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This press release by NASA: https://www.nasa.gov/feature/the-universe-s-first-type-of-molecule-is-found-at-last/

When the universe was still very young, only a few kinds of atoms existed. Scientists believe that around 100,000 years after the big bang, helium and hydrogen combined to make a molecule called helium hydride for the first time. Helium hydride should be present in some parts of the modern universe, but it has never been detected in space — until now.

Naively I would expect that the first molecule to form would be the $H_2$. A claim that $HeH^+$ was before that should mean that it has larger binding energy that allowed the molecules to withstand more radiation. However, if I check Wikipedia for dissociation energies of these molecules, I see $436\, kJ/mol$ for $H_2$ and $360 \, kJ/mol$ for $HeH^+$.

Additionally, Helium hydride is charged and should interact with lower energy photons that will destabilize the bond due to sheer quantity.

And finally, the temperature mentioned in the article is $4000 K \sim 0.34 eV$ — which is quite below the dissociation energy of molecular Hydrogen ($4.52 eV$). Why the formation of molecules was delayed so much? Is this one of those situations where baryon-to-photon ratio is important and photons overwhelm the baryons?

Could someone please explain this situation?

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"Atomic and molecular processes in the early Universe" 2002, Lepp et al describe this process (emphasis mine):

Molecular hydrogen was the first neutral molecule formed in the Universe and remains the most abundant. Because it does not have a dipole moment, molecular hydrogen cannot form directly by a radiative process. The most common reaction paths leading to $H_2$ formation in the early Universe use $H^+_2$ and $H^−$ as intermediaries....

The molecular ion HeH+ was the first to appear...and so at the earliest times it is the first to produce $H_2$. The $HeH^+$ is first converted to the $H^+_2$ molecular ion and then to $H_2$ through the series of reactions

$H^+$ + $He$$HeH^+$ + $ν$,
$HeH^+$ + $H$$H^+_2$ + $He$
$H^+_2$ + $H$$H_2$ + $H^+$

where both protons and He acted as catalyst, being returned to the gas once H2 is formed to start the process again.

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  • $\begingroup$ Thanks, this is sufficient to dig deeper $\endgroup$ – Andrii Magalich Apr 21 at 10:55

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