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?


"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.

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

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