Helium dimer
Names | |
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Other names
dihelium
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Identifiers | |
3D model (
JSmol ) |
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ChEBI | |
48 | |
PubChem CID
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Properties | |
He2 | |
Molar mass | 8.005204 g·mol−1 |
Appearance | colorless gas |
Thermochemistry | |
Std enthalpy of (ΔfH⦵298)formation |
-1.1×10−5 kcal/mol |
Related compounds | |
Related van der Waals molecules
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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The helium dimer is a
Two
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
Both helium atoms in the dimer can be
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.
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
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
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]
state | K | electronic angular momentum Λ | electronic spin S | Hund's coupling case | type | energy | dissociation energy eV | length pm | vibration levels |
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A1Σu | 1,3,5,7 | singlet | 2.5 | 103.9 | |||||
B1Πg | singlet | ||||||||
C1Σg | 0,2,4,6 | singlet | |||||||
a3Σu | 1,3,5,7 | triplet | |||||||
b3Πg | triplet | ||||||||
c3Σg | 0,2,4,6 | 0 | 1 | b | triplet | ||||
5Σ+g | quintet |
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
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
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]
References
- ^ "Substance Name: Dihelium". Toxnet.
- ^ S2CID 23043700.
- ^ a b c d Raunhardt, Matthias (2009). Generation and spectroscopy of atoms and molecules in metastable states (PDF) (Thesis). p. 84.
- ^ S2CID 118311511.
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- ^ a b 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.
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- ^ 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.
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- ^ Goldstein, E. (1913). "Über ein noch nicht beschriebenes, anscheinend dem Helium angehörendes Spektrum". Verhandlungen der Physikalischen Gessellschaft. 15 (10): 402–412.
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External links
- "Dihelium". NIST. November 1976.
- Jahnke, T (28 April 2015). "Interatomic and intermolecular Coulombic decay: the coming of age story". Journal of Physics B: Atomic, Molecular and Optical Physics. 48 (8): 082001. S2CID 56093209.
- Sprecher, D.; Liu, J.; Krähenmann, T.; Schäfer, M.; Merkt, F. (14 February 2014). "High-resolution spectroscopy and quantum-defect model for the gerade triplet np and nf Rydberg states of He2". The Journal of Chemical Physics. 140 (6): 064304. PMID 24527912. spectrum of He2