Photochemistry and Astrochemistry: Photochemical Pathways to Interstellar Complex Organic Molecules

Abstract

The interstellar medium is characterized by a rich and diverse chemistry. Many of its complex organic molecules are proposed to form through radical chemistry in icy grain mantles.Radicals form readily when interstellar ices (composed of water and other volatiles) are exposed to UV photons and other sources of dissociative radiation, and if sufficiently mobile the radicals can react to form larger, more complex molecules. The resulting complex organic molecules (COMs) accompany star and planet formation, and may eventually seed the origins of life on nascent planets. Experiments of increasing sophistication have demonstrated that known interstellar COMs as well as the prebiotically interesting amino acids can form through ice photochemistry. We review these experiments and discuss the qualitative and quantitative kinetic and mechanistic constraints they have provided. We finally compare the effects of UV radiation with those of three other potential sources of radical production and chemistry in interstellar ices: electrons, ions and X-rays.

A novel approach to measure photodesorption rates of interstellar ice analogues

“…Finally, the exact ice conditions -ice thickness, ice deposition temperature, and ice morphology -also play a role. For CO ice, Öberg et al (2009b) observed an ice deposition temperature dependence, as recently confirmed and extended over a wider range (7 to 20 K) by others, who concluded that this can be linked to the level of CO dipole orientation within the amorphous state (Muñoz Caro et al 2010, 2016.…”

“…This assumption was in line with wavelength dependent CO photodesorption rates recorded at the DESIRS beamline of the SOLEIL synchrotron facility (Fayolle et al 2011) that showed that the photodesorption of CO follows a wavelength dependent desorption induced by electronic transition (DIET) mechanism that is found to be more effective around 160 nm than at Ly-α. 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).…”

“…With the use of IR spectroscopy and/or mass spectrometry, the photodesorption rate for CO ice has been measured in a number of studies since 2007 yielding values from 10 −1 to 10 −3 molecules photon −1 (Öberg et al 2007, 2009bMuñoz Caro et al 2010, 2016Chen et al 2014). Wavelength dependent photodesorption studies of CO ice at 18 K using synchrotron radiation have unveiled also information about the underlying process, illustrating that it follows a DIET mechanism, and providing monochromatic photodesorption rates for the first time (Fayolle et al 2011).…”

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

“…Gasphase reactions are not efficient enough to explain many of the observed COM abundances, and it is generally accepted that such species form on the surfaces of icy dust grains by merging smaller species to larger and larger constituents. This idea, supported by numerous laboratory results and astrochemical simulations, indicates that surface reactions, triggered by accreting atoms, impacting cosmic rays and VUV photons, as well as by thermal processing, provide efficient pathways toward molecular complexity (Charnley & Rodgers 2008;Garrod et al 2008;Vasyunin & Herbst 2013b;Walsh et al 2014aWalsh et al , 2014bLinnartz et al 2015;Öberg 2016).…”

Importance of tunneling in H-abstraction reactions by OH radicals

“…Radicals in ices are important players in solid-state interstellar chemistry Öberg 2016). In particular, OH radicals seem to be abundant in ices (Chang & Herbst 2014;Lamberts et al 2014); they play a role during solidstate water formation processes Oba et al 2012;Lamberts et al 2013Lamberts et al , 2016a, are involved in the formation of solid CO 2 (Oba et al 2010;Noble et al 2011;Ioppolo et al 2013), and have been postulated to be crucial for the formation of complex organic molecules (Garrod 2013;Acharyya et al 2015).…”