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 cosmol ogical models. Atomic lithium is found to recombine at a redshift of a bout 450 but to remain significantly ionized with a fractional ionizat ion of approximately 0.3 as z --> 0 due to the scarcity of electrons a fter 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 tha n in previous studies. The inclusion of additional gas-phase chemical reactions results in a further reduction by another factor of about 10 0. The fractional abundance of LiH+ is predicted to approach 10% of th at 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). S uch small molecular abundances indicate that Thomson scattering betwee n cosmic background radiation (CBR) photons and primordial LiH and LiH + plays an insignificant role in erasing primary anisotropies in the C BR field.