Proton diffusion and hydrogen/deuterium exchange in amorphous solid water at temperatures from 114 to 134 K

Area of Science:

• Physical Chemistry
• Surface Science
• Materials Science

Background:

• Understanding proton behavior in amorphous solid water (ASW) is crucial for various chemical and physical processes.
• Previous studies have explored proton mobility but lacked detailed characterization within ASW films.

Purpose of the Study:

• To experimentally determine the reaction coefficient for hydrogen/deuterium (H/D) exchange.
• To characterize the diffusion of hydrated excess protons in ASW as a function of temperature.
• To investigate the distance-dependent distribution of protons within ASW films.

Main Methods:

• Water films were deposited on a Pt(111) substrate and exposed to hydrogen atoms to form hydrated protons.
• Proton diffusion and H/D exchange were monitored by observing D2O probe molecules using infrared spectroscopy upon heating.
• The spatial distribution of protons was probed by varying the distance of D2O molecules from the substrate.

Main Results:

• Proton diffusion and equilibrium distribution within the ASW film were observed between 114 K and 134 K.
• Proton concentration decayed with distance as x-2, consistent with an image charge electric field effect.
• Proton diffusion coefficients were estimated to range from 10^-20 m²/s at 114 K to 10^-18 m²/s at 134 K.
• An activation energy of 0.40 eV for proton diffusion was determined.

Conclusions:

• The diffusion of hydrated excess protons in ASW is significantly influenced by an image charge electric field.
• The activation energy suggests that proton diffusion is coupled to the dynamics of water molecules (translation and rotation).
• These findings provide critical insights into proton transport mechanisms in amorphous ice relevant to astrochemistry and materials science.