germany

Received: 22 April 2015
Accepted: 12 October 2016

Abstract

Context. Molecular hydrogen (H₂) is the most abundant molecule of the interstellar medium (ISM) in gas phase and it has been assumed to exist in solid state or as coating on grains.

Aims. Our goal is to show that solid H₂ can act as a hydrogenation agent, reacting with CN radicals to form HCN.

Methods. In a H₂ matrix, we studied the hydrogenation of the CN radical generated from the vacuum ultraviolet photolysis (VUV-photolysis) of C₂N₂ at 3.8 K. We modified the wavelengths and the host gas in order to be sure that CN radicals can abstract H from H₂ molecules.

Results. HCN monomers, dimers, and oligomers have been characterised by Fourier transform infrared spectroscopy (FTIR). H₂CN as well as CN radicals have also been clearly observed during the photolysis performed at 3.8 K.

Conclusions. H₂ is a hydrogenation reagent towards CN radicals producing HCN. This type of reaction should be taken into account for the reactivity at low temperature in contaminated H₂ ice macro-particles (CHIMPs), H₂ flakes or in the first sublayers of grains where solid H₂ has accumulated.