Branching ratio and energy distribution in grain-surface H₂ formation

Determine the exact branching ratio by which the energy released during H₂ formation on interstellar grains is converted into internal excitation, translation, and grain heating, and characterize how the resulting, potentially uneven energy distribution depends on the physical conditions in the photodissociation region and the nature of the grains.

Background

The paper discusses H₂ formation on the surfaces of interstellar grains, where the grains act as catalysts. It notes that energy equipartition among internal excitation, translation, and grain heating is commonly assumed, with one-third of the reaction energy assigned to internal excitation.

The authors explicitly identify the adopted branching ratio as unresolved and further state that the distribution of the released energy may be uneven and may vary with both the physical conditions in the photodissociation region and the properties of the grains. Resolving this issue is important for modeling H₂ excitation and interpreting infrared emission from photodissociation regions.