  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.

Astrophysics