HD in the primordial gas

We study the role of HD in the thermal balance of the primordial gas, beyond the redshift, z, at which the temperatures of radiation and matter have decoupled (z approximate to 300). Statistical arguments are used to derive the rate constants for the forward and reverse reactions, D+(H-2, HD)H+, involving reactant and product molecules in excited rotational states. The degree of chemical fractionation of HD is enhanced, compared with the value calculated by taking account of reactions between ground-state molecules only, by a factor of about 2. In spite of its low abundance (10(-3)), relative to H-2, HD contributes comparably to the rate of heating of the gas, through rotationally inelastic collisions with H and He. The much larger rate coefficients for collisional population transfer within HD, compared with H-2, and the tighter rotational level spacing are responsible for this finding. We conclude that HD is about as important as H-2 in the thermal balance of the primordial gas.