CO₂ formation in quiescent clouds: an experimental study of the CO + OH Pathway

Noble, J. A.; Dulieu, F.; Congiu, E. and Fraser, H. J. (2011). CO₂ formation in quiescent clouds: an experimental study of the CO + OH Pathway. Astrophysical Journal, 735(2) p. 121.

DOI: https://doi.org/10.1088/0004-637X/735/2/121

Abstract

The formation of CO₂ in quiescent regions of molecular clouds is not yet fully understood, despite CO₂ having an abundance of around 10%–34% H₂O. We present a study of the formation of CO₂ via the nonenergetic route CO + OH on nonporous H₂O and amorphous silicate surfaces. Our results are in the form of temperature-programmed desorption spectra of CO₂ produced via two experimental routes: O₂ + CO + H and O₃ + CO + H. The maximum yield of CO₂ is around 8% with respect to the starting quantity of CO, suggesting a barrier to CO + OH. The rate of reaction, based on modeling results, is 24 times slower than O₂ + H. Our model suggests that competition between CO₂ formation via CO + OH and other surface reactions of OH is a key factor in the low yields of CO₂ obtained experimentally, with relative reaction rates of k_CO+H « k_CO+OH< k _H2O2+_H < k_OH+H, k_O2+H. Astrophysically, the presence of CO₂ in low AV regions of molecular clouds could be explained by the reaction CO + OH occurring concurrently with the formation of H2O via the route OH + H.

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