Surface Temperature Dependence of Hydrogen Ortho-Para Conversion on Amorphous Solid Water.

Area of Science:

• Physical Chemistry
• Surface Science
• Astrochemistry

Background:

• The ortho-to-para conversion of hydrogen is crucial for understanding interstellar medium chemistry and thermal balance in molecular clouds.
• Amorphous solid water is a prevalent surface in astrophysical environments, acting as a potential catalyst for chemical reactions.

Purpose of the Study:

• To investigate the surface temperature dependence of the ortho-to-para conversion of H_{2} on amorphous solid water.
• To elucidate the underlying mechanism governing this conversion process at low temperatures.

Main Methods:

• Utilized a combination of photostimulated desorption and resonance-enhanced multiphoton ionization techniques.
• Probed the H_{2} conversion on amorphous solid water surfaces within a temperature range of 9.2–16 K.

Main Results:

• The conversion time exhibited a steep variation from approximately 4.1×10^{3} to 6.4×10^{2} seconds within a narrow 8 K temperature window.
• A significant surface temperature dependence of the ortho-to-para conversion rate was observed.

Conclusions:

• The study provides the first report on the surface temperature dependence of H_{2} ortho-to-para conversion on amorphous solid water.
• The two-phonon process is identified as the most probable mechanism dominating the conversion rate at low temperatures.
• These findings contribute to a better understanding of hydrogen conversion in astrophysical icy environments.