Last updated
Other names
argon hydride cation [1]
protonated argon [2]
3D model (JSmol)
  • InChI=1S/ArH/h1H/q+1 Yes check.svgY[ NIST ]
  • [ArH+]
Molar mass 40.956 g·mol−1
Conjugate base Argon
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Argonium (also called the argon hydride cation, the hydridoargon(1+) ion, or protonated argon; chemical formula ArH+) is a cation combining a proton and an argon atom. It can be made in an electric discharge, and was the first noble gas molecular ion to be found in interstellar space. [3]



Argonium is isoelectronic with hydrogen chloride. Its dipole moment is 2.18  D for the ground state. [4] The binding energy is 369 kJ mol−1 [5] (2.9 eV [6] ). This is smaller than that of H+
and many other protonated species, but more than that of H+
. [5]

Rotationless radiative lifetimes of different vibrational states vary with isotope and become shorter for the more rapid high-energy vibrations:

Lifetimes (ms) [7]

The force constant in the bond is calculated at 3.88 mdyne/Å2. [8]


But the reverse reaction happens:

Ar+ + H2 has a cross section of 10−18 m2 for low energy. It has a steep drop off for energies over 100 eV [9] Ar + H+
has a cross sectional area of 6×10−19 m2 for low energy H+
, but when the energy exceeds 10 eV yield reduces, and more Ar+ and H2 is produced instead. [9]

Ar + H+
has a maximum yield of ArH+ for energies between 0.75 and 1 eV with a cross section of 5×10−20 m2. 0.6 eV is needed to make the reaction proceed forward. Over 4 eV more Ar+ and H starts to appear. [9]

Argonium is also produced from Ar+ ions produced by cosmic rays and X-rays from neutral argon.

When ArH+ encounters an electron, dissociative recombination can occur, but it is extremely slow for lower energy electrons, allowing ArH+ to survive for a much longer time than many other similar protonated cations.

Because ionisation potential of argon atoms is lower than that of the hydrogen molecule (in contrast to that of helium or neon), the argon ion reacts with molecular hydrogen, but for helium and neon ions, they will strip an electron from a hydrogen molecule. [5]


Artificial ArH+ made from earthly argon contains mostly the isotope 40Ar rather than the cosmically abundant 36Ar. Artificially it is made by an electric discharge through an argon–hydrogen mixture. [10] Brault and Davis were the first to detect the molecule using infrared spectroscopy to observe vibration–rotation bands. [10]

Far infrared spectrum of 40Ar1H+ [10] 36Ar38Ar [4]
Transitionobserved frequency

The UV spectrum has two absorption points resulting in the ion breaking up. The 11.2 eV conversion to the B1Π state has a low dipole and so does not absorb much. A 15.8 eV to a repulsive A1Σ+ state is at a shorter wavelength than the Lyman limit, and so there are very few photons around to do this in space. [5]

Natural occurrence

ArH+ occurs in interstellar diffuse atomic hydrogen gas. For argonium to form, the fraction of molecular hydrogen H2 must be in the range 0.0001 to 0.001. Different molecular ions form in correlation with different concentrations of H2. Argonium is detected by its absorption lines at 617.525 GHz (J = 1→0), and 1234.602 GHz (J = 2→1). These lines are due to the isotopolog 36Ar1H+ undergoing rotational transitions. The lines have been detected in the direction of the galactic centre SgrB2(M) and SgrB2(N), G34.26+0.15, W31C (G10.62−0.39), W49(N), and W51e, however where absorption lines are observed, argonium is not likely to be in the microwave source, but instead in the gas in front of it. [5] Emission lines are found in the Crab Nebula. [6]

In the Crab Nebula ArH+ occurs in several spots revealed by emission lines. The strongest place is in the Southern Filament. This is also the place with the strongest concentration of Ar+ and Ar2+ ions. [6] The column density of ArH+ in the Crab Nebula is between 1012 and 1013 atoms per square centimeter. [6] Possible the energy required to excite the ions so that then can emit comes from collisions with electrons or hydrogen molecules. [6] Towards the Milky Way centre the column density of ArH+ is around 2×1013 cm−2. [5]

Two isotopologs of argonium 36ArH+ and 38ArH+ are known to be in a distant unnamed galaxy with z = 0.88582 (7.5 billion light years away) which is on the line of sight to the blazar PKS 1830−211. [4]

Electron neutralization and destruction of argonium outcompletes the formation rate in space if the H2 concentration is below 1 in 10−4. [11]


Using the McMath solar Fourier transform spectrometer at Kitt Peak National Observatory, James W. Brault and Sumner P. Davis observed ArH+ vibration-rotation infrared lines for the first time. [12] J. W. C. Johns also observed the infrared spectrum. [13]


Argon facilitates the reaction of tritium (T2) with double bonds in fatty acids by forming an ArT+ (tritium argonium) intermediate. [14] When gold is sputtered with an argon-hydrogen plasma, the actual displacement of gold is done by ArH+. [15]

Related Research Articles

Hydrogen Chemical element, symbol H and atomic number 1

Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula H2. It is colorless, odorless, tasteless, non-toxic, and highly combustible. Hydrogen is the most abundant chemical substance in the universe, constituting roughly 75% of all normal matter. Stars such as the Sun are mainly composed of hydrogen in the plasma state. Most of the hydrogen on Earth exists in molecular forms such as water and organic compounds. For the most common isotope of hydrogen (symbol 1H) each atom has one proton, one electron, and no neutrons.

In chemistry, hydronium (hydroxonium in traditional British English) is the common name for the aqueous cation H3O+, the type of oxonium ion produced by protonation of water. It is often viewed as the positive ion present when an Arrhenius acid is dissolved in water, as Arrhenius acid molecules in solution give up a proton (a positive hydrogen ion, H+) to the surrounding water molecules (H2O). In fact, acids must be surrounded by more than a single water molecule in order to ionize, yielding aqueous H+ and conjugate base. Three main structures for the aqueous proton have garnered experimental support: The Eigen cation, which is a tetrahydrate, H3O+(H2O)3; the Zundel cation, which is a symmetric dihydrate, H+(H2O)2; and the Stoyanov cation, an expanded Zundel cation, which is a hexahydrate: H+(H2O)2(H2O)4. Spectroscopic evidence from well-defined IR spectra overwhelmingly supports the Stoyanov cation as the predominant form. For this reason, it has been suggested that wherever possible, the symbol H+(aq) should be used instead of the hydronium ion.

Interstellar medium Matter and radiation in the space between the star systems in a galaxy

In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space. The energy that occupies the same volume, in the form of electromagnetic radiation, is the interstellar radiation field.

Hydroxyl radical Neutral form of the hydroxide ion (OH−)

The hydroxyl radical is the diatomic molecule
. The hydroxyl radical is very stable as a dilute gas, but it decays very rapidly in the condensed phase. It is pervasive in some situations. Most notably the hydroxyl radicals are produced from the decomposition of hydroperoxides (ROOH) or, in atmospheric chemistry, by the reaction of excited atomic oxygen with water. It is also important in the field of radiation chemistry, since it leads to the formation of hydrogen peroxide and oxygen, which can enhance corrosion and SCC in coolant systems subjected to radioactive environments.

Trihydrogen cation Ion

The trihydrogen cation or protonated molecular hydrogen is a cation with formula H+
, consisting of three hydrogen nuclei (protons) sharing two electrons.

Ethynyl radical Chemical compound

The ethynyl radical is an organic compound with the chemical formula C≡CH. It is a simple molecule that does not occur naturally on Earth but is abundant in the interstellar medium. It was first observed by electron spin resonance isolated in a solid argon matrix at liquid helium temperatures in 1963 by Cochran and coworkers at the Johns Hopkins Applied Physics Laboratory. It was first observed in the gas phase by Tucker and coworkers in November 1973 toward the Orion Nebula, using the NRAO 11-meter radio telescope. It has since been detected in a large variety of interstellar environments, including dense molecular clouds, bok globules, star forming regions, the shells around carbon-rich evolved stars, and even in other galaxies.

The hexatriynyl radical, C6H, is an organic radical molecule consisting of a chain of six carbon atoms terminated by a hydrogen.

Helium hydride ion Chemical compound

The helium hydride ion or hydridohelium(1+) ion or helonium is a cation (positively charged ion) with chemical formula HeH+. It consists of a helium atom bonded to a hydrogen atom, with one electron removed. It can also be viewed as protonated helium. It is the lightest heteronuclear ion, and is believed to be the first compound formed in the Universe after the Big Bang.

The dihydrogen cation or hydrogen molecular ion is a cation with formula H+
. It consists of two hydrogen nuclei (protons) sharing a single electron. It is the simplest molecular ion.


Diazenylium is the chemical N2H+, an inorganic cation that was one of the first ions to be observed in interstellar clouds. Since then, it has been observed for in several different types of interstellar environments, observations that have several different scientific uses. It gives astronomers information about the fractional ionization of gas clouds, the chemistry that happens within those clouds, and it is often used as a tracer for molecules that are not as easily detected (such as N2). Its 1–0 rotational transition occurs at 93.174 GHz, a region of the spectrum where Earth's atmosphere is transparent and it has a significant optical depth in both cold and warm clouds so it is relatively easy to observe with ground-based observatories. The results of N2H+ observations can be used not only for determining the chemistry of interstellar clouds, but also for mapping the density and velocity profiles of these clouds.

Cyano radical Chemical compound

The cyano radical (or cyanido radical) is a radical with molecular formula CN, sometimes written CN. The cyano radical was one of the first detected molecules in the interstellar medium, in 1938. Its detection and analysis was influential in astrochemistry. The discovery was confirmed with a coudé spectrograph, which was made famous and credible due to this detection. ·CN has been observed in both diffuse clouds and dense clouds. Usually, CN is detected in regions with hydrogen cyanide, hydrogen isocyanide, and HCNH+, since it is involved in the creation and destruction of these species (see also Cyanogen).

Triatomic hydrogen or H3 is an unstable triatomic molecule containing only hydrogen. Since this molecule contains only three atoms of hydrogen it is the simplest triatomic molecule and it is relatively simple to numerically solve the quantum mechanics description of the particles. Being unstable the molecule breaks up in under a millionth of a second. Its fleeting lifetime makes it rare, but it is quite commonly formed and destroyed in the universe thanks to the commonness of the trihydrogen cation. The infrared spectrum of H3 due to vibration and rotation is very similar to that of the ion, H+
. In the early universe this ability to emit infrared light allowed the primordial hydrogen and helium gas to cool down so as to form stars.

A hydrogen molecular ion cluster or hydrogen cluster ion is a positively charged cluster of hydrogen molecules. The hydrogen molecular ion and trihydrogen ion are well defined molecular species. However hydrogen also forms singly charged clusters with n up to 120.

Magnesium monohydride Chemical compound

Magnesium monohydride is a molecular gas with formula MgH that exists at high temperatures, such as the atmospheres of the Sun and stars. It was originally known as magnesium hydride, although that name is now more commonly used when referring to the similar chemical magnesium dihydride.

Helium is the smallest and the lightest noble gas and one of the most unreactive elements, so it was commonly considered that helium compounds cannot exist at all, or at least under normal conditions. Helium's first ionization energy of 24.57 eV is the highest of any element. Helium has a complete shell of electrons, and in this form the atom does not readily accept any extra electrons nor join with anything to make covalent compounds. The electron affinity is 0.080 eV, which is very close to zero. The helium atom is small with the radius of the outer electron shell at 0.29 Å. Helium is a very hard atom with a Pearson hardness of 12.3 eV. It has the lowest polarizability of any kind of atom, however, very weak van der Waals forces exist between helium and other atoms. This force may exceed repulsive forces, so at extremely low temperatures helium may form van der Waals molecules. Helium has the lowest boiling point of any known substance.

Neon compounds are chemical compounds containing the element neon (Ne) with other molecules or elements from the periodic table. Compounds of the noble gas neon were believed not to exist, but there are now known to be molecular ions containing neon, as well as temporary excited neon-containing molecules called excimers. Several neutral neon molecules have also been predicted to be stable, but are yet to be discovered in nature. Neon has been shown to crystallize with other substances and form clathrates or Van der Waals solids.

Argon compounds, the chemical compounds that contain the element argon, are rarely encountered due to the inertness of the argon atom. However, compounds of argon have been detected in inert gas matrix isolation, cold gases, and plasmas, and molecular ions containing argon have been made and also detected in space. One solid interstitial compound of argon, Ar1C60 is stable at room temperature. Ar1C60 was discovered by the CSIRO.

The magnesium argide ion, MgAr+ is an ion composed of one ionised magnesium atom, Mg+ and an argon atom. It is important in inductively coupled plasma mass spectrometry and in the study of the field around the magnesium ion. The ionization potential of magnesium is lower than the first excitation state of argon, so the positive charge in MgAr+ will reside on the magnesium atom. Neutral MgAr molecules can also exist in an excited state.

In chemistry, the decay technique is a method to generate chemical species such as radicals, carbocations, and other potentially unstable covalent structures by radioactive decay of other compounds. For example, decay of a tritium-labeled molecule yields an ionized helium atom, which might then break off to leave a cationic molecular fragment.


  1. NIST Computational Chemistry Comparison and Benchmark Database, NIST Standard Reference Database Number 101. Release 19, April 2018, Editor: Russell D. Johnson III.
  2. Neufeld, David A.; Wolfire, Mark G. (2016). "The Chemistry of Interstellar Argonium and Other Probes of the Molecular Fraction in Diffuse Clouds". The Astrophysical Journal. 826 (2): 183. arXiv: 1607.00375 . Bibcode:2016ApJ...826..183N. doi:10.3847/0004-637X/826/2/183. S2CID   118493563.
  3. Quenqua, Douglas (13 December 2013). "Noble Molecules Found in Space". The New York Times. Retrieved 26 September 2016.
  4. 1 2 3 Müller, Holger S. P.; Muller, Sébastien; Schilke, Peter; Bergin, Edwin A.; Black, John H.; Gerin, Maryvonne; Lis, Dariusz C.; Neufeld, David A.; Suri, Sümeyye (7 October 2015). "Detection of extragalactic argonium, ArH+, toward PKS 1830−211". Astronomy & Astrophysics. 582: L4. arXiv: 1509.06917 . Bibcode:2015A&A...582L...4M. doi:10.1051/0004-6361/201527254. S2CID   10017142.
  5. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Schilke, P.; Neufeld, D. A.; Müller, H. S. P.; Comito, C.; Bergin, E. A.; Lis, D. C.; Gerin, M.; Black, J. H.; Wolfire, M.; Indriolo, N.; Pearson, J. C.; Menten, K. M.; Winkel, B.; Sánchez-Monge, Á.; Möller, T.; Godard, B.; Falgarone, E. (4 June 2014). "Ubiquitous argonium (ArH+) in the diffuse interstellar medium: A molecular tracer of almost purely atomic gas". Astronomy & Astrophysics. 566: A29. arXiv: 1403.7902 . Bibcode:2014A&A...566A..29S. doi:10.1051/0004-6361/201423727. S2CID   44021593.
  6. 1 2 3 4 5 6 Barlow, M. J.; Swinyard, B. M.; Owen, P. J.; Cernicharo, J.; Gomez, H. L.; Ivison, R. J.; Krause, O.; Lim, T. L.; Matsuura, M.; Miller, S.; Olofsson, G.; Polehampton, E. T. (12 December 2013). "Detection of a Noble Gas Molecular Ion, 36ArH+, in the Crab Nebula". Science. 342 (6164): 1343–1345. arXiv: 1312.4843 . Bibcode:2013Sci...342.1343B. doi:10.1126/science.1243582. PMID   24337290. S2CID   37578581.
  7. Pavel Rosmus (1979). "Molecular Constants for the 1Σ+ Ground State of the ArH+ Ion". Theoretica Chimica Acta. 51 (4): 359–363. doi:10.1007/BF00548944. S2CID   98475430.
  8. Fortenberry, Ryan C. (June 2016). "Quantum astrochemical spectroscopy". International Journal of Quantum Chemistry. 117 (2): 81–91. doi: 10.1002/qua.25180 .
  9. 1 2 3 Phelps, A. V. (1992). "Collisions of H+, H+
    , H+
    , ArH+, H, H, and H2 with Ar and of Ar+ and ArH+ with H2 for Energies from 0.1 eV to 10 keV". J. Phys. Chem. Ref. Data. 21 (4). doi:10.1063/1.555917.
  10. 1 2 3 Brown, John M.; Jennings, D.A.; Vanek, M.; Zink, L.R.; Evenson, K.M. (April 1988). "The pure rotational spectrum of ArH+". Journal of Molecular Spectroscopy. 128 (2): 587–589. Bibcode:1988JMoSp.128..587B. doi:10.1016/0022-2852(88)90173-7.
  11. David A. Neufeld; Mark G. Wolfire (1 July 2016). "The chemistry of interstellar argonium and other probes of the molecular fraction in diffuse clouds". The Astrophysical Journal. 826 (2): 183. arXiv: 1607.00375 . Bibcode:2016ApJ...826..183N. doi:10.3847/0004-637X/826/2/183. S2CID   118493563.
  12. Brault, James W; Davis, Sumner P (1 February 1982). "Fundamental Vibration-Rotation Bands and Molecular Constants for the ArH+ Ground State (1Σ+ )". Physica Scripta. 25 (2): 268–271. Bibcode:1982PhyS...25..268B. doi:10.1088/0031-8949/25/2/004. S2CID   250825672.
  13. Johns, J.W.C. (July 1984). "Spectra of the protonated rare gases". Journal of Molecular Spectroscopy. 106 (1): 124–133. Bibcode:1984JMoSp.106..124J. doi:10.1016/0022-2852(84)90087-0.
  14. Peng, C. T. (April 1966). "Mechanism of Addition of Tritium to Oleate by Exposure to Tritium Gas". The Journal of Physical Chemistry. 70 (4): 1297–1304. doi:10.1021/j100876a053. PMID   5916501.
  15. Jiménez-Redondo, Miguel; Cueto, Maite; Doménech, José Luis; Tanarro, Isabel; Herrero, Víctor J. (3 November 2014). "Ion kinetics in Ar/H2 cold plasmas: the relevance of ArH+" (PDF). RSC Advances. 4 (107): 62030–62041. Bibcode:2014RSCAd...462030J. doi:10.1039/C4RA13102A. ISSN   2046-2069. PMC   4685740 . PMID   26702354.