Low-temperature chemistry induced by cosmic rays: positive and negative ions desorption from nitrile-bearing astrophysical ice analogues

TL;DR

This study investigates how cosmic ray analogs induce the ejection of positive and negative ions from nitrile-bearing ices, revealing complex fragmentation and ion emission processes relevant to astrochemical environments.

Contribution

It provides the first detailed analysis of secondary ion emission from nitrile ices under cosmic ray simulation, highlighting mechanisms of ion desorption in cold astrophysical conditions.

Findings

Identification of various ionic species emitted from nitrile ices.

Evidence of proton-transfer and electron affinity effects on ion desorption.

Proposal of sputtering and desorption as routes for molecular delivery to gas phase.

Abstract

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 onto icy surfaces. The secondary ions emitted from each sample were analysed by time-of-flight mass spectrometry (TOF-MS), using Plasma Desorption Mass Spectrometry (PDMS) 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 CHmCN- 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 ionised molecular species may be delivered to the gas phase where thermal desorption is negligible.