Ab-initio dynamic study of mechanisms for dust-mediated molecular hydrogen formation in space.

The reason for the abundance of molecular hydrogen (H) in space remains unresolved. Here we study collision dynamics under spacelike conditions to test H formation mechanisms where carbonaceous dust grains may have a catalytic role. Density functional theory molecular dynamics simulates atomic hydrogen capture and H formation on the surface of buckminsterfullerene as a carbonaceous cosmic dust model. Maximally localized Wannier functions are applied to examine the electronic bonding during transition states. The fullerene surface is shown to be effective at warm (50K) and low (10K) temperatures in achieving atomic H chemisorption, potentially explaining the observed broad temperature range for efficient H formation. We revise the Eley-Rideal mechanism and propose that both it and the Langmuir-Hinshelwood mechanism, induced by thermal hopping, contribute to bursts of H formation during energetic events. Additionally, we show how fullerene maintains the abundance of H in space by selectively preventing H molecules from capture.