Abstract
The formation of water (H2O) in the interstellar medium is intrinsically linked to grain-surface chemistry; thought to involve reactions between atomic (or molecular) hydrogen with atomic oxygen (O), molecular oxygen (O-2), and ozone (O-3). Laboratory precedent suggests that H2O is produced efficiently when O-2 ices are exposed to H atoms (similar to 100 K). This leads to the sequential generation of the hydroxyperoxyl radical (HO2), then hydrogen peroxide (H2O2), and finally H2O and a hydroxyl radical (OH); despite a barrier of similar to 2300K for the last step. Recent detection of the four involved species toward rho Oph A supports this general scenario; however, the precise formation mechanism remains undetermined. Here, solid O-2 ice held at 12K is exposed to a monoenergetic beam of 5 keV D+ ions. Products formed during the irradiation period are monitored through FTIR spectroscopy. O-3 is observed through seven archetypal absorptions. Three additional bands found at 2583, 2707, and 1195 cm(-1) correspond to matrix isolated DO2 (nu(1)) and D2O2 (nu(1),nu(5)), and D2O (nu(2)), respectively. During subsequent warming, the O-2 ice sublimates, revealing a broad band at 2472 cm(-1) characteristic of amorphous D2O (nu(1),nu(3)). Sublimating D-2, D2O, D2O2, and O-3 products were confirmed through their subsequent detection via quadrupole mass spectrometry. Reaction schemes based on both thermally accessible and suprathermally induced chemistries were developed to fit the observed temporal profiles are used to elucidate possible reaction pathways for the formation of D-2-water. Several alternative schemes to the hydrogenation pathway (O-2 -> HO2 -> H2O2 -> H2O) were identified; their astrophysical implications are briefly discussed.