2009ApJ...701..464O

Formation of H₂ O molecules through the codeposition of oxygen molecules and hydrogen atoms is examined in situ using IR spectroscopy at 10-40 K under various O₂ and H fluxes. It is found that H₂O and H₂O₂are continuously formed by reaction, even at 40 K. The H₂ O ice formed is amorphous, but has a compact (not microporous) structure compared to vapor-deposited amorphous H₂ O ice, because dangling OH bonds are not observed in the IR spectrum. This is consistent with astronomical observations in molecular clouds and theoretical predictions, which suggest that hydrogenation of O₂ is one of the potential routes for reproducing these IR spectral characteristics. The composition of the ice formed by codeposition varies with the O₂/H ratio and temperature. Although no data are available at present for the H₂O/H₂O₂ ratio of ice in molecular clouds, this study suggests that hydrogenation of O₂ has a potential to yield a H₂O/H₂O₂ratio of 5 or more in molecular clouds.