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Ones first thought would be that h atoms can simply

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Unformatted text preview: lens §9.7. 2. The Life Cycle of H2 1 We begin our discussion by investigating the formation and destruction of H2 molecules. A. FORMATION OF H2 Our discussion of molecular gas begins with investigation of how atoms can form molecules. One’s first thought would be that H atoms can simply combine to form H2: 2H → H 2 + γ . In fact this process is forbidden because there is no dipole moment. More precisely, one may view the two H atoms as forming an unbound vibrational state of € H2. The electrons for the initial atoms may form a spin singlet (term: 1 Σ + ) or a spin g triplet (term: 3 Σ + ); neither has an allowed decay to the ground state of H2 ( 1 Σ + ). g u A second possibility would be to use a 3 ­body reaction € 3H → H 2 + H € € or a catalyst: − H + e → H + γ, H - + H → H 2 + e− . € These reactions are slow: the first requires extremely high densities since it requires the simultaneous collision of 3 atoms, and the second requires ionization € (which is low in the cold regions most likely to form molecules) and has a small rate coefficient at low temperatures (selection rules require the initial electron to have odd angular momentum, which implies an angular momentum barrier). These reactions may be important in primordial galaxies, where there are no alternative pathways to form H2, but in the modern ISM H2 can form efficiently on dust grains. The formation of H2 on dust begins with a hydrogen atom being adsorbed onto its surface: the atom collides with the surface, and enough energy is dumped into vibrational modes of the grain that the atom can stick (forming a van der Waals bond). These H atoms then quantum ­mechanically tunnel among the possible adsorption sites on the grain surface, until they react to form H2. The bond energy of H2 (4.5 eV) is sufficient to detach the molecule from the grain and...
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This document was uploaded on 03/08/2014 for the course AY 102 at Caltech.

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