THE EFFECT OF H{sub 2}O ON ICE PHOTOCHEMISTRY

Journal Article · Sun Aug 01 00:00:00 EDT 2010 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/718/2/832· OSTI ID:21457116

Oeberg, Karin I ^[1]; Van Dishoeck, Ewine F ^[2]; Linnartz, Harold ^[3]; Andersson, Stefan ^[4]

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1. Harvard-Smithsonian Center for Astrophysics, MS 42, 60 Garden St, Cambridge, MA 02138 (United States)
2. Leiden Observatory, Leiden University, P.O. Box 9513, NL-2300 RA Leiden (Netherlands)
3. Sackler Laboratory for Astrophysics, Leiden Observatory, Leiden University, P.O. Box 9513, NL-2300 RA Leiden (Netherlands)
4. SINTEF Materials and Chemistry, P.O. Box 4760, NO-7465 Trondheim (Norway)

UV irradiation of simple ices is proposed to efficiently produce complex organic species during star formation and planet formation. Through a series of laboratory experiments, we investigate the effects of the H{sub 2}O concentration, the dominant ice constituent in space, on the photochemistry of more volatile species, especially CH{sub 4}, in ice mixtures. In the experiments, thin ({approx}40 ML) ice mixtures, kept at 20-60 K, are irradiated under ultra-high vacuum conditions with a broadband UV hydrogen discharge lamp. Photodestruction cross sections of volatile species (CH{sub 4} and NH{sub 3}) and production efficiencies of new species (C{sub 2}H{sub 6}, C{sub 2}H{sub 4}, CO, H{sub 2}CO, CH{sub 3}OH, CH{sub 3}CHO, and CH{sub 3}CH{sub 2}OH) in water-containing ice mixtures are determined using reflection-absorption infrared spectroscopy during irradiation and during a subsequent slow warm-up. The four major effects of increasing the H{sub 2}O concentration are: (1) an increase of the destruction efficiency of the volatile mixture constituent by up to an order of magnitude due to a reduction of back reactions following photodissociation, (2) a shift to products rich in oxygen, e.g., CH{sub 3}OH and H{sub 2}CO, (3) trapping of up to a factor of 5 more of the formed radicals in the ice, and (4) a disproportional increase in the diffusion barrier for the OH radical compared with the CH{sub 3} and HCO radicals. The radical diffusion temperature dependencies are consistent with calculated H{sub 2}O-radical bond strengths. All the listed effects are potentially important for the production of complex organics in H{sub 2}O-rich icy grain mantles around protostars and should thus be taken into account when modeling ice chemistry.

• OSTI ID: 21457116
• Journal Information: Astrophysical Journal, Vol. 718, Issue 2; Other Information: DOI: 10.1088/0004-637X/718/2/832; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTROPHYSICS
COSMOLOGY AND ASTRONOMY
ABSORPTION SPECTROSCOPY
AMMONIA
CARBON MONOXIDE
CROSS SECTIONS
DIMETHYL SULFIDE
ETHANE
ETHYLENE
HYDROGEN
HYDROXYL RADICALS
ICE
INFRARED SPECTRA
METHANE
METHANOL
OXYGEN
PHOTOCHEMISTRY
PHOTOLYSIS
PLANETS
PROTOSTARS
STARS
WATER
ALCOHOLS
ALKANES
ALKENES
CARBON COMPOUNDS
CARBON OXIDES
CHALCOGENIDES
CHEMICAL REACTIONS
CHEMISTRY
DECOMPOSITION
ELEMENTS
HYDRIDES
HYDROCARBONS
HYDROGEN COMPOUNDS
HYDROXY COMPOUNDS
NITROGEN COMPOUNDS
NITROGEN HYDRIDES
NONMETALS
ORGANIC COMPOUNDS
ORGANIC SULFUR COMPOUNDS
OXIDES
OXYGEN COMPOUNDS
PHOTOCHEMICAL REACTIONS
RADICALS
SPECTRA
SPECTROSCOPY
SULFIDES
SULFUR COMPOUNDS