Spectrally-resolved UV photodesorption of CH₄ in pure and layered ices

¹ Laboratoire d’Étude du Rayonnement et de la Matière en Astrophysique et Atmosphères (LERMA), Sorbonne Universités, UPMC Univ. Paris 06, Observatoire de Paris, PSL Research University, CNRS UMR 8112, 75005 Paris, France
e-mail: remi.dupuy@obspm.fr
² Laboratoire de Chimie Physique (LCP), CNRS UMR 8000, Univ. Paris Sud, 91400 Orsay, France

Received: 13 March 2017
Accepted: 14 April 2017

Abstract

Context. Methane is among the main components of the ice mantles of interstellar dust grains, where it is at the start of a rich solid-phase chemical network. Quantification of the photon-induced desorption yield of these frozen molecules and understanding of the underlying processes is necessary to accurately model the observations and the chemical evolution of various regions of the interstellar medium.

Aims. This study aims at experimentally determining absolute photodesorption yields for the CH₄ molecule as a function of photon energy. The influence of the ice composition is also investigated. By studying the methane desorption from layered CH₄:CO ice, indirect desorption processes triggered by the excitation of the CO molecules are monitored and quantified.

Methods. Tunable monochromatic vacuum ultraviolet light (VUV) light from the DESIRS beamline of the SOLEIL synchrotron is used in the 7–13.6 eV (177–91 nm) range to irradiate pure CH₄ or layers of CH₄ deposited on top of CO ice samples. The release of species in the gas phase is monitored by quadrupole mass spectrometry, and absolute photodesorption yields of intact CH₄ are deduced.

Results. CH₄ photodesorbs for photon energies higher than ~9.1 eV (~136 nm). The photodesorption spectrum follows the absorption spectrum of CH₄, which confirms a desorption mechanism mediated by electronic transitions in the ice. When it is deposited on top of CO, CH₄ desorbs between 8 and 9 eV with a pattern characteristic of CO absorption, indicating desorption induced by energy transfer from CO molecules.

Conclusions. The photodesorption of CH₄ from pure ice in various interstellar environments is around 2.0 ± 1.0 × 10^-3 molecules per incident photon. Results on CO-induced indirect desorption of CH₄ provide useful insights for the generalization of this process to other molecules co-existing with CO in ice mantles.