The dependences on z of fractional number densities of H + and He + ions are calculated with a proper allowance for two-photon decays of upper levels of hydrogen and parahelium and radiative transfer in intercombination line 2 3 P 1 ↔ 1 1 S 0 of helium. It is shown that for hydrogen this gives corrections for a degree of ionization in no more than a few percents but for helium this leads to a significant acceleration of recombination compared to the recent papers by Seager et al. (1999;2000) where these effects were ignored.

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Abstract. It is shown that taking into account a recoil for radiation scattering in the Ly-α line can lead to a noticable acceleration of primordial hydrogen recombination. Thus for ΛCDM model a decrease of ionization degree exceeds 1% for redshifts z in a range 800 -1050 achieving ≈ 1.3% at z = 900. Corresponding corrections to the cosmic microwave background power spectra can achieve 1.1% for T T spectra and 1.7% for EE ones. Radiative transfer in these calculations was treated in a quasistationary approximation. Numerical solutions are also obtained in diffusion approximation for a nonstationary problem of Ly-α line radiative transfer under partial frequency redistribution with a recoil. An evolution of a local line profile is traced to as well as an evolution of a relative number of uncompensated transitions from 2p state down to 1s one. It is shown that taking into account nonstationarity of Ly-α line radiative transfer can lead to an additional acceleration of primordial hydrogen recombination.