2020MNRAS.492.2140D

In cold core of dark molecular clouds, where the UV radiation from external sources is strongly attenuated, cosmic rays can induce chemical reactions on the surface of ice-covered grains promoting the ejection of the processed material to the gas phase. We report the positive and negative secondary ion emission from pure CH₃CN, C₂H₃CN, and i-C₃H₇CN ices due to the bombardment of heavy ions (²⁵²Cf fission fragments), simulating the incidence of cosmic rays on to icy surfaces. The secondary ions emitted from each sample were analysed by time-of-flight mass spectrometry, using Plasma Desorption Mass Spectrometry technique. Several ionic species were identified, indicating strong fragmentation on the frozen surface. Proton-transfer processes are suggested to play a role for positive ion desorption, as evidenced by the protonated RCNH⁺ parent molecules and (RCN)_nH⁺ ionic clusters. The high electron affinity of the cyano radical seems to contribute to the strong emission of CN^–, as well as anions attributed to the CH_mCN^– fragment and (RCN)_nCN^– cluster series. Sputtering and desorption of ion clusters (positive and negative) induced by heavy ion bombardment are suggested to constitute a route by which new neutral or ionized molecular species may be delivered to the gas phase where thermal desorption is negligible.