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.^[1] 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.^[2] Several boron-argon bonds with significant covalent interactions have been also reported.^[3][4] 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.^[5] Computation methods have been used to theoretically compute molecule parameters, and predict new stable molecules. Computational ab initio methods used have included CCSD(T), MP2 (Møller–Plesset perturbation theory of the second order), 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.^[5] 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.^[6]

Argonium

Argonium (ArH⁺) 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.^[1]

Argonium is formed when H₂⁺ reacts with Ar atoms:^[1]

Ar + H⁺
₂ → ArH⁺ + H^[1]

and it is also produced from Ar⁺ ions produced by cosmic rays and X-rays from neutral argon:

Ar⁺ + H₂ → *ArH⁺ + H^[1] 1.49 eV.^[7]

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⁻ → ArH* → Ar + H^[1]

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.^[8]

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.^[7] The column density of ArH⁺ in the Crab Nebula is between 10¹² and 10¹³ atoms per square centimeter.^[7] Possibly the energy required to excite the ions so that then can emit, comes from collisions with electrons or hydrogen molecules.^[7] Towards the Milky Way centre the column density of ArH⁺ is around 2×10¹³ cm⁻².^[1]

Cluster argon cations

The diargon cation, Ar⁺
₂ has a binding energy of 1.29 eV.^[9]

The triargon cation Ar⁺
₃ 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 Ar⁺
₂ bound to a neutral Ar atom.^[10]

Larger charged argon clusters are also detectable in mass spectroscopy. The tetraargon cation is also linear. Ar⁺
₁₃ icosahedral clusters have an Ar⁺
₃ core, whereas Ar⁺
₁₉ is dioctahedral with an Ar⁺
₄ core. The linear Ar⁺
₄ 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.^[11] The charged argon clusters absorb radiation, from the near infrared, through visible to ultraviolet. The charge core, Ar⁺
₂, Ar⁺
₃ or Ar⁺
₄ 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.^[11]

Negatively-charged argon clusters are thermodynamically unstable, and therefore cannot exist. Argon has a negative electron affinity.^[11]

Argon monohydride

Neutral argon hydride, also known as argon monohydride (ArH), 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.^[12] The X-ray excited argon atoms are in the 4p state.^[13]

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.^[14] 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.^[15]

Formation: Ar + ν → Ar*;  Ar* + H₂ → ArH* + H^[12]

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.^[12]

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 (33.8 cm⁻¹).^[16] The bond length in ArH* is calculated as 1.302 Å.^[17]

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 (band origin 17486.527 cm^−1[18]), 6p → 5s (band origin 21676.90 cm^−1[18]) 3dσ → 4p, 3dπ → 4p (6900 cm⁻¹), 3dδ → 4p (8200–8800 cm⁻¹), 4dσ → 4p (15075 cm⁻¹), 6s → 4p (7400–7950 cm⁻¹), 7s → 4p (predicted at 13970 cm⁻¹, but obscured), 8s → 4p (16750 cm⁻¹), 5dπ → 4p (16460 cm⁻¹), 5p → 6s (band origin 3681.171 cm⁻¹),^[19] 4f → 5s (20682.17 and 20640.90 cm⁻¹ band origin for ArD and ArH), 4f → 3dπ (7548.76 and 7626.58 ccm⁻¹), 4f → 3dδ (6038.47 and 6026.57 cm⁻¹), 4f → 3dσ (4351.44 cm⁻¹ for ArD).^[14] The transitions going to 5s, 3dπ → 5s and 5dπ → 5s, are strongly predissociated, blurring out the lines.^[19] 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.^[20] A band in the near infrared from 760 to 780 nm.^[21]

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⁺ (argonium) 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.^[22] 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.^[20]

Argon polyhydrides

The argon dihydrogen cation ArH⁺
₂ has been predicted to exist and to be detectable in the interstellar medium. However it has not been detected as of 2021^[update].^[23] ArH⁺
₂ 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 (8 kJ/mol), and ArH⁺
₂ readily loses a hydrogen atom to yield ArH⁺.^[24] The force constant of the ArH bond in this is 1.895 mdyne/Ų (1.895×10¹² Pa).^[25]

The argon trihydrogen cation ArH⁺
₃ has been observed in the laboratory.^[23][26] ArH₂D⁺, ArHD⁺
₂ and ArD⁺
₃ have also been observed.^[27] The argon trihydrogen cation is planar in shape, with an argon atom off the vertex of a triangle of hydrogen atoms.^[28]

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^[23] and to have a force constant of 2.988 mdyne/Ų (2.988×10¹² Pa).^[25]

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/Ų (1.866×10¹² Pa). 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^[25][29]

Argon dinitrogen cation

The argon dinitrogen linear cationic complex has also been detected in the lab:

Ar + N⁺
₂ → ArN⁺
₂ photodissociation→ Ar⁺ + N₂.^[23]

The dissociation yields Ar⁺, as this is a higher-energy state.^[9] The binding energy is 1.19 eV.^[9] 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 N⁺
₂ ion. The distance between one nitrogen and the argon atom is 2.2 Å.^[9] The vibrational band origin for the nitrogen bond in ArN⁺
₂ (V = 0 → 1) is at 2272.2564 cm⁻¹ compared with N₂⁺ at 2175 and N₂ at 2330 cm⁻¹.^[9]

In the process of photodissociation, it is three times more likely to yield Ar⁺ + N₂ compared to Ar + N⁺
₂.^[30]

ArHN⁺
₂

ArHN⁺
₂ has been produced in a supersonic jet expansion of gas and detected by Fourier transform microwave spectroscopy.^[26] The molecule is linear, with the atoms in the order Ar−H−N−N. The Ar−H distance is 1.864 Å. There is a stronger bond between hydrogen and argon than in ArHCO⁺.^[31]

The molecule is made by the following reaction:

ArH⁺ + N₂ → ArHN⁺
₂.^[31]

Bis(dinitrogen) argon cation

The argon ion can bond two molecules of dinitrogen (N₂) to yield an ionic complex with a linear shape and structure N=N−+Ar−N=N. The N=N bond length is 1.1014 Å, and the nitrogen to argon bond length is 2.3602 Å. 1.7 eV of energy is required to break this apart to N₂ and ArN⁺
₂. The band origin of an infrared band due to antisymmetric vibration of the N=N bonds is at 2288.7272 cm⁻¹. Compared to N₂ it is redshifted 41.99 cm⁻¹. The ground state rotational constant of the molecule is 0.034296 cm⁻¹.^[30]

Ar(N
₂)⁺
₂ is produced by a supersonic expansion of a 10:1 mixture of argon with nitrogen through a nozzle, which is impacted by an electron beam.^[30]

ArN₂O⁺

ArN₂O⁺ absorbs photons in four violet–ultraviolet wavelength bands leading to breakup of the molecule. The bands are 445–420, 415–390, 390–370, and 342 nm.^[32][33]

ArHCO⁺

ArHCO⁺ has been produced in a supersonic-jet expansion of gas and detected by Fabry–Perot-type Fourier transform microwave spectroscopy.^[26][34]

The molecule is made by this reaction

ArH⁺ + CO → ArHCO⁺.^[31]

Carbon dioxide–argon ion

ArCO⁺
₂ can be excited to form ArCO⁺
₂* where the positive charge is moved from the carbon dioxide part to the argon. This molecule may occur in the upper atmosphere. Experimentally the molecule is made from a low-pressure argon gas with 0.1% carbon dioxide, irradiated by a 150 V electron beam. Argon is ionized, and can transfer the charge to a carbon dioxide molecule.^[35] The dissociation energy of ArCO⁺
₂ is 0.26 eV.^[35]

ArCO⁺
₂ + CO₂ → Ar + CO
₂·CO⁺
₂ (yields 0.435 eV.)^[35]

van der Waals molecules

Neutral argon atoms bind very weakly to other neutral atoms or molecules to form van der Waals molecules. These can be made by expanding argon under high pressure mixed with the atoms of another element. The expansion happens through a tiny hole into a vacuum, and results in cooling to temperatures a few degrees above absolute zero. At higher temperatures the atoms will be too energetic to stay together by way of the weak London dispersion forces. The atoms that are to combine with argon can be produced by evaporation with a laser or alternatively by an electric discharge. The known molecules include AgAr, Ag₂Ar, NaAr, KAr, MgAr, CaAr, SrAr, ZnAr, CdAr, HgAr, SiAr,^[36] InAr, CAr,^[37] GeAr,^[38] SnAr,^[39] and BAr.^[40] SiAr was made from silicon atoms derived from Si(CH₃)₄.^[41]

In addition to the very weakly bound van der Waals molecules, electronically excited molecules with the same formula exist. As a formula these can be written ArX*, with the "*" indicating an excited state. The atoms are much more strongly bound with a covalent bond. They can be modeled as an ArX⁺ surrounded by a higher energy shell with one electron. This outer electron can change energy by exchanging photons and so can fluoresce. The widely used argon fluoride laser makes use of the ArF* excimer to produce strong ultraviolet radiation at 192 nm. The argon chloride laser using ArCl* produces even shorter ultraviolet at 175 nm, but is too feeble for application.^[42] The argon chloride in this laser comes from argon and chlorine molecules.^[43]

Argon clusters

Cooled argon gas can form clusters of atoms. Diargon, also known as the argon dimer, has a binding energy of 0.012 eV, but the Ar₁₃ and Ar₁₉ clusters have a sublimation energy (per atom) of 0.06 eV. For liquid argon, which could be written as Ar_∞, the energy increases to 0.08 eV. Clusters of up to several hundred argon atoms have been detected. These argon clusters are icosahedral in shape, consisting of shells of atoms arranged around a central atom. The structure changes for clusters with more than 800 atoms to resemble a tiny crystal with a face-centered cubic (fcc) structure, as in solid argon. It is the surface energy that maintains an icosahedral shape, but for larger clusters internal pressure will attract the atoms into an fcc arrangement.^[11] Neutral argon clusters are transparent to visible light.^[11]

Diatomic van der Waals molecules

Molecule	Binding energy ground Σ state (cm−1)	Binding energy excited Π state (cm−1)	Ground state bond length (Å)	Excited state bond length (Å)	CAS number[44]
ArH					30736-04-0
ArHe					12254-69-2
LiAr	42.5	925	4.89	2.48[45]
BAr					149358-32-7
ArNe					12301-65-4
NaAr	40	560			56633-38-6
MgAr	44	246			72052-59-6
AlAr					143752-09-4
SiAr[46]
ArCl					54635-29-9
Ar2					12595-59-4
KAr	42	373			12446-47-8
CaAr	62	134			72052-60-9
SrAr	68	136
NiAr					401838-48-0
ZnAr	96	706			72052-61-0
GaAr					149690-22-2
GeAr[38]
KrAr					51184-77-1
AgAr	90	1200
CdAr[47]	106	544			72052-62-1
InAr[48]					146021-90-1
SnAr[39]
ArXe					58206-67-0
AuAr					195245-92-2
HgAr	131	446			87193-95-1

ArO* is also formed when dioxygen trapped in an argon matrix is subjected to vacuum ultraviolet. It can be detected by its luminescence:

O₂ + hv → O⁺
₂ + e⁻;  O⁺
₂ + e⁻ → 2O*;  O* + Ar → ArO*.^[49]

Light emitted by ArO* has two main bands, one at 2.215 eV, and a weaker one at 2.195 eV.^[50]

Argon sulfide, ArS* luminesces in the near infrared at 1.62 eV. ArS is made from UV irradiated OCS in an argon matrix. The excited states lasts for 7.4 and 3.5 μs for spectrum peak and band respectively.^[51]

Triatomic van der Waals molecules

Zdroj:https://en.wikipedia.org?pojem=Argon_compounds
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