Non-thermal desorption of complex organic molecules

Efficient CH₃OH and CH₃COOCH₃ sputtering by cosmic rays^★

¹ Institut des Sciences Moléculaires d’Orsay (ISMO), UMR8214, CNRS – Université de Paris-Sud, Université Paris-Saclay, Bât 520, Rue André Rivière, 91405 Orsay, France
e-mail: emmanuel.dartois@u-psud.fr
² Institut de Physique Nucléaire d’Orsay (IPNO), CNRS-IN2P3, Université de Paris-Sud, Université Paris-Saclay, 91406 Orsay, France
³ Centre de Recherche sur les Ions, les Matériaux et la Photonique, CIMAP-CIRIL-GANIL, Normandie Université, ENSICAEN, UNICAEN, CEA, CNRS, 14000 Caen, France
⁴ Centre de Sciences Nucléaires et de Sciences de la Matière (CSNSM), CNRS/IN2P3, Université de Paris-Sud, Université Paris-Saclay, 91405 Orsay, France
⁵ Department of Physics, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India

Received: 6 December 2018
Accepted: 25 May 2019

Abstract

Context. The occurrence of complex organic molecules (COMs) in the gas phase at low temperature in the dense phases of the interstellar medium suggests that a non-thermal desorption mechanism is at work because otherwise, COMs should condense within a short timescale onto dust grains. Vacuum ultraviolet (VUV) photodesorption has been shown to be much less efficient for complex organic molecules, such as methanol, because mostly photoproducts are ejected. The induced photolysis competes with photodesorption for large COMs, which considerably lowers the efficiency to desorb intact molecules.

Aims. We pursue an experimental work that has already shown that water molecules, the dominant ice mantle species, can be efficiently sputtered by cosmic rays. We investigate the sputtering efficiency of complex organic molecules that are observed either in the ice mantles of interstellar dense clouds directly by infrared spectroscopy (CH₃OH), or that are observed in the gas phase by millimeter telescopes (CH₃COOCH₃) and that could be released from interstellar grain surfaces.

Methods. We irradiated ice films containing complex organic molecules (methanol and methyl acetate) and water with swift heavy ions in the electronic sputtering regime. We monitored the infrared spectra of the film as well as the species released to the gas phase with a mass spectrometer.

Results. We demonstrate that when methanol or methyl acetate is embedded in a water-ice mantle exposed to cosmic rays, a large portion is sputtered as an intact molecule, with a sputtering yield close to that of the main water-ice matrix. This must be even more true for the case of more volatile ice matrices, such as those that are embedded in carbon monoxide.

Conclusions. Cosmic rays penetrating deep into dense clouds provide an efficient mechanism to desorb complex organic molecules. Compared to the VUV photons, which are induced by the interaction of cosmic rays, a large portion desorb as intact molecules with a proportion corresponding to the time-dependent bulk composition of the ice mantle, the latter evolving with time as a function of fluence due to the radiolysis of the bulk.