Hirata, Christopher M. and Switzer, Eric R. (2008) Primordial helium recombination. II. Two-photon processes. Physical Review D, 77 (8). Art. No. 083007. ISSN 0556-2821. http://resolver.caltech.edu/CaltechAUTHORS:HIRprd08b
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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 He i line radiation; (iii) two-photon decays from highly excited levels in He i (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 10^4.
|Additional Information:||©2008 The American Physical Society. (Received 11 March 2007; published 30 April 2008) C.H. acknowledges support from the Institute for Advanced Study. E.S. acknowledges the support from grants from NASA No. LTSAA03-000-0090 and NSF No. PHY-0355328. We acknowledge useful conversations with Jens Chluba, Bruce Draine, Jim Peebles, Doug Scott, Uroš Seljak, and Rashid Sunyaev. We also thank Joanna Dunkley for critical readings and comments prior to publication.|
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|Deposited On:||15 Jul 2008 00:15|
|Last Modified:||26 Dec 2012 10:09|
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