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The chemistry and excitation of H2 and HD in the early Universe

Faure, A; Hily-Blant, P; Pineau des Forêts, G; Flower, D R

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A Faure

P Hily-Blant

G Pineau des Forêts


We have critically reviewed the literature pertaining to reactions that are significant for the chemistry of hydrogen-, deuterium-, and helium-bearing species in the homogeneous early Universe. For each reaction rate coefficient, we provide a fit in the modified-Arrhenius form, specifying the corresponding uncertainty and temperature range. This new network, limited to 21 reactions, should be the most reliable to date. Combined with accurate state-to-state rate coefficients for inelastic and reactive collisions involving H2 and HD, it allows us for the first time to follow the evolution of the abundances of atomic and molecular species, level populations of H2 and HD, and the ortho:para ratio (OPR) of H2, in a self-consistent fashion during the adiabatic expansion of the universe. The abundances of H2 and HD change only marginally compared to previous models, indicating that the uncertainties on the main reaction rate coefficients have essentially been removed. We also find that the adiabatic expansion has a dramatic effect on the OPR of H2, which freezes-out at redshifts z ≲ 50. In contrast, at higher redshifts, the populations of the rotational levels of H2 and HD are predicted to be fully thermalized at the temperature of the cosmic background radiation field, a result that conflicts with some recent, independent calculations. This new network allows the chemistry of primordial gas to be followed during the early phase of collapse towards Population III star progenitors.


Faure, A., Hily-Blant, P., Pineau des Forêts, G., & Flower, D. R. (2024). The chemistry and excitation of H2 and HD in the early Universe. Monthly Notices of the Royal Astronomical Society, 531(1), 340-354.

Journal Article Type Article
Acceptance Date Apr 9, 2024
Online Publication Date Apr 11, 2024
Publication Date May 13, 2024
Deposit Date Jul 8, 2024
Publicly Available Date Jul 8, 2024
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Volume 531
Issue 1
Pages 340-354
Public URL


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