Study of the photon-induced formation and subsequent desorption of CH₃OH and H₂CO in interstellar ice analogs

¹ Centro de Astrobiología (INTA-CSIC), Ctra. de Ajalvir, km 4, Torrejón de Ardoz, 28850 Madrid, Spain
e-mail: rmartin@cab.inta-csic.es
² NASA Ames Research Center, Moffett Field, Mountain View, CA 94035, USA
³ Bay Area Environmental Research Institute, Petaluma, CA 94952, USA

Received: 21 December 2015
Accepted: 6 February 2016

Abstract

Context. Methanol and formaldehyde are two simple organic molecules that are ubiquitously detected in the interstellar medium, in both the solid and gaseous phases. An origin in the solid phase and a subsequent nonthermal desorption into the gas phase is often invoked to explain their abundances in some of the environments where they are found. Experimental simulations under astrophysically relevant conditions have been carried out in the past four decades in order to find a suitable mechanism for that process.

Aims. In particular, photodesorption from pure methanol ice (and presumably from pure formaldehyde ice) has been found to be negligible in previous works, probably because both molecules are very readily dissociated by vacuum-UV photons. Therefore, we explore the in situ formation and subsequent photon-induced desorption of these species, studying the UV photoprocessing of pure ethanol ice, and a more realistic binary H₂O:CH₄ ice analog.

Methods. Experimental simulations were performed in an ultra-high vacuum chamber. Pure ethanol and binary H₂O:CH₄ ice samples deposited onto an infrared transparent window at 8 K were UV-irradiated using a microwave-discharged hydrogen flow lamp. Evidence of photochemical production of these two species and subsequent UV-photon-induced desorption into the gas phase were searched for by means of a Fourier transform infrared spectrometer and a quadrupole mass spectrometer, respectively. After irradiation, ice samples were warmed up to room temperature until complete sublimation was attained for detection of volatile products.

Results. Formation of CH₃OH was only observed during photoprocessing of the H₂O:CH₄ ice analog, accounting for ~4% of the initial CH₄ ice column density, but no photon-induced desorption was detected. Photochemical production of H₂CO was observed in both series of experiments. Formation of formaldehyde accounted for ≤45% conversion of the initial ethanol ice, but it could not be quantified during irradiation of the binary H₂O:CH₄ ice analogs. Photochemidesorption of formaldehyde, i.e., photon-induced formation on the ice surface and inmediate desorption, was observed, with a yield of ~6 × 10^-5 (molecules/incident photon) in the case of the pure ethanol ice experiments, and ~4.4 × 10^-5 (molecules/incident photon) when the H₂O:CH₄ ice analogs were photoprocessed. Photoprocessing of the ice analogs lead to formation of other species. Some of them were also found to desorb upon UV irradiation.

Conclusions. While certain C-bearing species, in particular H₂CO, were found to desorb upon irradiation, nonthermal desorption of CH₃OH was not observed. So far, there is no experimental evidence of any efficient CH₃OH desorption induced by UV photons. On the other hand, the observed photon-induced desorption of H₂CO could account for the total formaldehyde abundance observed in the Horsehead photodissociation-dominated region.