UV pumping of hyperfine transitions in the light elements, with   application to 21-cm hydrogen and 92-cm deuterium lines from the early   universe
  We present new analytic calculations of the coupling between ultraviolet resonance photons and the population of the hyperfine states in the light elements (H, D, He3+) which include several previously neglected physical processes. Among these are the backreaction of resonant scattering on the pumping radiation, the scattering of Ly_beta photons and the effect of local departure from pure Hubble flow. The application of the new treatment to the redshifted hydrogen 21 and deuterium 92 cm lines from the high-redshift universe results in an amplitude correction of up to an order of magnitude. We further show that the standard assumption that ultraviolet pumping drives the spin temperature towards the kinetic temperature does not hold for deuterium, whose spin temperature is generally negative. 
Leonid Chuzhoy and Paul R. Shapiro