Formation rates of complex organics in UV irradiated CH3OH-rich ices I: Experiments

By K. Oberg, R. Garrod, E. V. van Dishoeck, H. Linnartz. 2009

386 citations recorded by the source corpus.

Context. Gas-phase complex organic molecules are commonly detected in the warm inner regions of protostellar envelopes, so-called hot cores. Recent models show that photochemistry in ices followed by desorption may explain the observed abundances. There is, however, a general lack of quantitative data on UV-induced complex chemistry in ices. Aims. This study aims to experimentally quantify the UV-induced production rates of complex organics in CH3OH-rich ices under a variety of astrophysically relevant conditions. Methods. The ices are irradiated with a broad-band UV hydrogen microwave-discharge lamp under ultra-high vacuum conditions, at 20–70 K, and then heated to 200 K. The reaction products are identified by reflection-absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD), through comparison with RAIRS and TPD curves of pure complex species, and through the observed effects of isotopic substitution and enhancement of specific functional groups, such as CH3, in the ice. Results. Complex organics are readily formed in all experiments, both during irradiation and during the slow warm-up of the ices after the UV lamp is turned off. The relative abundances of photoproducts depend on the UV fluence, the ice temperature, and whether pure CH3OH ice or CH3OH:CH4/CO ice mixtures are used. C2H6 ,C H 3CHO, CH3CH2OH, CH3OCH3, HCOOCH3, HOCH2CHO and (CH2OH)2 are all detected in at least one experiment. Varying the ice thickness and the UV flux does not affect the chemistry. The derived product-formation yields and their dependences on different experimental parameters, such as the initial ice composition, are used to estimate the CH3OH photodissociation branching ratios in ice and the relative diffusion barriers of the formed radicals. At 20 K, the pure CH3OH photodesorption yield is 2.1(±1.0) × 10 −3 per incident UV photon, the photo-destruction cross section 2.6(±0.9) × 10 −18 cm 2 . Conclusions. Photochemistry in CH3OH ices is efficient enough to explain the observed abundances of complex organics around protostars. Some complex molecules, such as CH3CH2OH and CH3OCH3, form with a constant ratio in our ices and this can can be used to test whether complex gas-phase molecules in astrophysical settings have an ice-photochemistry origin. Other molecular ratios, e.g. HCO-bearing molecules versus (CH2OH)2, depend on the initial ice composition and temperature and can thus be used to investigate when and where complex ice molecules form.

Subfields: Astronomy and Astrophysics, Atmospheric Science, Spectroscopy

View Paper: https://www.semanticscholar.org/paper/98467c71e30a08287797cec03badea4160a77451

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The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Astronomy and Astrophysics

1. R. Garrod
2. E. Dishoeck
3. E. Herbst
4. P. Caselli
5. Cecilia Ceccarelli
6. E. Bergin
7. K. Oberg
8. V. Wakelam

In Atmospheric Science

1. R. Garrod
2. E. Herbst
3. P. Caselli
4. E. Dishoeck
5. V. Wakelam
6. Cecilia Ceccarelli
7. A. Tielens
8. E. Bergin

In Spectroscopy

1. R. Garrod
2. P. Caselli
3. E. Herbst
4. Cecilia Ceccarelli
5. E. Dishoeck
6. V. Wakelam
7. K. Oberg
8. E. Bergin

Citations in the corpus

• The molecular composition of the planet-forming regions of protoplanetary disks across the luminosity regime (2015, methodology): “We use experimentally determined photodesorption yields where available (see, e.g., Öberg et al. 2009a,b,c).”
• Astrochemistry and compositions of planetary systems (2020, background): “Early experiments demonstrated that UV photodesorption is highly efficient (Westley et al. 1995; Öberg et al. 2007, 2009a,c), while molecular dynamics simulations (Andersson et al. 2006; Andersson and van Dishoeck 2008) revealed that the process can be quite complex and occur through several…”
• Grain Surface Models and Data for Astrochemistry (2017, methodology): “Within this framework, it is also possible to include branching ratios of the expected products, for those species for which photodissociation is the primary process (e.g., H2O and CO2).”
• Gas and grain chemical composition in cold cores as predicted by the Nautilus three-phase model (2016, methodology): “…the last years, quantitative photo-desorption yields have been measured experimentally for pure ices sample using hydrogen discharge lamps: CO (Öberg et al. 2007; Muñoz Caro et al. 2010), H2O (Öberg et al. 2009c), N2, CO2 (Öberg et al. 2009a; Yuan & Yates 2013) and CH3OH (Öberg et al. 2009b).”
• The UMIST database for astrochemistry 2012 (2012, methodology): “…(1963), Nahar & Pradhan (1997), Nejad & Millar (1988), Neufeld & Wolfire (2009), Nguyen et al. (2006), Novotný et al. (2010), Öberg et al. (2005), Öberg et al. (2009a), Öjekull et al. (2004), Osamura et al. (1999), Pequignot & Aldrovandi (1986), Petrie et al. (1991), Petrie et al. (2003),…”
• Microscopic simulation of methanol and formaldehyde ice formation in cold dense cores (2009, methodology): “Higher temperatures allow the radicals to diffuse rapidly enough to form large molecules such as methyl formate and dimethyl ether (Garrod et al. 2008; Öberg et al. 2009d).”
• NON-RACEMIC AMINO ACID PRODUCTION BY ULTRAVIOLET IRRADIATION OF ACHIRAL INTERSTELLAR ICE ANALOGS WITH CIRCULARLY POLARIZED LIGHT (2011, background): “…such as carbon dioxide, formaldehyde, ammonium cyanate, formamide, and tentatively urea have been detected in both astrophysical ice environments and laboratory simulations (d’Hendecourt & Jourdain de Muizon 1989; Schutte et al. 1996; Schutte & Khanna 2003; Raunier et al. 2004; Oberg et al. 2009).”
• Our astrochemical heritage (2012, methodology): “…are experimentally found to promote the formation of organic species more complex than CH3OH (Gerakines et al. 1996; Bennett and Kaiser 2007; Öberg et al. 2009a, 2010b) up to amino acids (e.g., Bernstein et al. 2002; Muñoz Caro et al. 2002, 2004) and allow solid species to return into the…”
• Experimental evidence for glycolaldehyde and ethylene glycol formation by surface hydrogenation of CO molecules under dense molecular cloud conditions (2015, methodology): “In Öberg et al. (2009), for example, vacuum UV (VUV) irradiation of a pure methanol ice was shown to result in the formation of both glycolaldehyde and ethylene glycol.”
• The warm gas atmosphere of the HD 100546 disk seen by Herschel - Evidence of a gas-rich, carbon-poor atmosphere? (2012, methodology): “However, some “mild” photochemistry could lead to more complex, less volatile organics (Öberg et al. 2009a).”
• The Molecular Universe (2011, background): “Recent laboratory studies have focused on putting these ideas on a firm footing identifying the chemical routes involved (Öberg et al. 2009; Hudson et al. 2008).”
• Formation of ethylene glycol and other complex organic molecules in star-forming regions (2016, methodology): “The experiments of Öberg et al. (2009) indicate that different initial compositions of the ices could produce very different values of the [EG/GA] ratio: pure CH3OH ices produce a ratio >10, while ices with a composition CH3OH:CO 1:10 produce a ratio <0.25.”
• A THREE-PHASE CHEMICAL MODEL OF HOT CORES: THE FORMATION OF GLYCINE (2013, methodology): “Other authors have assumed a swapping mechanism tobe active in laboratory ice experiments (Öberg et al. 2009; Fayolle et al. 2011), which is also implicitly assumed in this model.”
• Complex organic molecules in protoplanetary disks (2013, methodology): “This is supported by recent estimates of the grain-surface photodestruction of pure methanol ice which also show that photochemistry occurs deep within the bulk ice (Öberg et al. 2009c).”
• Detection of complex organic molecules in a prestellar core: a new challenge for astrochemical models (2012, background): “According to Öberg et al. (2011), COMs could form sequentially and CH3OCHO and CH3CHO are formed by UV irradiation of cold CO-rich ices and CH3OCH3 forms in warmer (CH3OH-rich) ice after CO has evaporated (see also Öberg et al. 2009).”
• THE SPITZER ICE LEGACY: ICE EVOLUTION FROM CORES TO PROTOSTARS (2011, background): “This last and most source dependent ice chemistry phase is also predicted to result in complex ice formation that requires ice heating for diffusion of large radicals (Garrod et al. 2008; Öberg et al. 2009b).”
• A COLD COMPLEX CHEMISTRY TOWARD THE LOW-MASS PROTOSTAR B1-b: EVIDENCE FOR COMPLEX MOLECULE PRODUCTION IN ICES (2010, methodology): “The second reason for targeting B1-b is the detection of a large CH3OH gas phase column density of ∼2.3–2.5×1014 cm−2 toward the protostar (Öberg et al. 2009a; Hiramatsu et al. 2010) and thus ice evaporation.”
• H-atom addition and abstraction reactions in mixed CO, H2CO and CH3OH ices – an extended view on complex organic molecule formation (2016, methodology): “TPD QMS is used as the main diagnostic tool to detect these COMs because of (significant) spectral overlap of the stronger vibrational modes with H2CO and CH3OH infrared absorption features (Öberg et al. 2009).”