Novel approach to distinguish between vacuum UV-induced ice photodesorption and photoconversion

Investigation of CH₄, CH₃OH, and CH₃CN

¹ Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: bulak@strw.leidenuniv.nl
² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
³ Science Division, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA

Received: 11 December 2019
Accepted: 11 February 2020

Abstract

Context. In cold regions of the interstellar medium with intense ultraviolet radiation fields, photodesorption has been suggested as a nonthermal desorption mechanism promoting the transition of molecules from the solid state to the gas phase. Laboratory experiments measuring photodesorption rates are crucial in attempting to explain high molecular gas phase abundances of species that are expected to form in the solid state, such as methane, methanol, and acetonitrile, and to aid astrochemical modeling. Due to the convoluted competition between photodesorption and photoconversion, it is far from trivial to derive accurate photodesorption rates.

Aims. The aim of this study is to apply a new methodology to discriminate between the two processes. The method has been validated using the well-studied case of CO and extended to CH₄, CH₃OH, and CH₃CN.

Methods. Vacuum ultraviolet (VUV; photon energy of 7–10.2 eV) irradiated ices at 20 K are studied, first as a pure CH₄, CH₃OH, or CH₃CN ice and subsequently with an Ar coating on top. The latter is transparent to the VUV photons (wavelength below 200 nm), but it quenches the photodesorption process. Comparing the laser desorption post ionization time-of-flight mass spectrometry of the ices with and without the Ar coating provides information on the different interactions of the VUV photons with the ice.

Results. The newly developed experimental technique allowed for a derivation of photodesorption rates for ices at 20 K of: CO (3.1 ± 0.3)×10⁻³ mol. photon⁻¹, CH₄ (3.1 ± 0.5)×10⁻² mol. photon⁻¹, and upper limits for CH₃OH (< 6 × 10⁻⁵ mol. photon⁻¹) and CH₃CN (< 7.4 × 10⁻⁴ mol. photon⁻¹); in the latter case, no literature values have been reported yet. The newly introduced approach provides more insight into the photodesorption process, in particular, for commonly observed complex organic molecules (COMs). Photoconversion cross sections are presented in the 7–10.2 eV range. The possible role of photodesorption and photoconversion in the formation of interstellar COMs is discussed.