Formation of CO2 and H2CO3 through non-energetic surface reactions at lowtemperatures.
Y. Oba,1 N. Watanabe,1 A. Kouchi,1 T. Hama,1 and V. Pirronello2
1Institute of Low Temperature Science, Hokkaido University, Japan
2DMFCI, Università di Catania, Italy

INTRODUCTION
Carbon dioxide (CO2) is one of the most abundant solid components in icy grain
mantles of dense molecular clouds. It is well known that CO2 can be synthesized from purecarbon monoxide (CO) or CO-containing ices through energetic processes such as UV
irradiation, ion irradiation, and electron irradiation [e.g. 1]. However, recent detections ofabundant CO2 in dense clouds observed toward background stars, in which the UV field isrelatively weak, suggest that some reactions without UV irradiation may also contribute to theCO2 formation in these environments [2]
In contrast to CO2, carbonic acid (H2CO3) has never been detected either in the gas
phase or solid phase in molecular clouds. However, it is considered that H2CO3 may have arole in acid-base chemistry, such as in reactions with NH3 to yield NH4+, in astrophysicalenvironments [3]. Therefore, elucidating the formation pathways of H2CO3 and its
astrophysical significance is necessary for understanding chemical reaction networks related
to the presence of CO in molecular clouds.
In the present study, we demonstrate formation of CO2 and H2CO3 through surface
reactions of CO with non-energetic OH radicals at low temperatures.
EXPERIMENTAL
Experiments were performed using the Apparatus for SUrface Reaction in Astrophysics(ASURA) system. ASURA consists of a main chamber, an atomic source, and a Fouriertransform infrared (FTIR) spectrometer. Hydroxyl (OH) radicals, produced by dissociatingwater molecules in microwave-induced plasma, were cooled down to 100 K prior to reactwith CO. OH and CO were codeposited onto an Al substrate (10–40 K) and the reaction
products were monitored in-situ by infrared reflection-absorption spectroscopy.
RESULTS
When CO molecules were simultaneously introduced together with OH radicals onto
the substrate, formation of CO2 was clearly identified in the reaction product even at 40 K. In
addition to CO2, several peaks were observed in the IR spectrum. It has been theoreticallydemonstrated that gas phase synthesis of CO2 by CO + OH proceeds through the following
pathways: CO + OH → trans-HOCO → cis-HOCO → CO2 + H [4]. Detection of both HOCO
radicals in the reaction product at 10 K further supports an assumption that CO2 formed
through the above reaction pathways in the present experiment.
After the sample was warmed up to 220 K, some peaks were still observed in the IRspectrum, which were attributable to carbonic acid (H2CO3). Formation of H2CO3 was also
demonstrated by the temperature-programmed desorption spectra of the reaction product. It is
likely that H2CO3 formed through reactions of trans- and cis-HOCO radicals with OH.
The present study demonstrated that CO2 and H2CO3 can form inside dense molecular
clouds without external energy inputs from UV, ions, and electrons.
References
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[2]
[3]
[4]

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