CO₂ formation in quiescent clouds: AN experimental study of the CO + OH pathway

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 CO ₂ 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 . Astrophysically, the presence of CO₂ in low A_V regions of molecular clouds could be explained by the reaction CO+ OH occurring concurrently with the formation of H₂O via the route OH + H.