Diargon

Diargon
Names
	Other names argon dimer
Identifiers
CAS Number	12595-59-4 ;
3D model (JSmol)	Interactive image;
PubChem CID	6432035;
	InChI InChI=1S/2Ar Key: XMPZLAQHPIBDSO-UHFFFAOYSA-N;
	SMILES [Ar].[Ar];
Properties
Chemical formula	Ar2
Molar mass	79.896 g·mol−1
Appearance	transparent gas
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Diargon or the argon dimer is a molecule containing two argon atoms. Normally this is only very weakly bound together by van der Waals forces. However in an excited state, or ionised state, the two atoms can be more tightly bound together, with significant spectral features. At cryogenic temperatures argon gas can have a few percent of diargon molecules.^[1]

Properties

The ionisation energy of the neutral molecule is 14.4558 eV (or 116593 cm⁻¹).^[2]

The dissociation energy of neutral Ar₂ in the ground state is 98.7 cm⁻¹^[3] which is hundreds of times weaker than that of typical molecules.^[1] Dissociation energy of Ar₂⁺ is 1.3144 eV or 10601 cm⁻¹.^[4]

The Ar₂ molecule can exist in a number of different vibration and rotational states. If the molecule is not rotating theere are eight different vibration states. But if the molecule pins fast, vibration is more likely to shake it apart, and at the 30th rotational level there are only two stable and one metastable state of vibration. In combination there are 170 different possibilities that are stable. In the metastable states, energy will be released if the molecule breaks apart into two separate atoms, but some extra energy is required to overcome the attraction between the atoms. Quantum tunneling can result in the molecule breaking apart with no extra energy. However this takes time, which can vary from 10⁻¹¹ seconds to several centuries.^[1] Molecules crashing into each other also results in breakup of van der Waals molecules. At standard conditions this only takes about 100 picoseconds.^[1]

Excited states

Neutral

99.6% of the argon isotopes are ⁴⁰Ar, so the spectrum observed in natural argon dimer will be due to the ⁴⁰Ar⁴⁰Ar isotopomer.^[5] The following table lists different excited states.^[6]

Parameter	T_e	ω_e	ω_ex_e	ω_ey_e	B_e	α_e	γ_e	D_e	β_e	r_e	ν₀₀	R_e Å	ref
H											112033.9
G											110930.9
F 0⁺ _g											108492.2
E											107330
D											106029.5
C 0⁺ _g											95050.7
B¹Σ_u⁺ 0⁺ _g											93241.26
A³Σ_2u⁺1_u											92393.3
X¹Σ_g⁺		31.92	3.31	0.11	0.060	0.004		76.9				3.8	^[5]^[1]

Cation

Parameter	breakup	T_e	ω_e	ω_ex_e	ω_ey_e	B_e	α_e	γ_e	D_e	β_e	r_e	ν₀₀	R_e Å	ref
D²Σ_1/2u⁺	Ar¹S₀ + Ar⁺²P_1/2
C²Π_1/2u	Ar¹S₀ + Ar⁺²P_1/2	128 004	58.9	1.4					622 cm⁻¹					^[7]
B²Π_1/2g	Ar¹S₀ + Ar⁺²P_3/2
C²Π_3/2u	Ar¹S₀ + Ar⁺²P_3/2	126884							311 cm⁻¹					^[7]
B²Π_3/2g⁺	Ar¹S₀ + Ar⁺²P_3/2								0.104 eV					^[3]
A²Σ_1/2u⁺	Ar¹S₀ + Ar⁺²P_3/2	116591	307.0	2.05					622 cm⁻¹ ?1.361 eV					^[4]^[3]^[7]

References

^ ^a ^b ^c ^d ^e Ewing, George E. (June 1975). "Structure and Properties of van der Waals Molecules". Accounds of Chemical Research. 8 (6): 185–192.
^ Dehmer, P. M.; Pratt, S. T. (15 January 1982). "Photoionization of argon clusters". The Journal of Chemical Physics. 76 (2): 843–853. doi:10.1063/1.443056.
^ ^a ^b ^c Pradeep, T.; Niu, B.; Shirley, D. A. (April 1993). "Photoelectron spectroscopy of rare gas dimers revisited: Vibrationally resolved photoelectron spectrum of argon dimer" (PDF). The Journal of Chemical Physics. 98 (7): 5269–5275. doi:10.1063/1.464926.
^ ^a ^b Signorell, R.; Wüest, A.; Merkt, F. (22 December 1997). "The first adiabatic ionization potential of Ar2". The Journal of Chemical Physics. 107 (24): 10819–10822. doi:10.1063/1.474199.
^ ^a ^b Docken, Kate K.; Schafer, Trudy P. (June 1973). "Spectroscopic information on ground-state Ar2, Kr2, and Xe2 from interatomic potentials". Journal of Molecular Spectroscopy. 46 (3): 454–459. doi:10.1016/0022-2852(73)90057-X.
^ "Argon dimer". webbook.nist.gov. Retrieved 19 February 2018. {{cite web}}: More than one of |work= and |website= specified (help)
^ ^a ^b ^c Signorell, R.; Merkt, F. (8 December 1998). "The first electronic states of Ar2+ studied by high resolution photoelectron spectroscopy". The Journal of Chemical Physics. 109 (22): 9762–9771. doi:10.1063/1.477646.

[ewing-1] Ewing, George E. (June 1975). "Structure and Properties of van der Waals Molecules". Accounds of Chemical Research. 8 (6): 185–192.

[2] Dehmer, P. M.; Pratt, S. T. (15 January 1982). "Photoionization of argon clusters". The Journal of Chemical Physics. 76 (2): 843–853. doi:10.1063/1.443056.

[prad-3] Pradeep, T.; Niu, B.; Shirley, D. A. (April 1993). "Photoelectron spectroscopy of rare gas dimers revisited: Vibrationally resolved photoelectron spectrum of argon dimer" (PDF). The Journal of Chemical Physics. 98 (7): 5269–5275. doi:10.1063/1.464926.

[sig-4] Signorell, R.; Wüest, A.; Merkt, F. (22 December 1997). "The first adiabatic ionization potential of Ar2". The Journal of Chemical Physics. 107 (24): 10819–10822. doi:10.1063/1.474199.

[kate-5] Docken, Kate K.; Schafer, Trudy P. (June 1973). "Spectroscopic information on ground-state Ar2, Kr2, and Xe2 from interatomic potentials". Journal of Molecular Spectroscopy. 46 (3): 454–459. doi:10.1016/0022-2852(73)90057-X.

[6] "Argon dimer". webbook.nist.gov. Retrieved 19 February 2018. {{cite web}}: More than one of |work= and |website= specified (help)

[sign-7] Signorell, R.; Merkt, F. (8 December 1998). "The first electronic states of Ar2+ studied by high resolution photoelectron spectroscopy". The Journal of Chemical Physics. 109 (22): 9762–9771. doi:10.1063/1.477646.

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