Helium dimer

helium dimer
Names
Other names
dihelium
Identifiers
12184-98-4 
ChEBI CHEBI:33685
48
Jmol-3D images Image
PubChem 135094814
Properties
He2
Molar mass 8.0052 g/mol
Appearance colorless gas
Thermochemistry
Std enthalpy of
formation (ΔfHo298)
 1.1×10−5 kcal/mol.
Related compounds
Related van der Waals molecules
 LiHe NeHe2
Related compounds
 He3
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 
Infobox references

The helium dimer is a van der Waals molecule with formula He2 consisting of two helium atoms.[2] This molecule is the largest with two atoms, and is only very weakly bound together. The bond is so weak that it will break if the molecule rotates, or vibrates too much. It can only exist at very low cryogenic temperatures.

Two excited helium atoms can also bond to each other in a form called an excimer. This was discovered from a spectrum of helium that contained bands first seen in 1912. He2* was the first known Rydberg molecule.[3]

Dihelium ions also exist with a negative, positive and double positive charge. Two helium atoms can be confined together without bonding in the cage of a fullerene.

Molecule

Based on molecular orbital theory, He2 should not exist, and a chemical bond cannot form between the atoms. However, the van der Waals force exists between helium atoms as shown by the existence of liquid helium, and at a certain range of distances between atoms the attraction exceeds the repulsion. So a molecule composed of two helium atoms bound by the van der Waals force can exist.[4]

He2 is the largest known molecule of two atoms when in its ground state, due to its extremely long bond length.[4] The He2 molecule has a large separation distance between the atoms of about 52 Å. This is the largest for a diatomic molecule without ro-vibronic excitation. The binding energy is only about 1.3 mK, 10−7eV[5][6] or 1.1×10−5 kcal/mol.[7] The bond is 5000 times weaker than the covalent bond in the hydrogen molecule.[8]

Both helium atoms in the dimer can be ionized by a single photon with energy 63.86 eV. The proposed mechanism for this double ionization is that the photon ejects an electron from one atom, and then that electron hits the other helium atom and ionizes that as well.[9] The dimer then explodes as two helium cations ions repel each other with the same speed but opposite directions.[9]

A dihelium molecule bound by Van der Waals forces was first proposed by John Clarke Slater in 1928.[10]

Formation

The helium dimer can be formed in small amounts when helium gas expands and cools as it passes through a nozzle in a gas beam.[2] Only the isotope 4He can form molecules like this; 4He3He and 3He3He do not exist, as they do not have a stable bound state.[5] The amount of the dimer formed in the gas beam is of the order of one percent.[9]

Molecular ions

He2+ is a related ion bonded by a half covalent bond. It can be formed in a helium electrical discharge. It recombines with electrons to form an electronically excited He2(a3Σ+u) excimer molecule.[11] Both of these molecules are much smaller with more normal sized interatomic distances.

The helium dication dimer He22+ is extremely repulsive and would release much energy when it dissociated, around 835kJ/mol.[12] Dynamical stability of the ion was predicted by Linus Pauling.[13] An energy barrier of 33.2 kcal/mol prevents immediate decay. This ion is isoelectronic with the hydrogen molecule.[14][15] He22+ is the smallest possible molecule with a double positive charge. It is detectable using mass spectroscopy.[12][16]

The negative helium dimer He2− is metastable, lasting over 1μs, and can be formed when He2+ captures two electrons from cesium atoms in vapour or nitrogen molecules.[17]

The molecular helium anion is also found in liquid helium that has been excited by electrons with an energy level higher than 22 eV. This takes place firstly by penetration of liquid He, taking 1.2 eV, followed by excitation of a He atom electron to the 3P level which takes 19.8 eV. The electron can the combine with another helium atom and the excited helium atom to form He2−. He2− repels helium atoms, and so has a void around it. It will tend to migrate to a surface.[18]

Excimers

In a normal helium atom two electrons are found in the 1s orbital. However if sufficient energy is added, one electron can be elevated to a higher energy level. This high energy electron can become a valence electron, and the electron that remains in the 1s orbital is a core electron. Two excited helium atoms can react with a covalent bond to form a molecule called dihelium that lasts for short times of the order of a microsecond up to second or so.[3] Excited helium atoms in the 23S state can last for up to an hour, and react like alkali metal atoms.[19]

The first clues that dihelium exists were noticed in 1900 when W. Heuse observed a band spectrum in a helium discharge. However no information about the nature of the spectrum was published. independently E. Goldstein from Germany and W. E. Curtis from London published details of the spectrum in 1913.[20][21] Curtis was called away to military service in World War I, and the study of the spectrum was continued by Alfred Fowler. Fowler recognised that the doubled headed bands fell into two sequences analogous to principal and diffuse series in line spectra.[22]

The emission band spectrum shows a number of bands that degrade towards the red, meaning that the lines thin out and the spectrum weakens towards the longer wavelengths. Only one band with a green band head at 5732 Å degrades towards the violet. Other strong band heads are at 6400 (red), 4649, 4626, 4546, 4157.8, 3777, 3677, 3665, 3356.5, and 3348.5 Å. There are also some headless bands and extra lines in the spectrum.[20] Weak bands are found with heads at 5133 and 5108.[22]

If the valence electron is in 2s 3s or 3d, a 1Σu state results, and if in 2p 3p or 4p, a 1Σg state results.[23] The ground state is X1Σg+.[24]

The three lowest triplet states of He2 have designations a3Σu, b3Πg and c3Σg.[25] The a3Σu state with no vibration (v=0) has a long metastable lifetime of 18 s, much longer than the lifetime for other states or inert gas excimers.[3] The explanation is that the a3Σu state has no electron orbital angular momentum, as all the electrons are in S orbitals for the helium sate.[3]

The lower lying singlet states of He2 are A1Σu, B1Πg and C1Σg.[26] The excimer molecules are much smaller and more tightly bound. For the A1Σu state the binding energy is around 2.5 eV, with a separation of 1.039 Å. The C1Σg state has a binding energy 0.643 eV and the separation between atoms is 1.091 Å.[23] These two states have a repulsive range of distances with a maximum around 3 Å, where if the excited atoms approach, they have to overcome an energy barrier.[23]

The spectrum of the He2 excimer contains bands due to a great number of lines due to transitions between different rotation rates and vibrational states, combined with different electronic transitions. The lines can be grouped into P, Q and R branches. But the even numbered rotational levels do not have Q branch lines, due to both nuclei being spin 0. Numerous electronic states of the molecule have been studied, including Rydberg states with the number of the shell up to 25.[27]

Helium discharge lamps produce vacuum ultraviolet radiation from helium molecules. When high energy protons hit helium gas it also produces UV emission at around 600 Å by the decay of excited highly vibrating molecules of He2 in the A1Σu state to the ground state.[28] The UV radiation from excited helium molecules is used in the the pulsed discharge ionization detector (PDHID) which is capable of detecting the contents of mixed gases at levels below parts per billion.[29]

The Hopfield continuum is a band of ultraviolet light between 600 and 1000 Å in wavelength formed by photodissociation of helium molecules.[28]

One mechanism for formation of the helium molecules is firstly a helium atom becomes excited with one electron in the 21S orbital. This excited atom meets two other non excited helium atoms in a three body association and reacts to form a A1Σu state molecule with maximum vibration and a helium atom.[28]

Helium molecules in the quintet state 5Σ+g can be formed by the reaction of two spin polarised helium atoms in He(23S1) states. This molecule has a high energy level of 20 eV. The highest vibration level allowed is v=14. [30]

Clusters

He2 can form van der Waals compounds with other atoms forming bigger clusters such as 24MgHe2 and 40CaHe2.[31]

The helium-4 trimer (4He3), a cluster of three helium atoms, is predicted to be in an Efimov state.[32][33]

Cage

Two helium atoms can fit inside larger fullerenes, including C70 and C84. These can be detected by the nuclear magnetic resonance of 3He having a small shift, and by mass spectrometry. C84 with enclosed helium can contain 20% He2@C84, whereas C78 has 10% and C76 has 8%. The larger cavities are more likely to hold more atoms.[34] Even when the two helium atoms are placed closely to each other in a small cage, there is no chemical bond between them.[8][35] The presence of two He atoms in a C60 fullerene cage is only predicted to have a small effect on the reactivity of the fullerene.[36]

References

  1. ↑ "Substance Name: Dihelium". Toxnet.
  2. ↑ 2.0 2.1 Schöllkopf, W; Toennies, JP (25 November 1994). "Nondestructive mass selection of small van der waals clusters.". Science (New York, N.Y.) 266 (5189): 1345–8. PMID 17772840.
  3. ↑ 3.0 3.1 3.2 3.3 Raunhardt, Matthias (2009). Generation and spectroscopy of atoms and molecules in metastable states (PDF) (Thesis). p. 84.
  4. ↑ 4.0 4.1 Kolganova, Elena; Motovilov, Alexander; Sandhas, Werner (November 2004). "Scattering length of the helium-atom–helium-dimer collision". Physical Review A 70 (5). doi:10.1103/PhysRevA.70.052711.
  5. ↑ 5.0 5.1 Al Taisan, Nada Ahmed (May 2013). Spectroscopic Detection of the Lithium Helium (LiHe) van der Waals Molecule (PDF) (Thesis).
  6. ↑ Grisenti, R.; Schöllkopf, W.; Toennies, J.; Hegerfeldt, G.; Köhler, T.; Stoll, M. (September 2000). "Determination of the Bond Length and Binding Energy of the Helium Dimer by Diffraction from a Transmission Grating". Physical Review Letters 85 (11): 2284–2287. doi:10.1103/PhysRevLett.85.2284.
  7. ↑ Toennies, J. Peter. "Spectroscopy without Photons: Diffraction of Weakly Bound Complexes from Nano-Gratings".
  8. ↑ 8.0 8.1 Cerpa, Erick; Krapp, Andreas; Flores-Moreno, Roberto; Donald, Kelling J.; Merino, Gabriel (9 February 2009). "Influence of Endohedral Confinement on the Electronic Interaction between He atoms: A He2@C20H20 Case Study". Chemistry – A European Journal 15 (8): 1985–1990. doi:10.1002/chem.200801399.
  9. ↑ 9.0 9.1 9.2 Havermeier, T.; Jahnke, T.; Kreidi, K.; Wallauer, R.; Voss, S.; Schöffler, M.; Schössler, S.; Foucar, L.; Neumann, N.; Titze, J.; Sann, H.; Kühnel, M.; Voigtsberger, J.; Malakzadeh, A.; Sisourat, N.; Schöllkopf, W.; Schmidt-Böcking, H.; Grisenti, R. E.; Dörner, R. (April 2010). "Single Photon Double Ionization of the Helium Dimer". Physical Review Letters 104 (15). doi:10.1103/PhysRevLett.104.153401.
  10. ↑ Slater, J. (September 1928). "The Normal State of Helium". Physical Review 32 (3): 349–360. Bibcode:1928PhRv...32..349S. doi:10.1103/PhysRev.32.349.
  11. ↑ CALLEAR, A. B.; HEDGES, R. E. M. (16 September 1967). "Metastability of Rotationally Hot Dihelium at 77° K". Nature 215 (5107): 1267–1268. doi:10.1038/2151267a0.
  12. ↑ 12.0 12.1 Guilhaus, Michael; Brenton, A. Gareth; Beynon, John H.; Rabrenović, Mila; von Ragué Schleyer, Paul (1985). "He2 2+, the experimental detection of a remarkable molecule". Journal of the Chemical Society, Chemical Communications (4): 210. doi:10.1039/C39850000210.
  13. ↑ Pauling, Linus (1933). "The Normal State of the Helium Molecule-Ions He2+ and He2++". The Journal of Chemical Physics 1 (1): 56. doi:10.1063/1.1749219.
  14. ↑ Olah, George A.; Klumpp, Douglas A. Superelectrophiles and Their Chemistry. p. 12. Retrieved 19 February 2015.
  15. ↑ Dunitz, J. D.; Ha, T. K. (1972). "Non-empirical SCF calculations on hydrogen-like molecules: the effect of nuclear charge on binding energy and bond length". Journal of the Chemical Society, Chemical Communications (9): 568. doi:10.1039/C39720000568.
  16. ↑ Guilhaus, M; Brenton, A G; Beynon, J H; Rabrenovic, M; Schleyer, P Von Rague (14 September 1984). "First observation of He 2 2+ : charge stripping of He2+ using a double-focusing mass spectrometer". Journal of Physics B: Atomic and Molecular Physics 17 (17): L605–L610. doi:10.1088/0022-3700/17/17/010.
  17. ↑ Krishnamurthy, Vidhya; Mathur, D. (April 1990). "An experimental and theoretical study of the negatively charged helium dimer, He2−". Chemical Physics Letters 167 (5): 475–478. doi:10.1016/0009-2614(90)85034-a.
  18. ↑ Rodríguez-Cantano, Rocío; González-Lezana, Tomás; Villarreal, Pablo; Gianturco, Franco A. (14 March 2015). "A configurational study of helium clusters doped with He∗− and He2∗−". The Journal of Chemical Physics 142 (10): 104303. doi:10.1063/1.4913958.
  19. ↑ Vrinceanu, D.; Sadeghpour, H. (June 2002). "He(1 ^{1}S)–He(2 ^{3}S) collision and radiative transition at low temperatures". Physical Review A 65 (6). doi:10.1103/PhysRevA.65.062712.
  20. ↑ 20.0 20.1 Curtis, W. E. (19 August 1913). "A New Band Spectrum Associated with Helium". Proceedings of the Royal Society of London. Series A 89 (608): 146–149. Retrieved 14 February 2015.
  21. ↑ Goldstein, E. (1913). "Über ein noch nicht beschriebenes, anscheinend dem Helium angehörendes Spektrum". Verhandlungen der Physikalischen Gessellschaft 15 (10): 402–412.
  22. ↑ 22.0 22.1 Fowler, Alfred (1 March 1915). "A New Type of Series in the Band Spectrum Associated with Helium". Proceedings of the Royal Society of London. Series A 91 (627): 208–216.
  23. ↑ 23.0 23.1 23.2 Guberman, S.L.; Goddard, W.A. (15 June 1972). "On the origin of energy barriers in the excited states of He2". Chemical Physics Letters 14 (4): 460–465. doi:10.1016/0009-2614(72)80240-9.
  24. ↑ Kristensen, Martin; Keiding, Søren R.; van der Zande, Wim J. (December 1989). "Lifetime determination of the long-lived B 1Πg state in He2* by photofragment spectroscopy". Chemical Physics Letters 164 (6): 600–604. doi:10.1016/0009-2614(89)85266-2.
  25. ↑ Hazell, I.; Norregaard, A.; Bjerre, N. (July 1995). "Highly Excited Rotational and Vibrational Levels of the Lowest Triplet States of He2: Level Positions and Fine Structure". Journal of Molecular Spectroscopy 172 (1): 135–152. doi:10.1006/jmsp.1995.1162.
  26. ↑ Focsa, C.; Bernath, P.F.; Colin, R. (September 1998). "The Low-Lying States of He2". Journal of Molecular Spectroscopy 191 (1): 209–214. doi:10.1006/jmsp.1998.7637.
  27. ↑ Panock, R.; Freeman, R.R.; Storz, R.H.; Miller, Terry A. (September 1980). "Observation of laser driven transitions to high rydberg states of He2". Chemical Physics Letters 74 (2): 203–206. doi:10.1016/0009-2614(80)85142-6.
  28. ↑ 28.0 28.1 28.2 Hill, Peter (November 1989). "Ultraviolet continua of helium molecules". Physical Review A 40 (9): 5006–5016. doi:10.1103/PhysRevA.40.5006.
  29. ↑ Cai, Huamin; Stearns, Stanley D. (April 2013). "Pulsed discharge helium ionization detector with multiple combined bias/collecting electrodes for gas chromatography". Journal of Chromatography A 1284: 163–173. doi:10.1016/j.chroma.2013.01.100.
  30. ↑ Beams, Timothy J.; Peach, Gillian; Whittingham, Ian B. (18 July 2006). "Spin-dipole-induced lifetime of the least-bound 5Σ+g state of He(23S1)+He(23S1)". Physical Review A 74 (1). doi:10.1103/PhysRevA.74.014702.
  31. ↑ Liu, Min-min; Han, Hui-li; Li, Cheng-bin; Gu, Si-hong (October 2013). "Binding energies and geometry of the 24Mg-He2 and 40Ca-He2 triatomic systems". Physical Review A 88 (4). doi:10.1103/PhysRevA.88.042503.
  32. ↑ Kolganova, Elena A. (26 Nov 2010). "Helium Trimer in the Framework of Faddeev Approach" (PDF). Physics of Particles and Nuclei 41 (7). pp. 1108–1110. doi:10.1134/S1063779610070282. Retrieved 28 February 2015.
  33. ↑ Kolganova, E. A.; Motovilov, A. K.; Sandhas, W. (4 May 2011). "The 4He Trimer as an Efimov System". Few-Body Systems 51 (2–4): 249–257. arXiv:1104.1989. doi:10.1007/s00601-011-0233-x.
  34. ↑ Wang, Guan-Wu; Saunders, Martin; Khong, Anthony; Cross, R. James (April 2000). "A New Method for Separating the Isomeric C84 Fullerenes". Journal of the American Chemical Society 122 (13): 3216–3217. doi:10.1021/ja994270x.
  35. ↑ Krapp, Andreas; Frenking, Gernot (5 October 2007). "Is This a Chemical Bond? A Theoretical Study of Ng2@C60 (Ng=He, Ne, Ar, Kr, Xe)". Chemistry – A European Journal 13 (29): 8256–8270. doi:10.1002/chem.200700467.
  36. ↑ Osuna, SÃlvia; Swart, Marcel; SolÃ, Miquel (7 December 2009). "Reactivity and Regioselectivity of Noble Gas Endohedral Fullerenes Ng@C60 and Ng2@C60(Ng=He-Xe)" (PDF). Chemistry – A European Journal 15 (47): 13111–13123. doi:10.1002/chem.200901224.

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