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Session 12 - Cosmology, Large-Scale Structure and Distance Scales.
Display session, Monday, June 10
Great Hall,

[12.10] Stimulated Radiative Association and its Effect on the Production of LiH in the Early Universe

A. Dalgarno (CfA), P. C. Stancil (UNLV)

The production of LiH in the postrecombination epoch was previously investigated by Stancil et al. (1996, ApJ, 458, 401) who incorporated quantal rate coefficients for the (spontaneous) radiative association reaction Li + H \to LiH + \nu (Dalgarno et al. 1996, ApJ, 458, 397) into a comprehensive lithium chemistry. The fractional abundance [LiH]/[H] was found to approach \sim 10^-16 for a redshift z\sim 10; an abundance so small as to limit the utility of LiH in erasing cosmic background radiation (CBR) anisotropies through Thompson scattering (Maoli et al. 1994, ApJ, 425, 374) or to discriminate between various big bang nucleosynthesis models (Signore et al. 1994, ApJS, 92, 535).

It was suggested by Dubrovich (1995, private communication) that the production of LiH could be enhanced by stimulated radiative association due to the microwave CBR field. Using a fully quantal approach (cf. Dalgarno et al. 1996), we have determined stimulated radiative association rate coefficients for the formation of LiH over a range of matter temperatures T_m and black-body radiation temperatures T_r. We find, with a slight T_m dependence, the total rate coefficients to be enhanced by factors of about 1.1, 1.3, and 4 for T_r = 500, 1000, and 5000 K, respectively. Below T_r = 100 K there is no significant enhancement.

Inclusion of stimulated radiative association into the early universe chemistry results in a slight increase of the LiH fractional abundance by a maximum of \sim25% for \sim 350 > z > 150. For higher z, the enhancement is negligible since the fractional abundance is small due to a high dissociation efficiency. For z < 100, the enhancement is also insignificant since T_r<550 K and LiH is predominantly formed through the associative detachment reaction Li^- + H \to LiH + e^-. The conclusions in Stancil et al. (1996) regarding the attenuation of CBR anisotropies and constraints on the lithium primordial abundance remain unchanged.

Funding provided by NSF grants OSR-9353227 and AST 93-01099.

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