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 $436\,\rm kJ/mol$ for H$_2$ and $360 \,\rm kJ/mol$ 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 $4000\rm\, K \sim 0.34 \, eV$ — which is quite below the dissociation energy of molecular Hydrogen ($4.52 \,\rm eV$). Why was the formation of molecules 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?

  • $\begingroup$ See the question that has been closed as a duplicate for an alternative explanation. $\endgroup$
    – ProfRob
    Mar 20, 2020 at 14:25
  • $\begingroup$ @RobJeffries could you share the link please? $\endgroup$ Mar 20, 2020 at 14:31
  • $\begingroup$ physics.stackexchange.com/questions/530851/… $\endgroup$
    – ProfRob
    Mar 20, 2020 at 14:32
  • $\begingroup$ @RobJeffries nice, thanks! $\endgroup$ Mar 20, 2020 at 14:35

1 Answer 1


In "Atomic and molecular processes in the early Universe" (eprint) 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

\begin{align} \rm H^+ + He & \to \rm HeH^+ + \nu \\ \rm HeH^+ + H & \to \rm H^+_2 + He \\ \rm H^+_2 + H & \to \rm H_2 + H^+, \end{align}

where both protons and He acted as catalyst, being returned to the gas once H$_2$ is formed to start the process again.

  • 1
    $\begingroup$ There is also a H + H$^{*}$ interaction that produces H$_2$.. The important point is that all these reaction routes to H$_2$ require hydrogen atoms. $\endgroup$
    – ProfRob
    Mar 20, 2020 at 14:29

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