Non-thermal ion desorption from an acetonitrile (CH₃CN) astrophysical ice analogue studied by electron stimulated ion desorption

Abstract

The incidence of high-energy radiation onto icy surfaces constitutes an important route for leading new neutral or ionized molecular species back to the gas phase in interstellar and circumstellar environments, especially where thermal desorption is negligible. In order to simulate such processes, an acetonitrile ice (CH3CN) frozen at 120 K is bombarded by high energy electrons, and the desorbing positive ions are analyzed by time-of-flight mass spectrometry (TOF-MS). Several fragment and cluster ions were identified, including the Hn=1-3(+), CHn=0-3(+)/NHn=0-1(+); C2Hn=0-3(+)/CHn=0-3N(+), C2Hn=0-6N(+) ion series and the ion clusters (CH3CN)n=1-2(+) and (CH3CN)n=1-2H(+). The energy dependence on the positive ion desorption yield indicates that ion desorption is initiated by Coulomb explosion following Auger electronic decay. The results presented here suggest that non-thermal desorption processes, such as desorption induced by electronic transitions (DIET) may be responsible for delivering neutral and ionic fragments from simple nitrile-bearing ices to the gas-phase, contributing to the production of more complex molecules. The derived desorption yields per electron impact may contribute to chemical evolution models in different cold astrophysical objects, especially where the abundance of CH3CN is expected to be high.

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

“…Thus, hydrogenation induced by heavy ion bombardment may lead to the desorption of amines from condensed nitriles. As previously observed by electron stimulated ion desorption (ESID) of CH 3 CN (Ribeiro et al 2015), the high intensity peak centred at m/z = 42 in Fig. 2a is ascribed to the desorption of protonated acetonitrile, CH 3 CNH + .…”

Surface Science Investigations of Icy Mantle Growth on Interstellar Dust Grains in Cooling Environments

“…To date, the vast majority of electron/ion irradiation experiments on interstellar ice analogues focus on the chemistry induced upon exposure to ionizing bombardment; [131][132][133][134][135][136] fewer look at the desorbing species. [137][138][139][140] It is even rarer to see both processes compared, as in the work of Abdulgalil et al 141 This study reports that ion/electron induced chemistry is dominant in those environments where the ices are relatively thick (hundreds of layers thick), while EPD is significant when ices are thinner, which is the case as diffuse clouds transition to being dense clouds when reactive accretion has not yet reached a steady state. Consistency with what is known of the radiolysis of liquids and vapours 145,146 would suggest that H/H 2 production is likely to be an important process in photolysis and radiolysis of both simple (e.g.…”

On Cosmic-Ray-driven Grain Chemistry in Cold Core Models

“…These non-thermal desorption mechanisms, such as sputtering, desorption induced by electronic transitions (DIET), electron stimulated ion desorption (ESID), and Auger stimulated ion desorption (ASID) (Ribeiro et al 2015) are particularly promising since they could provide a means of enriching gas-phase abundances at low temperatures, and are therefore a natural complement to the nonthermal chemistry described here.…”