Other names
argon dimer
3D model (JSmol)
  • InChI=1S/2Ar
  • [Ar].[Ar]
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).

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 (a van der Waals molecule). 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]


Interaction energy of argon dimer
Interaction energy of argon dimer

Two argon atoms are attracted together by van der Waals forces when far from each other. When they are close, electrostatic forces repel them. There is a balance point where the van der Waals force matches the opposing repelling force, where energy is at a minimum, represented as the trough in the graph of interaction energy versus distance. This distance is the ground state of the unexcited argon dimer. In a vibrating molecule, the distance between the atoms bounces backwards and forwards from one side of the trough to the other. Faster vibrations will force the state up to higher levels in the energy trough. If the vibration is too much the molecule will break up. In a rotating molecule, the centrifugal force drags the atoms apart, but can still be overcome by the attractive force. But if the rotation is too much the atoms break apart.


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] The 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, there are eight different vibration states. But if the molecule spins 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 the van der Waals molecules. At standard conditions this only takes about 100 picoseconds.[1]

Excited states


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 Te ωe ωexe ωeye Be αe γe De βe re ν00 Re Å ref
H 112033.9
G 110930.9
F 0+
E 107330
D 106029.5
C 0+
B1Σu+ 0+
A3Σ2u+1u 92393.3
X1Σg+ 31.92 3.31 0.11 0.060 0.004 76.9 3.8 [1][5]


Parameter breakup Te ωe ωexe ωeye Be αe γe De βe re ν00 Re Å ref
D2Σ1/2u+ Ar1S0 + Ar+2P1/2
C2Π1/2u Ar1S0 + Ar+2P1/2 128 004 58.9 1.4 622 cm−1 [7]
B2Π1/2g Ar1S0 + Ar+2P3/2
C2Π3/2u Ar1S0 + Ar+2P3/2 126884 311 cm−1 [7]
B2Π3/2g+ Ar1S0 + Ar+2P3/2 0.104 eV [3]
A2Σ1/2u+ Ar1S0 + Ar+2P3/2 116591 307.0 2.05 622 cm−1 ?1.361 eV [3][4][7]


  1. ^ a b c d e Ewing, George E. (June 1975). "Structure and Properties of van der Waals Molecules". Accounts of Chemical Research. 8 (6): 185–192. doi:10.1021/ar50090a001.
  2. ^ Dehmer, P. M.; Pratt, S. T. (15 January 1982). "Photoionization of argon clusters". The Journal of Chemical Physics. 76 (2): 843–853. Bibcode:1982JChPh..76..843D. doi:10.1063/1.443056.
  3. ^ 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. Bibcode:1993JChPh..98.5269P. doi:10.1063/1.464926.
  4. ^ 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. Bibcode:1997JChPh.10710819S. doi:10.1063/1.474199.
  5. ^ 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. Bibcode:1973JMoSp..46..454D. doi:10.1016/0022-2852(73)90057-X.
  6. ^ "Argon dimer". NIST Standard Reference Database 69: NIST Chemistry WebBook. Retrieved 19 February 2018.
  7. ^ 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. Bibcode:1998JChPh.109.9762S. doi:10.1063/1.477646.

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