Universe's First Molecule Is Shrouded in Mystery

Universe's First Molecule Is Shrouded in Mystery
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Somewhere, out there, is the first molecule thought to have formed in the universe. But strangely, scientists have yet to see it in the wild.

That molecule is the helium hydride ion.

Roughly 300,000 years after the Big Bang, the first atoms formed. Back then, there was only hydrogen, helium, and lithium. In this time of chemical simplicity, when the average temperature of the universe was a scorching 4000 degrees Kelvin, helium bonded with an ion of hydrogen. It was the first time two different elements joined together: the original cosmic love story.

13.7 billion years later, chemists here on Earth created the helium hydride ion (HeH+) in the laboratory, concocting an extraordinary molecule that defies the odds. Helium is the most unreactive substance in existence. With two electrons completely filling its innermost energy layer, the element has no desire to seek out electrons from, or share its own, with other elements. Yet, with some prodding, helium renounces its selfish ways to form the stable helium hydride ion. The molecule contains just two electrons: one is solely for helium and the other is shared between helium and the single hydrogen proton. The arrangement renders the helium hydride ion extremely prone to react. In fact, the molecule is the strongest known acid in existence.

Reactivity may be one reason the helium hydride ion has evaded detection by astronomers and astrophysicists. In the lab, it exists only in isolation, as it will protonate any molecule it contacts. Another reason is that the helium hydride ion seems very prone to photodissociation, whereby a photon of light smacks the molecule and grants it enough energy to break the link between the conjoined atoms -- in this case, helium and hydrogen. The helium hydride ion is also notoriously difficult to spot with a spectroscope.

"One of its prominent lines, the ‘fingerprints’ used to identify chemical elements optically, overlaps with lines in the spectrum of the CH methylidene radical," Chemistry World's Brian Clegg explained.

All of these difficulties have repeatedly thwarted astronomers' efforts to detect the helium hydride ion. To date, there have been several attempts, non conclusive. Astronomers will keep trying, however. Finding the elusive helium hydride ion, likely within a nebula or a white dwarf star, would inform long-held models of primordial star formation. It's thought that the molecule acts as a molecular coolant of sorts.

Models predict that the universe is flooded with helium hydride ions. But models can be wrong, a fact that atomic physicist Jérôme Loreau readily admits.

"The abundance of HeH+ mystery remains very much unsolved and it is hoped that our calculations, as well as observations from infrared telescopes such as the Herschel Space Telescope and the Spitzer Space Telescope, can shed some new light on the issue. Indeed, observations... could invalidate current models of the appearance of the first molecules, with consequences on our theories of the formation of stars and galaxies in the early universe."

(Image: NASA)

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