Laser desorption time-of-flight mass spectrometry of vacuum UV photo-processed methanol ice

Abstract

Context. Methanol in the interstellar medium mainly forms upon sequential hydrogenation of solid CO. With typical abundances of up to 15% (with respect to water) it is an important constituent of interstellar ices where it is considered as a precursor in the formation of large and complex organic molecules (COMs), e.g. upon vacuum UV (VUV) photo-processing or exposure to cosmic rays. Aims. This study aims at detecting novel complex organic molecules formed during the VUV photo-processing of methanol ice in the laboratory using a technique more sensitive than regular surface diagnostic tools. In addition, the formation kinetics of the main photo-products of methanol are unravelled for an astronomically relevant temperature (20 K) and radiation dose. Methods. The VUV photo-processing of CH 3 OH ice is studied by applying laser desorption post-ionisation time-of-flight mass spectrometry (LDPI TOF-MS), and analysed by combining molecule-specific fragmentation and desorption features. Results. The mass spectra correspond to fragment ions originating from a number of previously recorded molecules and from new COMs, such as the series (CO) x H + y , with x = 3 and y < 3x − 1, to which prebiotic glycerin belongs. The formation of these large COMs has not been reported in earlier photolysis studies and suggests that such complex species may form in the solid state under interstellar conditions.

Production of complex organic molecules:H-atom addition versus UV irradiation

“…6 compares the laboratory data taken in this work (left-hand panels) with some selected literature data (right-hand panels) of (i) hydrogenation of ice mixtures including CO/H2CO/CH3OH at 15 K by Chuang et al (2016), (ii) UV irradiation of pre-deposited CH3 OH and CH3OH:CO ice at 20 K by Ö berg et al (2009), studied by RAIR and QMS techniques and (iii) UV exposure of pre-deposited pure CH3OH ice at 20 K by Paardekooper et al (2016a), studied by laser desorption post-ionization mass spectrometry. For the hydrogenation experiment of CO:CH3OH ice mixtures presented here (exp.…”

“…A large number of laboratory studies have been performed on the photo-desorption rates of CO and CH3OH ice (Ö berg et al 2007Fayolle et al 2011;Muñoz-Caro et al 2010;Chen et al 2014;Bertin et al 2016;Paardekooper et al 2016a;Cruz-Diaz et al 2016), and even though there exist some discrepancies between the exact values, roughly within an order of 10, this does not prevent the ability to assess the role of photo-desorption in the present experiment. With photo-desorption rates of the order of 1 × 10 −2 (1 × 10 −3 ) for pure CO ice and less than 1 × 10 −4 for pure CH3OH ice, typically for ices with temperatures around 15-20 K, an estimate can be made.…”

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

“…Instead, "nonenergetic" H-atom addition and abstraction reactions provide the reactive intermediates to generate glycerol and likely glyceraldehyde. So far, similar results for oxygen-rich three-carbon-bearing COMs were only presented after VUV photon or cosmic-ray irradiation of CH 3 OH-containing ices, where various reactive intermediates, i.e., ·CH 2 OH, CH 3 O·, ·CH 3 , HĊO, etc., are obtained by CH 3 OH dissociation with impinging particles Maity et al 2015;Paardekooper et al 2016a). The latter process is effective but lacks the selectivity of an H-atom-induced reaction scheme as shown in Figure 1.…”

“…Laboratory studies show that in this case COMs are efficiently formed through recombination of carbon-bearing CH 3 OH dissociation products (Allamandola et al 1988;Hudson & Moore 2000;Öberg et al 2009;Modica & Palumbo 2010;Jheeta et al 2013;Henderson & Gudipati 2015;Maity et al 2015;Abou Mrad et al 2016;Paardekooper et al 2016a;Chuang et al 2017). Whereas the "nonenergetic" scenarios are thought to dominate in dark clouds, the latter ones will be more prominent during later stages of the star and planet formation process.…”

A novel approach to measure photodesorption rates of interstellar ice analogues

“…Therefore, the SED of the microwave powered H 2 -discharge lamp is used as characterised recently, using an absolute calibrated VUV spectrometer (Ligterink et al 2015). The resulting spectrum has been presented in Paardekooper et al (2016). The emission profile predominately contains photons at Ly-α and also has contributions from molecular hydrogen at 160 nm.…”