Issue |
A&A
Volume 480, Number 3, March IV 2008
|
|
---|---|---|
Page(s) | 629 - 645 | |
Section | Cosmology (including clusters of galaxies) | |
DOI | https://doi.org/10.1051/0004-6361:20077921 | |
Published online | 17 January 2008 |
Two-photon transitions in hydrogen and cosmological recombination
1
Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching bei München, Germany e-mail: jchluba@mpa-garching.mpg.de
2
Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia
Received:
21
May
2007
Accepted:
22
November
2007
We study the two-photon process for the transitions and
in hydrogen up to large n.
For
we provide simple analytic fitting formulae to describe the
non-resonant part of the two-photon emission profiles. Combining these
with the analytic form of the cascade-term yields a simple and accurate
description of the full two-photon decay spectrum, which only involves a
sum over a few intermediate states.
We demonstrate that the cascade term naturally leads to a nearly Lorentzian
shape of the two-photon profiles in the vicinity of the resonances.
However, due to quantum-electrodynamical corrections, the two-photon
emission spectra deviate significantly from the Lorentzian shape in the very
distant wings of the resonances. We investigate up to which distance the
two-photon profiles are close to a Lorentzian and discuss the role of the
interference term.
We then analyze how the deviation of the two-photon profiles from the
Lorentzian shape affects the dynamics of cosmological hydrogen
recombination.
Since in this context the escape of photons from the Lyman-α
resonance plays a crucial role, we concentrate on the two-photon corrections in
the vicinity of the Lyman-α line.
Our computations show that the changes in the ionization history
due to the additional two-photon process from high shell (
) likely do
not reach the percent-level. For conservative assumptions we find a
correction
at redshift
.
This is numerically similar to the result of another recent study;
however, the physics leading to this conclusion is rather different. In
particular, our calculations of the effective two-photon decay rates
yield significantly different values, where the destructive
interference of the resonant and non-resonant terms plays a crucial role
in this context. We also show that the bulk of the corrections to the
ionization history is only due to the 3s and 3d-states and that the
higher states do not contribute significantly.
Key words: atomic processes / atomic data / radiation mechanisms: general / cosmology: theory / cosmic microwave background
© ESO, 2008
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