Argonium

Argonium, an ion combining a proton and an argon atom (and so also called protonated argon), (ArH⁺) can be made in an electric discharge, and was the first noble gas molecular ion to be found in interstellar space.^[1]

Properties

Argonium is isoelectronic with hydrogen chloride. Its dipole moment is 2.18 D for the ground state.^[2] The binding energy is 369 kJ mol^−1[3] (2.9 eV^[4]). This is smaller than that of H+
3 and many other protonated species, but less than H+
2.^[3]

Lifetimes of different vibrational states vary with isotope and become shorter for the more rapid high-energy vibrations. For ArH⁺ v=1 2.28, v=2 1.2, v=3 0.85, v=4 0.64, v=5 0.46 ms For ArD⁺ 9.09, 4.71, 3.27, 2.55, 2.11 ms^[5]

The force constant in the bond is calculated at 3.88 mdyne/Ų.^[6]

Reactions

• ArH⁺ + H₂ → Ar + H+
  3^[3]
• ArH⁺ + C → Ar + CH⁺
• ArH⁺ + N → Ar + NH⁺
• ArH⁺ + O → Ar + OH⁺
• ArH⁺ + CO → Ar + COH^+[3]

But the reverse reaction happens:

• Ar + H+
  2 → ArH⁺ + H.^[3]
• Ar + H+
  3 → *ArH⁺ + H₂^[3]

Ar⁺ + H₂ has a cross section of 10⁻¹⁸ m² for low energy. It seriously drops off for energies over 100 eV^[7] Ar + H+
2 has a cross sectional area of 6×10⁻¹⁹ m² for low energy H+
2, but when the energy exceeds 10 eV yield reduces, and more Ar⁺ and H₂ is produced instead.^[7]

Ar + H+
3 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 4eV more Ar⁺ and H starts to appear.^[7]

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

• Ar⁺ + H₂ → *ArH⁺ + H^[3] 1.49 eV^[4]

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.

• ArH⁺ + e⁻ → Ar + H^[3]

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.^[3]

• Ar⁺ + H₂ → ArH⁺ + H^[3]
• Ne⁺ + H₂ → Ne + H⁺ + H (dissociative charge transfer)^[3]
• He⁺ + H₂ → He + H⁺ + H^[3]

Spectrum

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.^[8] Brault and Davis were the first to detect the molecule using infrared spectroscopy to observe vibration-rotation bands.^[8]

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 Lymann limit, and so there are very few photons around to do this in space.^[3]

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.^[3] Emission lines are found in the Crab Nebula.^[4]

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.^[4] The column density of ArH⁺ in the Crab Nebula is between 10¹² and 10¹³ atoms per square centimeter.^[4] Possible the energy required to excite the ions so that then can emit comes from collisions with electrons or hydrogen molecules.^[4] Towards the Milky Way centre the column density of ArH⁺ is around 2×10¹³ cm⁻².^[3]

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.^[2]

Electron neutralization and destruction of argonium outcompletes the formation rate in space if the H₂ concentration is below 1 in 10⁻⁴.^[9]

History

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.^[10] J. W. C. Johns also observed the infrared spectrum.^[11]

Use

Argon facilitates the reaction of tritium (T₂) with double bonds in fatty acids by forming a ArT+ (tritium argonium) intermediate.^[12] When gold is sputtered with an argon-hydrogen plasma, the actual displacement of gold is done by ArH⁺.^[13]

Other things

Southern hemisphere of Vesta, showing Rheasilvia crater

Argonium is the name of a major tectonic scarp on the edge of the Rheasilvia impact basin in the Quadrangle Av-12 (sextilia) on 4 Vesta.^[14] Argonium is often an erroneous name for Argon.

References