The lithium chemistry of the early universe

A comprehensive chemistry of lithium in the postrecombination epoch is presented, and calculations are carried out of the abundances of Li, Li+, Li-, LiH, and LiH+ as a function of redshift z for several cosmological models. Atomic lithium is found to recombine at a redshift of about 450 but to remain significantly ionized with a fractional ionization of approximately 0.3 as z --> 0 due to the scarcity of electrons after hydrogen recombination. With the inclusion of a new quantal rate coefficient for the radiative association of lithium and hydrogen, the calculated fractional abundance of LiH is about 100 times smaller than in previous studies. The inclusion of additional gas-phase chemical reactions results in a further reduction by another factor of about 100. The fractional abundance of LiH+ is predicted to approach 10% of that of the neutral LiH as z --> 0, but LiH+ is much less abundant than LiH for z > 30. The fraction of lithium that is taken up in molecular form in the postrecombination epoch is predicted to be about 10(-7). Such small molecular abundances indicate that Thomson scattering between cosmic background radiation (CBR) photons and primordial LiH and LiH+ plays an insignificant role in erasing primary anisotropies in the CBR field.