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Helonium... the first molecule of the Universe!

What is Helonium?
Credit: Mondolithic studios
Helonium or Helium hydride is a molecular ion consisting of a helium atom and a hydrogen ion (HeH+) and it is considered to be the first molecule of the universe. It is viewed as a protonated helium and it is the lightest heteronuclear ion.

Discovery
Helonium was first produced in the laboratory in Berkeley in 1925, under vacuum conditions with the aid of electricity by injecting protons of known energy into a rarefied mixture of hydrogen and helium. Although it is stable in isolation, it cannot be prepared in bulk because it is extremely reactive.


Properties
  • It is the strongest known acid with pKa -63 (for comparison hydrochloric acid pKa is -6.3)
  • It is isoelectronic with molecular hydrogen H2
  • Its covalent bond is dative with a length of 0.772 A
  • 80% of the electron charge is closer to the helium nucleus than to the hydrogen nucleus
  • Unlike the helium hydride ion, the neutral helium hydride molecule HeH is not stable in the ground state
Detection
HeH+ has been conjectured to exist in the interstellar medium since the 1970s and its first detection was reported in an article published in the journal Nature in April 2019.

Helonium detected in the nebula NGC 7027 in the Cygnus constellation, located around 3,000 light-years from earth. NGC 7027 is about 600 years old and one of the brightest planetary nebulae and its shell of released stellar material is still rather compact and dense, because of these characteristics it is rendered a natural candidate for a search for HeH+.

How it was detected?
The deployment of the German Receiver for Astronomy at Terahertz Frequencies (GREAT) heterodyne spectrometer onboard the Stratospheric Observatory for Infrared Astronomy (SOFIA) has now opened up new opportunities.
Sofia is a Boeing 747SP aircraft modified to carry a 2.7-meter (106-inch) reflecting telescope. Flying into the stratosphere at 38,000-45,000 feet puts SOFIA above 99 percent of Earth’s infrared-blocking atmosphere, allowing astronomers to study the solar system and beyond in ways that are not possible with ground-based telescopes.

SOFIA is gathering data to study:
  • Star birth and death
  • ​Formation of new solar systems
  • Identification of complex molecules in space
  • Planets, comets, and asteroids in our solar system
  • Nebulas and galaxies
  • Celestial magnetic fields
  • Black holes at the center of galaxies
The latest advances in terahertz technologies have enabled the operation of the high-resolution spectrometer upGREAT at frequencies above 2 THz, allowing the HeH+ J = 1–0 line to be targeted at 149.137 µm (where J is total angular momentum) was impaired by the limited resolving power of the spectrometer (Δλ = 0.6 µm), which did not allow the HeH+ transition to be separated from the nearby Λ-doublet of the methylidyne radical (CH) at 149.09 µm and 149.39 µm.

Credit: Scientific America

Importance of Helonium
Helonium formation opens up the path for molecular hydrogen and polyatomic hydrogen ions formation, which will react with O and C for the formation of water and any other chemical compound required for life's existence.


References:  Wikipedia ~ Nature (Gunsen et al 2019)  ~ Nasa.gov

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