Helium dimer

Last updated

Contents

Helium dimer
Helium-dimer-2D-model.png
Names
Other names
dihelium
Identifiers
3D model (JSmol)
ChEBI
48
PubChem CID
  • [1] :InChI=1S/He2/c1-2
    Key: GHVQTHCLRQIINU-UHFFFAOYSA-N
  • [He][He]
Properties
He2
Molar mass 8.005204 g·mol−1
Appearancecolorless gas
Thermochemistry
-1.1×10−5 kcal/mol
Related compounds
Related van der Waals molecules
 LiHe NeHe2 He3
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
X mark.svgN (what is  Yes check.svgYX mark.svgN ?)

The helium dimer is a van der Waals molecule with formula He2 consisting of two helium atoms. [2] This chemical is the largest diatomic molecule—a molecule consisting of two atoms bonded together. The bond that holds this dimer together 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. Written as He2* with the * meaning an excited state, it is the first known Rydberg molecule. [3]

Several dihelium ions also exist, having net charges of negative one, positive one, and positive two. 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] The existence of this molecule was proposed as early as 1930. [5]

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 5200  pm (= 52 ångström). This is the largest for a diatomic molecule without ro-vibronic excitation. The binding energy is only about 1.3 mK, 10−7eV [6] [7] [8] or 1.1×10−5 kcal/mol. [9] According to high-level theoretical calculations, the bond is 5000 times weaker than the covalent bond in the hydrogen molecule. [10]

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. [11] The dimer then explodes as two helium cations ions repel each other with the same speed but opposite directions. [11]

A dihelium molecule bound by Van der Waals forces was first proposed by John Clarke Slater in 1928. [12]

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. [6] The amount of the dimer formed in the gas beam is of the order of one percent. [11]

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. [13] Both of these molecules are much smaller with more normally sized interatomic distances. He2+ reacts with N2, Ar, Xe, O2, and CO2 to form cations and neutral helium atoms. [14]

The helium dication dimer He22+ is extremely repulsive and releases much energy when it dissociates, around 835 kJ/mol. [15] Dynamical stability of the ion was predicted by Linus Pauling. [16] An energy barrier of 138.91 kJ/mol prevents immediate decay. This ion is isoelectronic with the hydrogen molecule. [17] [18] He22+ is the smallest possible molecule with a double positive charge. It is detectable using mass spectroscopy. [15] [19]

The negative helium dimer He2− is metastable and was discovered by Bae, Coggiola and Peterson in 1984 by passing He2+ through cesium vapor. [20] Subsequently, H. H. Michels theoretically confirmed its existence and concluded that the 4Πg state of He2− is bound relative to the a2Σ+u state of He2. [21] The calculated electron affinity is 0.233 eV compared to 0.077 eV for the He−[4P∘] ion. The He2− decays through the long-lived 5/2g component with τ~350 μsec and the much shorter-lived 3/2g, 1/2g components with τ~10 μsec. The 4Πg state has a 1σ2g1σu2σg2πu electronic configuration, its electron affinity E is 0.18±0.03 eV, and its lifetime is 135±15 μsec; only the v=0 vibrational state is responsible for this long-lived state. [22]

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 then 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 the surface of liquid helium. [23]

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. [24]

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. [25] [26] 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 double headed bands fell into two sequences analogous to principal and diffuse series in line spectra. [27]

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. [25] Weak bands are found with heads at 5133 and 5108. [27]

If the valence electron is in a 2s 3s, or 3d orbital, a 1Σu state results; if it is in 2p 3p or 4p, a 1Σg state results. [28] The ground state is X1Σg+. [29]

The three lowest triplet states of He2 have designations a3Σu, b3Πg and c3Σg. [30] 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 state. [3]

The lower lying singlet states of He2 are A1Σu, B1Πg and C1Σg. [31] The excimer molecules are much smaller and more tightly bound than the van der Waals bonded helium dimer. For the A1Σu state the binding energy is around 2.5 eV, with a separation of the atoms of 103.9 pm. The C1Σg state has a binding energy 0.643 eV and the separation between atoms is 109.1 pm. [28] These two states have a repulsive range of distances with a maximum around 300 pm, where if the excited atoms approach, they have to overcome an energy barrier. [28] The singlet state A1Σ+u is very unstable with a lifetime only nanoseconds long. [32]

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. [33]

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. [34] The UV radiation from excited helium molecules is used in the pulsed discharge ionization detector (PDHID) which is capable of detecting the contents of mixed gases at levels below parts per billion. [35]

The Hopfield continuum is a band of ultraviolet light between 600 and 1000 Å in wavelength formed by photodissociation of helium molecules. [34]

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. [34]

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. [36]

In liquid helium the excimer forms a solvation bubble. In a 3d state a He*
2 molecule is surrounded by a bubble 12.7 Å in radius at atmospheric pressure. When pressure is increased to 24 atmospheres the bubble radius shrinks to 10.8 Å. This changing bubble size causes a shift in the fluorescence bands. [37]

stateKelectronic angular momentum Λelectronic spin SHund's coupling casetypeenergydissociation energy eVlength pmvibration levels
A1Σu1,3,5,7singlet2.5103.9
B1Πgsinglet
C1Σg0,2,4,6singlet
a3Σu1,3,5,7triplet
b3Πgtriplet
c3Σg0,2,4,601btriplet
5Σ+gquintet

Magnetic condensation

In very strong magnetic fields, (around 750,000 Tesla) and low enough temperatures, helium atoms attract, and can even form linear chains. This may happen in white dwarfs and neutron stars. [38] The bond length and dissociation energy both increase as the magnetic field increases. [39]

Use

The dihelium excimer is an important component in the helium discharge lamp.

A second use of dihelium ion is in ambient ionization techniques using low temperature plasma. In this helium atoms are excited, and then combine to yield the dihelium ion. The He2+ goes on to react with N2 in the air to make N2+. These ions react with a sample surface to make positive ions that are used in mass spectroscopy. The plasma containing the helium dimer can be as low as 30 °C in temperature, and this reduces heat damage to samples. [40]

Clusters

He2 has been shown to form van der Waals compounds with other atoms forming bigger clusters such as 24MgHe2 and 40CaHe2. [41]

The helium-4 trimer (4He3), a cluster of three helium atoms, is predicted to have an excited state which is an Efimov state. [42] [43] This has been confirmed experimentally in 2015. [44]

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. [45] Even when the two helium atoms are placed closely to each other in a small cage, there is no chemical bond between them. [10] [46] 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. [47] The effect is to have electrons withdrawn from the endohedral helium atoms, giving them a slight positive partial charge to produce He2δ+, which have a stronger bond than uncharged helium atoms. [48] However, by the Löwdin definition there is a bond present. [49]

The two helium atoms inside the C60 cage are separated by 1.979 Å and the distance from a helium atom to the carbon cage is 2.507 Å. The charge transfer gives 0.011 electron charge units to each helium atom. There should be at least 10 vibrational levels for the He-He pair. [49]

Related Research Articles

<span class="mw-page-title-main">Fullerene</span> Allotrope of carbon

A fullerene is an allotrope of carbon whose molecules consist of carbon atoms connected by single and double bonds so as to form a closed or partially closed mesh, with fused rings of five to seven atoms. The molecules may be hollow spheres, ellipsoids, tubes, or other shapes.

<span class="mw-page-title-main">Noble gas</span> Group of low-reactive, gaseous chemical elements

The noble gases are the naturally occurring members of group 18 of the periodic table: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). Under standard conditions, these elements are odorless, colorless, monatomic gases with very low chemical reactivity and cryogenic boiling points.

<span class="mw-page-title-main">Buckminsterfullerene</span> Cage-like allotrope of carbon

Buckminsterfullerene is a type of fullerene with the formula C60. It has a cage-like fused-ring structure (truncated icosahedron) made of twenty hexagons and twelve pentagons, and resembles a football. Each of its 60 carbon atoms is bonded to its three neighbors.

<span class="mw-page-title-main">Ionization</span> Process by which atoms or molecules acquire charge by gaining or losing electrons

Ionization is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule is called an ion. Ionization can result from the loss of an electron after collisions with subatomic particles, collisions with other atoms, molecules and ions, or through the interaction with electromagnetic radiation. Heterolytic bond cleavage and heterolytic substitution reactions can result in the formation of ion pairs. Ionization can occur through radioactive decay by the internal conversion process, in which an excited nucleus transfers its energy to one of the inner-shell electrons causing it to be ejected.

Dilithium, Li2, is a strongly electrophilic, diatomic molecule comprising two lithium atoms covalently bonded together. Li2 is known in the gas phase. It has a bond order of 1, an internuclear separation of 267.3 pm and a bond energy of 102 kJ/mol or 1.06 eV in each bond. The electron configuration of Li2 may be written as σ2.

<span class="mw-page-title-main">Excimer</span> Excited dimeric molecule containing a noble gas

An excimer is a short-lived polyatomic molecule formed from two species that do not form a stable molecule in the ground state. In this case, formation of molecules is possible only if such atom is in an electronic excited state. Heteronuclear molecules and molecules that have more than two species are also called exciplex molecules. Excimers are often diatomic and are composed of two atoms or molecules that would not bond if both were in the ground state. The lifetime of an excimer is very short, on the order of nanoseconds.

In chemistry, noble gas compounds are chemical compounds that include an element from the noble gases, group 18 of the periodic table. Although the noble gases are generally unreactive elements, many such compounds have been observed, particularly involving the element xenon.

<span class="mw-page-title-main">Endohedral fullerene</span> Fullerene molecule with additional atoms, ions, or clusters enclosed within itself

Endohedral fullerenes, also called endofullerenes, are fullerenes that have additional atoms, ions, or clusters enclosed within their inner spheres. The first lanthanum C60 complex called La@C60 was synthesized in 1985. The @ (at sign) in the name reflects the notion of a small molecule trapped inside a shell. Two types of endohedral complexes exist: endohedral metallofullerenes and non-metal doped fullerenes.

Penning ionization is a form of chemi-ionization, an ionization process involving reactions between neutral atoms or molecules. The Penning effect is put to practical use in applications such as gas-discharge neon lamps and fluorescent lamps, where the lamp is filled with a Penning mixture to improve the electrical characteristics of the lamps.

This page deals with the electron affinity as a property of isolated atoms or molecules. Solid state electron affinities are not listed here.

<span class="mw-page-title-main">Helium hydride ion</span> Chemical compound

The helium hydride ion, hydridohelium(1+) ion, or helonium is a cation (positively charged ion) with chemical formula HeH+. It consists of a helium atom bonded to a hydrogen atom, with one electron removed. It can also be viewed as protonated helium. It is the lightest heteronuclear ion, and is believed to be the first compound formed in the Universe after the Big Bang.

Interatomic Coulombic decay (ICD) is a general, fundamental property of atoms and molecules that have neighbors. Interatomic (intermolecular) Coulombic decay is a very efficient interatomic (intermolecular) relaxation process of an electronically excited atom or molecule embedded in an environment. Without the environment the process cannot take place. Until now it has been mainly demonstrated for atomic and molecular clusters, independently of whether they are of van-der-Waals or hydrogen bonded type.

An excimer lamp is a source of ultraviolet light based on spontaneous emission of excimer (exciplex) molecules.

Lithium helide is a compound of helium and lithium with the formula LiHe. The substance is a cold low-density gas made of Van der Waals molecules, each composed of a helium atom and lithium atom bound by van der Waals force. The preparation of LiHe opens up the possibility to prepare other helium dimers, and beyond that multi-atom clusters that could be used to investigate Efimov states and Casimir retardation effects.

Helium is the smallest and the lightest noble gas and one of the most unreactive elements, so it was commonly considered that helium compounds cannot exist at all, or at least under normal conditions. Helium's first ionization energy of 24.57 eV is the highest of any element. Helium has a complete shell of electrons, and in this form the atom does not readily accept any extra electrons nor join with anything to make covalent compounds. The electron affinity is 0.080 eV, which is very close to zero. The helium atom is small with the radius of the outer electron shell at 0.29 Å. Helium is a very hard atom with a Pearson hardness of 12.3 eV. It has the lowest polarizability of any kind of atom, however, very weak van der Waals forces exist between helium and other atoms. This force may exceed repulsive forces, so at extremely low temperatures helium may form van der Waals molecules. Helium has the lowest boiling point of any known substance.

<span class="mw-page-title-main">Carbon peapod</span> Hybrid nanomaterial

Carbon peapod is a hybrid nanomaterial consisting of spheroidal fullerenes encapsulated within a carbon nanotube. It is named due to their resemblance to the seedpod of the pea plant. Since the properties of carbon peapods differ from those of nanotubes and fullerenes, the carbon peapod can be recognized as a new type of a self-assembled graphitic structure. Possible applications of nano-peapods include nanoscale lasers, single electron transistors, spin-qubit arrays for quantum computing, nanopipettes, and data storage devices thanks to the memory effects and superconductivity of nano-peapods.

Neon compounds are chemical compounds containing the element neon (Ne) with other molecules or elements from the periodic table. Compounds of the noble gas neon were believed not to exist, but there are now known to be molecular ions containing neon, as well as temporary excited neon-containing molecules called excimers. Several neutral neon molecules have also been predicted to be stable, but are yet to be discovered in nature. Neon has been shown to crystallize with other substances and form clathrates or Van der Waals solids.

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.

<span class="mw-page-title-main">Dirubidium</span> Chemical compound

Dirubidium is a molecular substance containing two atoms of rubidium found in rubidium vapour. Dirubidium has two active valence electrons. It is studied both in theory and with experiment. The rubidium trimer has also been observed.

<span class="mw-page-title-main">Diargon</span> Chemical compound

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.

References

  1. ↑ "Substance Name: Dihelium". Toxnet.
  2. 1 2 Schöllkopf, W; Toennies, JP (25 November 1994). "Nondestructive mass selection of small van der Waals clusters". Science. 266 (5189): 1345–8. Bibcode:1994Sci...266.1345S. doi:10.1126/science.266.5189.1345. PMID   17772840. S2CID   23043700.
  3. 1 2 3 4 Raunhardt, Matthias (2009). Generation and spectroscopy of atoms and molecules in metastable states (PDF) (Thesis). p. 84.
  4. 1 2 Kolganova, Elena; Motovilov, Alexander; Sandhas, Werner (November 2004). "Scattering length of the helium-atom–helium-dimer collision". Physical Review A. 70 (5): 052711. arXiv: physics/0408019 . Bibcode:2004PhRvA..70e2711K. doi:10.1103/PhysRevA.70.052711. S2CID   118311511.
  5. ↑ Glockler, Geo. (1937). "Complex formation". Transactions of the Faraday Society. 33: 224. doi:10.1039/TF9373300224.(subscription required)
  6. 1 2 Al Taisan, Nada Ahmed (May 2013). Spectroscopic Detection of the Lithium Helium (LiHe) van der Waals Molecule (PDF) (Thesis). Archived from the original (PDF) on 4 March 2016. Retrieved 9 February 2015.
  7. ↑ 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. Bibcode:2000PhRvL..85.2284G. doi:10.1103/PhysRevLett.85.2284. PMID   10977992.
  8. ↑ Zeller, S.; Kunitski, M.; Voigtsberger, J.; Kalinin, A.; Schottelius, A.; Schober, C.; Waitz, M.; Sann, H.; Hartung, A.; Bauer, T.; Pitzer, M.; Trinter, F.; Goihl, C.; Janke, C.; Richter, M.; Kastirke, G.; Weller, M.; Czasch, A.; Kitzler, M.; Braune, M.; Grisenti, R. E.; Schöllkopf, W.; Schmidt, L. Ph H.; Schöffer, M.; Williams, J. B.; Jahnke, T.; Dörner, R. (20 December 2016). "Imaging the He2 quantum halo state using a free electron laser". Proceedings of the National Academy of Sciences. 113 (51): 14651–14655. arXiv: 1601.03247 . Bibcode:2016PNAS..11314651Z. doi: 10.1073/pnas.1610688113 . ISSN   0027-8424. PMC   5187706 . PMID   27930299.
  9. ↑ Toennies, J. Peter. "Spectroscopy without Photons: Diffraction of Weakly Bound Complexes from Nano-Gratings". Archived from the original on 4 March 2016. Retrieved 9 February 2015.
  10. 1 2 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. PMID   19021178.
  11. 1 2 3 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): 153401. arXiv: 1006.2667 . Bibcode:2010PhRvL.104o3401H. doi:10.1103/PhysRevLett.104.153401. PMID   20481987. S2CID   13319551.
  12. ↑ 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.
  13. ↑ Callear, A. B.; Hedges, R. E. M. (16 September 1967). "Metastability of Rotationally Hot Dihelium at 77° K". Nature. 215 (5107): 1267–1268. Bibcode:1967Natur.215.1267C. doi:10.1038/2151267a0. S2CID   4251449.
  14. ↑ Jahani, H.R.; Gylys, V.T.; Collins, C.B.; Pouvesle, J.M.; Stevefelt, J. (March 1988). "The importance of three-body processes to reaction kinetics at atmospheric pressures. III. Reactions of He/sub 2//sup +/ with selected atomic and molecular reactants". IEEE Journal of Quantum Electronics. 24 (3): 568–572. doi:10.1109/3.162.
  15. 1 2 Guilhaus, Michael; Brenton, A. Gareth; Beynon, John H.; Rabrenović, Mila; von Ragué Schleyer, Paul (1985). "He22+, the experimental detection of a remarkable molecule". Journal of the Chemical Society, Chemical Communications (4): 210–211. doi:10.1039/C39850000210.
  16. ↑ Pauling, Linus (1933). "The Normal State of the Helium Molecule-Ions He2+ and He2++". The Journal of Chemical Physics. 1 (1): 56–59. Bibcode:1933JChPh...1...56P. doi:10.1063/1.1749219.
  17. ↑ Olah, George A.; Klumpp, Douglas A. (3 January 2008). Superelectrophiles and Their Chemistry. John Wiley & Sons. p. 12. ISBN   9780470185117 . Retrieved 19 February 2015.
  18. ↑ 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): 568–569. doi:10.1039/C39720000568.
  19. ↑ Guilhaus, M.; Brenton, A. G.; Beynon, J. H.; Rabrenovic, M.; Schleyer, P. von Rague (14 September 1984). "First observation of He22+: charge stripping of He2+ using a double-focusing mass spectrometer". Journal of Physics B: Atomic and Molecular Physics. 17 (17): L605–L610. Bibcode:1984JPhB...17L.605G. doi:10.1088/0022-3700/17/17/010.
  20. ↑ Bae, Y. K.; Coggiola, M. J.; Peterson, J. R. (27 February 1984). "Observation of the Molecular Helium Negative Ion He2−". Physical Review Letters. 52 (9): 747–750. Bibcode:1984PhRvL..52..747B. doi:10.1103/PhysRevLett.52.747.
  21. ↑ Michels, H. H. (16 April 1984). "Electronic Structure of the Helium Molecular Anion He2−". Physical Review Letters. 52 (16): 1413–1416. Bibcode:1984PhRvL..52.1413M. doi:10.1103/PhysRevLett.52.1413.
  22. ↑ Andersen, T. (1995). "Lifetimes of negative ions determined in a storage ring". Physica Scripta. 1995 (T59): 230–235. Bibcode:1995PhST...59..230A. doi:10.1088/0031-8949/1995/T59/031. ISSN   1402-4896. S2CID   250868275.
  23. ↑ 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∗−" (PDF). The Journal of Chemical Physics. 142 (10): 104303. Bibcode:2015JChPh.142j4303R. doi:10.1063/1.4913958. hdl: 10261/128098 . PMID   25770536.
  24. ↑ 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): 062712. Bibcode:2002PhRvA..65f2712V. doi:10.1103/PhysRevA.65.062712.
  25. 1 2 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. Bibcode:1913RSPSA..89..146C. doi: 10.1098/rspa.1913.0073 . JSTOR   93468.
  26. ↑ Goldstein, E. (1913). "Über ein noch nicht beschriebenes, anscheinend dem Helium angehörendes Spektrum". Verhandlungen der Physikalischen Gessellschaft. 15 (10): 402–412.
  27. 1 2 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. Bibcode:1915RSPSA..91..208F. doi:10.1098/rspa.1915.0011. JSTOR   93423. S2CID   95790902.
  28. 1 2 3 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. Bibcode:1972CPL....14..460G. doi:10.1016/0009-2614(72)80240-9.
  29. ↑ 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. Bibcode:1989CPL...164..600K. doi:10.1016/0009-2614(89)85266-2.
  30. ↑ 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. Bibcode:1995JMoSp.172..135H. doi:10.1006/jmsp.1995.1162.
  31. ↑ Focsa, C.; Bernath, P.F.; Colin, R. (September 1998). "The Low-Lying States of He2". Journal of Molecular Spectroscopy. 191 (1): 209–214. Bibcode:1998JMoSp.191..209F. doi:10.1006/jmsp.1998.7637. PMID   9724597.
  32. ↑ Carter, F.W.; Hertel, S.A.; Rooks, M.J.; McClintock, P.V.E.; McKinsey, D.N.; Prober, D.E. (4 May 2016). "Calorimetric observation of single He∗ 2 excimers in a 100 mK He bath". Journal of Low Temperature Physics. 186 (3): 183–196. arXiv: 1605.00694v1 . doi:10.1007/s10909-016-1666-x. PMC   7346980 . PMID   32669743.
  33. ↑ 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. Bibcode:1980CPL....74..203P. doi:10.1016/0009-2614(80)85142-6.
  34. 1 2 3 Hill, Peter (November 1989). "Ultraviolet continua of helium molecules". Physical Review A. 40 (9): 5006–5016. Bibcode:1989PhRvA..40.5006H. doi:10.1103/PhysRevA.40.5006. PMID   9902760.
  35. ↑ 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. PMID   23484651.
  36. ↑ 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): 014702. arXiv: physics/0604189 . Bibcode:2006PhRvA..74a4702B. doi:10.1103/PhysRevA.74.014702. S2CID   117149989.
  37. ↑ Bonifaci, Nelly; Li, Zhiling; Eloranta, Jussi; Fiedler, Steven L. (4 November 2016). "Interaction of Helium Rydberg State Molecules with Dense Helium". The Journal of Physical Chemistry A. 120 (45): 9019–9027. Bibcode:2016JPCA..120.9019B. doi:10.1021/acs.jpca.6b08412. PMID   27783517.
  38. ↑ Lai, Dong (29 August 2001). "Matter in strong magnetic fields". Reviews of Modern Physics. 73 (3): 629–662. arXiv: astro-ph/0009333 . Bibcode:2001RvMP...73..629L. doi:10.1103/RevModPhys.73.629. S2CID   119492595.
  39. ↑ Lange, K. K.; Tellgren, E. I.; Hoffmann, M. R.; Helgaker, T. (19 July 2012). "A Paramagnetic Bonding Mechanism for Diatomics in Strong Magnetic Fields". Science. 337 (6092): 327–331. Bibcode:2012Sci...337..327L. doi:10.1126/science.1219703. PMID   22822146. S2CID   5431912.
  40. ↑ Seró, R.; Núñez, Ó.; Moyano, E. (2016). Ambient Ionisation–High-Resolution Mass Spectrometry. Comprehensive Analytical Chemistry. Vol. 71. pp. 51–88. doi:10.1016/bs.coac.2016.01.003. ISBN   9780444635723. ISSN   0166-526X.
  41. ↑ 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): 042503. Bibcode:2013PhRvA..88d2503L. doi:10.1103/PhysRevA.88.042503.
  42. ↑ Kolganova, Elena A. (26 November 2010). "Helium Trimer in the Framework of Faddeev Approach" (PDF). Physics of Particles and Nuclei. 41 (7): 1108–1110. Bibcode:2010PPN....41.1108K. doi:10.1134/S1063779610070282. S2CID   120976241 . Retrieved 28 February 2015.
  43. ↑ 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 . Bibcode:2011FBS....51..249K. doi:10.1007/s00601-011-0233-x. S2CID   119266992.
  44. ↑ Kunitski, Maksim; Zeller, Stefan; Voigtsberger, Jörg; Kalinin, Anton; Schmidt, Lothar Ph. H.; Schöffler, Markus; Czasch, Achim; Schöllkopf, Wieland; Grisenti, Robert E.; Jahnke, Till; Blume, Dörte; Dörner, Reinhard (May 2015). "Observation of the Efimov state of the helium trimer". Science. 348 (6234): 551–555. arXiv: 1512.02036 . Bibcode:2015Sci...348..551K. doi:10.1126/science.aaa5601. PMID   25931554. S2CID   206635093.
  45. ↑ 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.
  46. ↑ 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. PMID   17639524.
  47. ↑ Osuna, Sílvia; Swart, Marcel; Solà, 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. PMID   19859923.
  48. ↑ Kryachko, Eugene S.; Nikolaienko, Tymofii Yu. (15 July 2015). "He2@C60: Thoughts of the concept of a molecule and of the concept of a bond in quantum chemistry". International Journal of Quantum Chemistry. 115 (14): 859–867. doi:10.1002/qua.24916.
  49. 1 2 Dolgonos, G. A.; Kryachko, E. S.; Nikolaienko, T. Yu (18 June 2018). "До питання Не–Не зв'язку у ендоедральному фулерені Не2@C60 (On the Problem of He–He Bond in the Endohedral Fullerene He2@C60)". Ukrainian Journal of Physics. 63 (4): 288. doi: 10.15407/ujpe63.4.288 . ISSN   2071-0194. Open Access logo PLoS transparent.svg
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.