Trihydrogen Cation - History

History

H₃+ was first discovered by J.J. Thomson in 1911. While studying the resultant species of plasma discharges, he discovered something very odd. Using an early form of mass spectrometry, he discovered a large abundance of a molecular ion with a mass-to-charge ratio of 3. He stated that the only two possibilities were C4+ or H₃+. Since C4+ would be very unlikely and the signal grew stronger in pure hydrogen gas, he correctly assigned the species as H₃+.

The formation pathway was discovered by Hogness & Lunn in 1925. They also used an early form of mass spectrometry to study hydrogen discharges. They found that as the pressure of hydrogen increased, the amount of H₃+ increased linearly and the amount of H₂+ decreased linearly. In addition, there was little H+ at any pressure. This data suggested the proton exchange formation pathway discussed below.

In 1961, Martin et al. first suggested that H₃+ may be present in interstellar space given the large amount of hydrogen in interstellar space and its reaction pathway was exothermic (~1.5 eV). This led to the suggestion of Watson and Herbst & Klemperer in 1973 that H₃+ is responsible for the formation of many observed molecular ions.

It was not until 1980 that the first spectrum of H₃+ was discovered by Takeshi Oka, which was of the ν₂ fundamental band using a technique called frequency modulation detection. This started the search for interstellar H₃+. Emission lines were detected in the late 1980s and early 1990s in the ionospheres of Jupiter, Saturn, and Uranus.

In 1996, H₃+ was finally detected in the interstellar medium (ISM) by Geballe & Oka in two molecular interstellar clouds in the sightlines GL2136 and W33A. In 1998, H₃+ was unexpectedly detected by McCall et al. in a diffuse interstellar cloud in the sightline Cygnus OB2#12. In 2006 Oka announced that H₃+ was ubiquitous in interstellar medium, and that the Central Molecular Zone contained a million times the concentration of ISM generally.