Chemistry in the Early Universe

Abstract

The chemical processes which led to the formation of molecules in the early universe are described. Molecular hydrogen is formed by two sequences. in one, radiative attachment to form H − is followed by associative detachment and in the other radiative association to form H 2 + is followed by a chemical reaction with H. Trace amounts of HD and the molecular ion LiH + are formed by the reaction of D + with H 2 and by the radiative association of Li + and H. The H 2 molecular fraction in the expanding universe is about 10 −6 . Because they are cooling agents, hydrogen molecules are significant in the collapse of pre-galactic clouds. With increasing density the hydrogen gas is converted to molecular hydrogen by three-body recombination and emission from the rotational and vibrational levels cools the cloud and slows the rise in temperature of the gravitationally collapsing object. Ultimately though, the radiation is trapped and the temperature rises. The H 2 molecules are then destroyed by collision-induced dissociation. At still higher temperatures collisional ionization occurs, initiated most probably by the process of associative ionization. Molecules may be significant also in galaxy formation if the explosive amplification model is correct. Heavy elements may be present in the shell of gas swept up by the blast waves of the exploded seed galaxies and chemical processes leading to the formation of molecular hydrides may take place, in a scenario analogous to the aftermath of an interstellar dissociative shock.