Insights into H2 formation in space from ab initio molecular dynamics

Area of Science:

• Astrochemistry
• Interstellar Medium Physics
• Surface Science

Background:

• Molecular hydrogen (H2) is crucial for interstellar cloud physics and chemistry.
• H2 formation mechanisms are debated for various space environments, from cold clouds to warm regions.
• Carbonaceous dust grain surfaces are the primary suspected sites for H2 formation.

Purpose of the Study:

• Investigate direct Eley-Rideal (E-R) hydrogen recombination on dust grain surfaces.
• Explore the roles of lattice dynamics, surface corrugation, and H-dimer formation.
• Determine the dominant H2 formation mechanism in interstellar conditions.

Main Methods:

• Ab initio molecular dynamics simulations.
• Inclusion of lattice dynamics and surface corrugation effects.
• Analysis of competing H-dimer formation pathways.

Main Results:

• The Eley-Rideal reaction is found to be the dominant H2 formation pathway.
• This mechanism explains observed H2 abundances at interstellar medium energies.
• Facile sticking at specific surface sites (defects, edges) is crucial for the E-R mechanism.

Conclusions:

• The Eley-Rideal mechanism, considering surface complexities, can account for interstellar molecular hydrogen abundance.
• Special surface sites on dust grains play a significant role in facilitating H2 formation.
• Direct recombination offers a viable explanation for H2 in diverse interstellar environments.