Experimental Studies on the Formation of D₂o and D₂o₂by Implantation of Energetic D⁺ions Into Oxygen Ices

The formation of water (H 2 O) 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 H 2 O is produced efficiently when O 2 ices are exposed to H atoms (∼100 K).This leads to the sequential generation of the hydroxyperoxyl radical (HO 2 ), then hydrogen peroxide (H 2 O 2 ), and finally H 2 O and a hydroxyl radical (OH); despite a barrier of ∼2300 K for the last step.Recent detection of the four involved species toward ρ Oph A supports this general scenario; however, the precise formation mechanism remains undetermined.Here, solid O 2 ice held at 12 K 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 DO 2 (ν 1 ) and D 2 O 2 (ν 1 , ν 5 ), and D 2 O (ν 2 ), respectively.During subsequent warming, the O 2 ice sublimates, revealing a broad band at 2472 cm -1 characteristic of amorphous D 2 O (ν 1 , ν 3 ).Sublimating D 2 , D 2 O, D 2 O 2 , 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 →HO 2 →H 2 O 2 →H 2 O) were identified; their astrophysical implications are briefly discussed.