# Was helium hydride really the first molecule?

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?

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

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.
• 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. – ProfRob Mar 20 '20 at 14:29