Cosmological production of H2 before the formation of the first galaxies

Previous calculations of the pre-galactic chemistry have found that a small amount of H-2, x[H-2]equivalent to n[H-2]/n[H] approximate to 2.6 x 10(-6), is produced catalytically through the H-, H-2(+) and HeH+ mechanisms. We revisit this standard calculation taking into account the effects of the non-thermal radiation background produced by cosmic hydrogen recombination, which is particularly effective at destroying H- via photodetachment. We also take into consideration the non-equilibrium level populations of H-2(+), which occur since transitions among the rotational-vibrational levels are slow compared to photodissociation. The new calculation predicts a final H-2 abundance of x[H-2] approximate to 6 x 10(-7) for the standard cosmology. This production is due almost entirely to the H- mechanism, with similar to 1 per cent coming from HeH+ and similar to 0.004 per cent from H-2(+). We evaluate the heating of the diffuse pre-galactic gas from the chemical reactions that produce H-2 and from rotational transitions in H-2, and find them to be negligible.