Non-thermal photons and H2 formation in the early Universe

The cosmological recombination of H and He at z similar or equal to 10(3) and the formation of H-2 during the dark ages produce a non-thermal photon excess in the Wien tail of the cosmic microwave background blackbody spectrum. Here, we compute the effect of these photons on the H- photodetachment and H-2(+) photodissociation processes. We discuss the implications for the chemical evolution of the Universe in the post-recombination epoch, emphasizing how important a detailed account of the full vibrational manifold of H-2 and H-2(+) in the chemical network is. We find that the final abundances of H-2, H-2(+), H-3(+) and HD are significantly smaller than in previous calculations that neglected the effect of non-thermal photons. The suppression is mainly caused by extra hydrogen recombination photons and could affect the formation rate of first stars. We provide simple analytical approximations for the relevant rate coefficients and briefly discuss the additional effect of dark matter annihilation on the considered reaction rates.