Molecular hydrogen production from amorphous solid water during low energy electron irradiation

Area of Science:

• Astrochemistry
• Materials Science
• Surface Science

Background:

• Amorphous solid water (ASW) is a crucial component in interstellar ices.
• Understanding molecular formation and diffusion in ASW is vital for astrochemical models.
• Electron irradiation is a key process for simulating space environments and driving chemical reactions.

Purpose of the Study:

• To investigate the production of molecular hydrogen isotopologues (H₂, HD, D₂) from layered ASW films.
• To determine how electron irradiation energy and depth of buried D₂O layers influence isotopologue yields.
• To model the diffusion and escape mechanisms of produced molecules.

Main Methods:

• Irradiation of layered H₂O/D₂O and D₂O/H₂O thin films with low-energy electrons (400–500 eV).
• Analysis of molecular hydrogen isotopologue production as a function of irradiation time and depth.
• Utilizing ultrahigh vacuum conditions to maintain sample integrity.
• Development of a random-walk model to interpret diffusion effects.

Main Results:

• H₂ production is consistent, reflecting the H₂O content, with a slight dip when products escape the D₂O layer.
• HD and D₂ production show distinct peaks corresponding to escape from the buried D₂O layer.
• Peak broadening for HD and D₂ indicates diffusion, with greater broadening at deeper formation depths.
• The random-walk model qualitatively explains the observed peak profiles.

Conclusions:

• Electron irradiation effectively produces molecular hydrogen isotopologues in ASW.
• Diffusion plays a significant role in the spatial distribution of HD and D₂ within the ASW film.
• The study provides insights into molecular processing in icy bodies relevant to astrochemistry.