Uv Photodesorption of Methanol in Pure and Co-Rich Ices: Desorption Rates of the Intact Molecule and of the Photofragments

Abstract

Wavelength dependent photodesorption rates have been determined using synchrotron radiation, for condensed pure and mixed methanol ice in the 7 -14 eV range. The VUV photodesorption of intact methanol molecules from pure methanol ices is found to be of the order of 10 −5 molecules/photon, that is two orders of magnitude below what is generally used in astrochemical models. This rate gets even lower (< 10 −6 molecules/photon) when the methanol is mixed with CO molecules in the ices. This is consistent with a picture in which photodissociation and recombination processes are at the origin of intact methanol desorption from pure CH 3 OH ices. Such low rates are explained by the fact that the overall photodesorption process is dominated by the desorption of the photofragments CO, CH 3 , OH, H 2 CO and CH 3 O/CH 2 OH, whose photodesorption rates are given in this study. Our results suggest that the role of the photodesorption as a mechanism to explain the observed gas phase abundances of methanol in cold media is probably overestimated. Nevertheless, the photodesorption of radicals from methanol-rich ices may stand at the origin of the gas phase presence of radicals such as CH 3 O, therefore opening new gas phase chemical routes for the formation of complex molecules.

A novel approach to measure photodesorption rates of interstellar ice analogues

“…The CO photodesorption rate furthermore strongly depends on the deposition temperature, as has been discussed by several groups (Öberg et al 2009b;Muñoz Caro et al 2010, 2016. In parallel, many more studies have been performed on other pure ices, including H 2 O, N 2 , CO 2 , O 2 (O 3 ), and CH 3 OH, as well as a few mixed ices, CO:N 2 , CO:H 2 O, and CO:CH 3 OH (Öberg et al 2009a;Hama et al 2010;Chen et al 2011;Bahr & Baragiola 2012;Bertin et al 2012Bertin et al , 2013Bertin et al , 2016Fayolle et al 2013;Yuan & Yates 2013;Fillion et al 2014;Zhen & Linnartz 2014;Martín-Doménech et al 2015). In many of these studies it became clear that upon VUV photolysis, molecules not only photodesorb, but may also be involved in photo-induced reactions (Öberg 2016) substantially complicating the analysis, as photoproducts may photodesorb as well (see e.g.…”

Grain Surface Models and Data for Astrochemistry

“…The general consensus from several independent groups, is that pure methanol ice does not photodesorb intact. Photofragments only were observed, with a deduced upper limit to intact methanol photodesorption ∼10 −6 − 10 −5 molecules per incident photon, i.e., at least two orders of magnitude less than that found for water ice (Cruz-Diaz et al 2016;Bertin et al 2016). This low photodesorption yield was found for both pure and mixed (with CO) methanol ice, showing also that the DIET mechanism appears not to work for large molecules such as methanol.…”

“…The former experiments were conducted in the 100-275 nm range at 77 K using a Hg-Xe high pressure arc lamp, whereas the latter were conducted using an ArF 193 nm laser at relatively high fluence between 90 and 135 K. It was not until the experiments by Westley et al (1995a) and Westley et al (1995b) that a robust quantification of water ice photodesorption at low temperatures (35-100 K) was reported. They determined a yield of (3-8 × 10 −3 ) photon -Bertin et al (2016) at Lyman-α (121.6 nm). Photodesorption was revisited much later by (Öberg et al 2007, 2009a,b) who investigated the photodesorption yields of pure CO, N 2 , CO 2 , and H 2 O ices under ultra-high vacuum conditions (base pressure better than 10 −9 mbar) at low temperatures using a broadband hydrogen microwave discharge lamp (120-170 nm).…”

Formation of Glycerol through Hydrogenation of CO Ice under Prestellar Core Conditions

“…COMs produced on icy dust grains in this early stage of star formation can, in turn, be locked on the grain surface and participate in active energetic processing during later stages, or even be released into the gas phase by shocks, heat, or photodesorption (Charnley & Rodgers 2008;Caselli & Ceccarelli 2012;Walsh et al 2014aWalsh et al , 2014bJiménez-Serra et al 2016). Their efficient release into the gas phase by heat and photodesorption is unlikely, however, due to the low volatility of such complex species and a tendency of even the simplest COMs (e.g., CH 3 OH) to photodissociate and desorb as fragments, rather than as an intact molecule (Bertin et al 2016). As a consequence, these heavy-weight COMs have a higher chance to remain on grain surfaces during the later stages of ice evolution.…”