FORMATION OF COMPACT AMORPHOUS H₂O ICE BY CODEPOSITION OF HYDROGEN ATOMS WITH OXYGEN MOLECULES ON GRAIN SURFACES

Abstract

Formation of H2O 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 H2O2 are continuously formed by reaction, even at 40 K. The H2O ice formed is amorphous, but has a compact (not microporous) structure compared to vapor-deposited amorphous H2O 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/H2O2 ratio of 5 or more in molecular clouds.