LAUSR

Context. Molecular hydrogen (H 2) 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 H 2 can act as a hydrogenation agent, reacting with CN radicals to form HCN. Methods. In a H 2 matrix, we studied the hydrogenation of the CN radical generated from the vacuum ultraviolet photolysis (VUV-photolysis) of C 2 N 2 at 3.8 K. We modified the wavelengths and the host gas in order to be sure that CN radicals can abstract H from H 2 molecules. Results. HCN monomers, dimers, and oligomers have been characterised by Fourier transform infrared spectroscopy (FTIR). H 2 CN as well as CN radicals have also been clearly observed during the photolysis performed at 3.8 K. Conclusions. H 2 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 H 2 ice macro-particles (CHIMPs), H 2 flakes or in the first sublayers of grains where solid H 2 has accumulated.