Formation of Compact Amorphous H2o Ice by Codeposition of Hydrogen Atoms With Oxygen Molecules on Grain Surfaces

Formation of H 2 O molecules through the codeposition of oxygen molecules and hydrogen atoms is examined in situ using IR spectroscopy at 10–40 K under various O 2 and H fluxes. It is found that H 2 O and H 2 O 2 are continuously formed by reaction, even at 40 K. The H 2 O ice formed is amorphous, but has a compact (not microporous) structure compared to vapor-deposited amorphous H 2 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 O 2 is one of the potential routes for reproducing these IR spectral characteristics. The composition of the ice formed by codeposition varies with the O 2 /H ratio and temperature. Although no data are available at present for the H 2 O/H 2 O 2 ratio of ice in molecular clouds, this study suggests that hydrogenation of O 2 has a potential to yield a H 2 O/H 2 O 2 ratio of 5 or more in molecular clouds.