Brief Introduction to H₃⁺

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from "H₃⁺- an Ion with Many Talents", B. J. McCall & T. Oka, Science, 287,1941-1942 (2000).

The H₃⁺ ion plays an important role in diverse fields from chemistry to astronomy. Not only does this most fundamental of molecular ions serve as a benchmark for quantum chemists, it was recently discovered both in molecular clouds (1) and in the diffuse interstellar medium (2), and it provides a tool for characterizing Jupiter's atmosphere from afar. At a recent discussion meeting in London (3), chemists, physicists, and astronomers came together to take stock of what is known about H₃⁺ and take a glimpse into its future.

The H₃⁺ molecular ion consists of three protons bound by two electrons and can be thought of as a hydrogen molecule (H₂) with an extra proton attached (H⁺). This ion is the dominant positively charged ion in molecular hydrogen plasmas and was first identified in 1911 by J. J. Thomson, using an early form of mass spectrometry (4). Because H₃⁺ lacks a stable electronic excited state (necessary for electronic spectroscopy) and a permanent dipole moment (necessary for rotational spectroscopy), the only spectroscopic probe of this ion is its infrared rotation-vibration spectrum, which was first observed in the laboratory in 1980 (5).

In the two decades since this initial spectroscopic observation, over 600 spectral lines of H₃⁺ in low-energy ro-vibrational states have been detected. Using state-of-the-art computers, theoretical spectroscopists are now able to reproduce this laboratory spectrum with high accuracy from first principles and provide predictions of new lines to help guide laboratory work. Because H₃⁺ is the simplest polyatomic molecule, these calculations for H₃⁺ serve as a benchmark for calculations on other polyatomic molecules, such as water. In contrast to the low-energy spectrum, theorists have not yet been able to assign any of the over 27,000 spectral lines in the H₃⁺ near-dissociation spectrum (6). If the sensitivity of the low-energy experiments can be substantially increased so that higher energy bands can be studied, and if the near-dissociation experiments can reach lower energies using visible lasers, the two techniques may eventually converge, leading to a complete theoretical understanding of this ion.

A controversy surrounds the recombination of H₃⁺ with electrons (7), the dominant destruction mechanism in some plasmas. In the past three decades, laboratory measurements of this recombination rate have differed by four orders of magnitude. The situation has improved, but discrepancies between different experiments remain, and the rate is still uncertain to within a factor of 10. To make matters worse, the best theoretical estimates of the recombination rate are 100 times lower than the experimental data. This enigma extends from the laboratory to interstellar space: Because the recombination process is the dominant destruction mechanism for H₃⁺ in diffuse clouds, the uncertainty in the electron recombination rate translates to a large uncertainty in the size of the diffuse clouds where H₃⁺ has been measured with the use of its infrared spectrum (2). The importance of reconciling theory and experiment and of reducing the present uncertainty in the value of the H₃⁺ electron recombination rate cannot be overemphasized.

H₃⁺ also plays an important role in planetary science. Ever since it was first spectroscopically detected in emission from Jupiter's aurora (8), the ion has served as a useful remote probe of Jupiter's upper atmosphere (9). Because the strongest spectral lines of H₃⁺ are in a spectral region where few other molecules have lines, H₃⁺ can be observed on Jupiter with only an infrared camera with a narrow filter. With this technique, Jupiter's aurora can be imaged with ground-based telescopes, and the images can be used to create and evaluate detailed models of the jovian magnetosphere and the interaction between Jupiter and its moon Io. As the only hydrogenic species in this environment with efficient spontaneous emission, H₃⁺ is the dominant coolant of the jovian ionosphere. During the meeting, it was proposed that H₃⁺ may also play a major role in the energy budget and the overall dynamics of the jovian magnetosphere. It was even suggested that H₃⁺ emission might be detectable from Jupiter-like planets orbiting other stars such as τ Boötis.

In interstellar space, H₃⁺ forms the basis for an extensive network of ion-molecule reactions that are responsible for the creation of most of the molecules observed in interstellar space (10). This scheme of interstellar chemistry was directly confirmed when the infrared spectrum of H₃⁺ was observed in molecular clouds (1). Thanks to improvements in astronomical spectrometers, the detection of interstellar H₃⁺ is now almost routine, and observations of H₃⁺ can now be combined with those of other important molecules such as H₂ and CO to characterize the physical and chemical conditions in interstellar clouds. The observations of dense molecular clouds are generally in accord with theoretical models of interstellar chemistry.

The present understanding of the chemistry of diffuse clouds is, in contrast, quite primitive. In addition to the long-standing enigmas of the high abundance of CH⁺ and the ubiquitous but unexplained diffuse interstellar bands, the H₃⁺ ion presents a new mystery. Given the current experimental values of the H₃⁺ electron recombination rate, the observations (2) suggest that H₃⁺ in diffuse clouds extends for unreasonably long distances (over a thousand light years).

The physics and chemistry of H₃⁺, combined with the low density and temperature of interstellar space, lead to interesting phenomena such as extraordinary deuterium fractionation, bistability of chemical models, and radiative thermalization through forbidden rotational transitions. The discussions between astronomers, physicists, and chemists about the various processes in which H₃⁺ plays the pivotal role were inspiring, but, as expected, there are still more questions than answers. Hopefully, more astronomical observations and laboratory and theoretical studies will provide solutions to these problems in the coming years.

References and Notes

1.) T. R. Geballe and T. Oka, Nature 384, 334 (1996).
2.) B. J. McCall, T. R. Geballe, K. H. Hinkle, T. Oka, Science 279, 1910 (1998).
3.) "Astronomy, Physics, and Chemistry of H₃⁺," Royal Society Discussion Meeting, Royal Society, London, UK, 9 to 10 February 2000. See also h3plus.uchicago.edu. Proceedings will be published in the Philos. Trans. R. Soc. London, Ser. A.

4.) J. J. Thomson, Philos. Mag. 21, 225 (1911).
5.) T. Oka, Phys. Rev. Lett. 45, 531 (1980).
6.) I. R. McNab, Adv. Chem. Phys. LXXXIX, 1 (1995).
7.) M. Larsson, Annu. Rev. Phys. Chem. 48, 151 (1997).
8.) L. Trafton, D. F. Lester, K. L. Thompson, Astrophys. J. 343, L73 (1989); P. Drossart et al., Nature 340, 539 (1989).
9.) J. E. P. Connerney, R. Baron, T. Satoh, T. Owen, Science 262, 1035 (1993) .
10.) E. Herbst and W. Klemperer, Astrophys. J. 185, 505 (1973); W. D. Watson, Astrophys. J. 183, L17 (1973).