Non-thermal desorption of complex organic molecules

Cosmic-ray sputtering of CH₃OH embedded in CO₂ ice^⋆

¹ Institut des Sciences Moléculaires d’Orsay (ISMO), UMR8214, CNRS, Université Paris-Saclay, Bât. 520, Rue André Rivière, 91405 Orsay, France
e-mail: emmanuel.dartois@universite-paris-saclay.fr
² Institut de Physique Nucléaire d’Orsay (IPNO), CNRS, Université Paris-Saclay, 91406 Orsay, France
³ Univ. Grenoble Alpes, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), 38000 Grenoble, France
⁴ Centre de Recherche sur les Ions, les Matériaux et la Photonique, CIMAP-CIRIL-GANIL, Normandie Université, ENSICAEN, UNICAEN, CEA, CNRS, 14000 Caen, France

Received: 16 October 2019
Accepted: 17 December 2019

Abstract

Aims. Methanol ice is embedded in interstellar ice mantles present in dense molecular clouds. We aim to measure the sputtering efficiencies starting from different ice mantles of varying compositions experimentally, in order to evaluate their potential impact on astrochemical models. The sputtering yields of complex organic molecules is of particular interest, since few mechanisms are efficient enough to induce a significant feedback to the gas phase.

Methods. We irradiated ice film mixtures made of methanol and carbon dioxide of varying ratios with swift heavy ions in the electronic sputtering regime. We monitored the evolution of the infrared spectra as well as the species released to the gas phase with a mass spectrometer. Methanol (¹²C) and isotopically labelled ¹³C-methanol were used to remove any ambiguity on the measured irradiation products.

Results. The sputtering of methanol embedded in carbon dioxide ice is an efficient process leading to the ejection of intact methanol in the gas phase. We establish that when methanol is embedded in a carbon-dioxide-rich mantle exposed to cosmic rays, a significant fraction (0.2–0.3 in this work) is sputtered as intact molecules. The sputtered fraction follows the time-dependent bulk composition of the ice mantle, the latter evolving with time due to the radiolysis-induced evolution of the bulk. If methanol is embedded in a carbon dioxide ice matrix, as the analyses of the spectral shape of the CO₂ bending mode observations in some lines of sight suggest, the overall methanol sputtering yield is higher than if embedded in a water ice mantle. The sputtering is increased by a factor close to the dominant ice matrix sputtering yield, which is about six times higher for pure carbon dioxide ice when compared to water ice. These experiments are further constraining the cosmic-ray-induced ice mantle sputtering mechanisms important role in the gas-phase release of complex organic molecules from the interstellar solid phase.