TITLE:
Primordial helium recombination II: two-photon processes.
AUTHOR(S):
Christopher M. Hirata (IAS); Eric R. Switzer (Princeton).
DATE:
2007 Feb 06 (arXiv, posted, v1);
2007 Mar 11 (Phys. Rev. D, submitted);
2008 Feb 12 (revised);
2008 Feb 24 (Phys. Rev. D, accepted);
2008 Apr 30 (Phys. Rev. D, published).
AVAILABILITY:
arXiv astro-ph/0702144 (free);
APS (requires subscription).
PUBLICATION INFORMATION:
Phys. Rev. D 77, 083007 (2008).
ABSTRACT:
Interpretation of precision measurements of the cosmic microwave background (CMB) will require a detailed understanding of the recombination era, which
determines such quantities as the acoustic oscillation scale and the Silk damping scale. This paper is the second in a series devoted to the subject of
helium recombination, with a focus on two-photon processes in He I. The standard treatment of these processes includes only the spontaneous two-photon
decay from the 21S level. We extend this treatment by including five additional effects, some of which have been suggested in recent papers
but whose impact on He I recombination has not been fully quantified. These are: (i) stimulated two-photon decays; (ii) two-photon absorption of
redshifted HeI line radiation; (iii) two-photon decays from highly excited levels in HeI (n1S and n1D, with n>=3); (iv) Raman
scattering; and (v) the finite width of the 21Po resonance. We find that effect (iii) is highly suppressed when one takes into
account destructive interference between different intermediate states contributing to the two-photon decay amplitude. Overall, these effects are found
to be insignificant: they modify the recombination history at the level of several parts in 104.
ADS BIBLIOGRAPHIC CODE: 2008PhRvD..77h3007H
COMMENTS:
This is the second in a series of three papers on helium recombination; see Paper I and Paper
III.