Argon compounds

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.
Argon ionises at 15.76 eV, which is higher than hydrogen, but lower than helium, neon or fluorine. Molecules containing argon can be van der Waals molecules held together very weakly by London dispersion forces. Ionic molecules can be bound by charge induced dipole interactions. With gold atoms there can be some covalent interaction. Several boron-argon bonds with significant covalent interactions have been also reported. Experimental methods used to study argon compounds have included inert gas matrices, infrared spectroscopy to study stretching and bending movements, microwave spectroscopy and far infrared to study rotation, and also visible and ultraviolet spectroscopy to study different electronic configurations including excimers. Mass spectroscopy is used to study ions. Computation methods have been used to theoretically compute molecule parameters, and predict new stable molecules. Computational ab initio methods used have included CCSD, MP2, CIS and CISD. For heavy atoms, effective core potentials are used to model the inner electrons, so that their contributions do not have to be individually computed. More powerful computers since the 1990s have made this kind of in silico study much more popular, being much less risky and simpler than an actual experiment. This article is mostly based on experimental or observational results.
The argon fluoride laser is important in photolithography of silicon chips. These lasers make a strong ultraviolet emission at 192 nm.

Argonium

is an ion combining a proton and an argon atom. It is found in interstellar space in diffuse atomic hydrogen gas where the fraction of molecular hydrogen H₂ is in the range of 0.0001 to 0.001.
Argonium is formed when H₂⁺ reacts with Ar atoms:
and it 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.
Artificial ArH⁺ made from earthly Ar contains mostly the isotope ⁴⁰Ar rather than the cosmically abundant ³⁶Ar. Artificially it is made by an electric discharge through an argon-hydrogen mixture.

Natural occurrence

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. The column density of ArH⁺ in the Crab Nebula is between 10¹² and 10¹³ atoms per square centimeter. Possibly the energy required to excite the ions so that then can emit, comes from collisions with electrons or hydrogen molecules. Towards the Milky Way centre the column density of ArH⁺ is around.

Cluster argon cations

The diargon cation, has a binding energy of 1.29 eV.
The triargon cation is linear, but has one Ar−Ar bond shorter than the other. Bond lengths are 2.47 and 2.73 ångströms. The dissociation energy to Ar and Ar₂⁺ is 0.2 eV. In line with the molecule's asymmetry, the charge is calculated as +0.10, +0.58 and +0.32 on each argon atom, so that it greatly resembles bound to a neutral Ar atom.
Larger charged argon clusters are also detectable in mass spectroscopy. The tetraargon cation is also linear. icosahedral clusters have an core, whereas is dioctahedral with an core. The linear core has +0.1 charge on the outer atoms, and +0.4 charge on each or the inner atoms. For larger charged argon clusters, the charge is not distributed on more than four atoms. Instead the neutral outer atoms are attracted by induced electric polarization. The charged argon clusters absorb radiation, from the near infrared, through visible to ultraviolet. The charge core,, or is called a chromophore. Its spectrum is modified by the first shell of neutral atoms attached. Larger clusters have the same spectrum as the smaller ones. When photons are absorbed in the chromophore, it is initially electronically excited, but then energy is transferred to the whole cluster in the form of vibration. Excess energy is removed by outer atoms evaporating from the cluster one at a time. The process of destroying a cluster by light is called photofragmentation.
Negatively-charged argon clusters are thermodynamically unstable, and therefore cannot exist. Argon has a negative electron affinity.

Argon monohydride

Neutral argon hydride, also known as argon monohydride, was the first discovered noble gas hydride. J. W. C. Johns discovered an emission line of ArH at 767 nm and announced the find in 1970. The molecule was synthesized using X-ray irradiation of mixtures of argon with hydrogen-rich molecules such as H₂, H₂O, CH₄ and CH₃OH. The X-ray excited argon atoms are in the 4p state.
Argon monohydride is unstable in its ground state, 4s, as a neutral inert gas atom and a hydrogen atom repel each other at normal intermolecular distances. When a higher-energy-level ArH* emits a photon and reaches the ground state, the atoms are too close to each other, and they repel and break up. However a van der Waals molecule can exist with a long bond. However, excited ArH* can form stable Rydberg molecules, also known as excimers. These Rydberg molecules can be considered as a protonated argon core, surrounded by an electron in one of many possible higher energy states.
Instead of dihydrogen, other hydrogen containing molecules can also have a hydrogen atom abstracted by excited argon, but note that some molecules bind hydrogen too strongly for the reaction to proceed. For example, acetylene will not form ArH this way.
In the van der Waals molecule of ArH, the bond length is calculated to be about 3.6 Å and the dissociation energy calculated to be 0.404 kJ/mol. The bond length in ArH* is calculated as 1.302 Å.
The spectrum of argon monohydride, both ArH* and ArD*, has been studied. The lowest bound state is termed A²Σ⁺ or 5s. Another low lying state is known as 4p, made up of C²Σ⁺ and B²π states. Each transition to or from higher level states corresponds to a band. Known bands are 3p → 5s, 4p → 5s, 5p → 5s, 6p → 5s 3dσ → 4p, 3dπ → 4p, 3dδ → 4p, 4dσ → 4p, 6s → 4p, 7s → 4p, 8s → 4p, 5dπ → 4p, 5p → 6s, 4f → 5s, 4f → 3dπ, 4f → 3dδ, 4f → 3dσ. The transitions going to 5s, 3dπ → 5s and 5dπ → 5s, are strongly predissociated, blurring out the lines. In the UV spectrum a continuous band exists from 200 to 400 nm. This band is due to two different higher states: B²Π → A²Σ⁺ radiates over 210–450 nm, and E²Π → A²Σ⁺ is between 180 and 320 nm. A band in the near infrared from 760 to 780 nm.
Other ways to make ArH include a Penning-type discharge tube, or other electric discharges. Yet another way is to create a beam of ArH⁺ ions and then neutralize them in laser-energized caesium vapour. By using a beam, the lifetimes of the different energy states can be observed, by measuring the profile of electromagnetic energy emitted at different wavelengths. The E²π state of ArH has a radiative lifetime of 40 ns. For ArD the lifetime is 61 ns. The B²Π state has a lifetime of 16.6 ns in ArH and 17 ns in ArD.

Argon polyhydrides

The argon dihydrogen cation has been predicted to exist and to be detectable in the interstellar medium. However it has not been detected as of 2021. is predicted to be linear in the form Ar−H−H. The H−H distance is 0.94 Å. The dissociation barrier is only 2 kcal/mol, and readily loses a hydrogen atom to yield ArH⁺. The force constant of the ArH bond in this is 1.895 mdyne/Å².
The argon trihydrogen cation has been observed in the laboratory. ArH₂D⁺, and have also been observed. The argon trihydrogen cation is planar in shape, with an argon atom off the vertex of a triangle of hydrogen atoms.

Argoxonium

The argoxonium ion ArOH⁺ is predicted to be bent molecular geometry in the 1¹A′ state. ³Σ⁻ is a triplet state 0.12 eV higher in energy, and ³A″ is a triplet state 0.18 eV higher. The Ar−O bond is predicted to be 1.684 Å long and to have a force constant of 2.988 mdyne/Å².

ArNH⁺

ArNH⁺ is a possible ionic molecule to detect in the lab, and in space, as the atoms that compose it are common. ArNH⁺ is predicted to be more weakly bound than ArOH⁺, with a force constant in the Ar−N bond of 1.866 mdyne/Å². The angle at the nitrogen atom is predicted to be 97.116°. The Ar−N lengths should be 1.836 Å and the N−H bond length would be 1.046 Å

Argon dinitrogen cation

The argon dinitrogen linear cationic complex has also been detected in the lab:
The dissociation yields Ar⁺, as this is a higher-energy state. The binding energy is 1.19 eV. The molecule is linear. The distance between two nitrogen atoms is 1.1 Å. This distance is similar to that of neutral N₂ rather than that of ion. The distance between one nitrogen and the argon atom is 2.2 Å. The vibrational band origin for the nitrogen bond in is at 2272.2564 cm⁻¹ compared with N₂⁺ at 2175 and N₂ at 2330 cm⁻¹.
In the process of photodissociation, it is three times more likely to yield Ar⁺ + N₂ compared to Ar +.