The abstract of the new Nature paper Astrophysical detection of the helium hydride ion HeH+ tell us that infrared spectroscopy from SOFIA detected

...rotational ground-state transition of HeH+ at a wavelength of 149.1 micrometres in the planetary nebula NGC 7027.

It also says:

In this metal-free and low-density environment, neutral helium atoms formed the Universe’s first molecular bond in the helium hydride ion HeH+ through radiative association with protons. As recombination progressed, the destruction of HeH+ created a path to the formation of molecular hydrogen.

Question: Why was helium hydride (HeH+) the universe's first molecule? Why was it the hydrogen helium cation rather than the dihydrogen cation?

  • 4
    $\begingroup$ My guess is that the large ionization potential of He caused He+ to grab electrons while it was still too hot for H+ to do so, but H already outnumbered He by 12 to 1 at that time, so the neutral He was more likely to bump into an H+ than another He[+] (and HH[+] couldn't form until it was cool enough for some neutral H). $\endgroup$ – amI Apr 18 at 2:15
  • $\begingroup$ @amI that's a pretty convincing-sounding guess! $\endgroup$ – uhoh Apr 18 at 23:38
  • $\begingroup$ @aml -- That's pretty much exactly right, as I understand it. $\endgroup$ – Peter Erwin Apr 21 at 9:06
  • $\begingroup$ @ami Seems reasonable that He+ would be the first with a bound electron, but He+ + H+ would give $HeH^{2+}$ and I don't think that is actually bonded. Where does the second electron come from? $\endgroup$ – Steve Linton Apr 21 at 10:04
  • 2
    $\begingroup$ @SteveLinton -- It's "He + H+ -> HeH+", not "He+ + H+ -> HeH+". The idea is that first you get He+ (no molecules form), and a little later you get He (neutral), which can bond with H+. And then a little later the H+ starts recombining and the rules change. $\endgroup$ – Peter Erwin Apr 21 at 10:26

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