Helium compounds

Impurity helium condensates

Impurity helium condensates (IHCs) (or impurity helium gels) are deposited as a snow-like gel in liquid helium when various atoms or molecules are absorbed on the surface of superfluid helium. Atoms can include H, N, Na, Ne, Ar, Kr, Xe, alkalis or alkaline earths. The impurities form nanoparticle clusters coated with localised helium held by van der Waals force. Helium atoms are unable to move towards or away from the impurity, but perhaps can move perpendicularly around the impurity. The snow like solid is structured like an aerogel. When free atoms are included in the condensate a high energy density can be achieved, up to 860 J cm⁻¹ or 5 kJ g⁻¹. These condensates were first investigated as a possible rocket fuel. The mixtures are given a notation involving square brackets so that [N]/[He] represents a nitrogen atom impurity in helium.

[N]/[He] atomic nitrogen impurity helium is produced when a radio frequency discharge in a nitrogen helium mixture is absorbed into superfluid helium, it can have up to 4% nitrogen atoms included. The substance resembles crumbly snow and condenses and settles from the liquid helium. It also contains variable proportions of N₂ molecules. This substance is a high energy solid, with as much power as conventional explosives. When it is heated above 2.19 K (the lambda point of helium), the solid decomposes and explodes. This substance is not a true compound, but more like a solid solution. E. B. Gordon et al. suggested that this material may exist in 1974. The localised helium shells around an individual atom are termed van der Waals spheres. However the idea that the nitrogen atoms are dispersed in the helium has been replaced by the concept of nitrogen atoms attached to the surface of clusters of nitrogen molecules. The energy density of the solid can be increased by pressing it.

Other inert gas impurity helium condensates can also be made from a gas beam into superfluid helium. [Ne]/[He] decomposes at 8.5 K with release of heat and formation of solid neon. Its composition approximates NeHe₁₆.

[Ar]/[He] contains 40–60 helium atoms per argon atom.

[Kr]/[He] contains 40–60 helium atoms per krypton atom and is stable up to 20 K.

[Xe]/[He] contains 40–60 helium atoms per xenon atom.

[N₂]/[He] contains 12—17 He atoms per N₂ molecule. It is stable up to 13 K

[N]/[Ne]/[He] Formed from a gas beam generated from a radio-frequency electric discharge in mixtures of neon, nitrogen and helium blown into superfluid He. Additional inert gas stabilises more nitrogen atoms. It decomposes around 7 K with a blue green light flash. Excited nitrogen atoms in the N(²D) state can be relative long lasting, up to hours, and give off a green luminescence.

[H₂]/[He], or [D₂]/[He] when dihydrogen or dideuterium is absorbed into superfluid helium, filaments are formed. When enough of these form, the solid resembles cotton, rather than snow. Using H₂ results in the product floating and stopping further production, but with deuterium, or a half-half mixture, it can sink and accumulate. Atomic hydrogen in impurity helium decays fairly rapidly due to quantum tunneling (H + H → H₂). Atomic deuterium dimerises slower (D + D → D₂), but reacts very quickly with any diprotium present. (D + H₂ → HD + H). Atomic hydrogen solids are further stabilised by other noble gases such as krypton. Lowering temperatures into the millikelvin range can prolong the lifetime of atomic hydrogen condensates. Condensates containing heavy water or deuterium are under investigation for the production of ultracold neutrons. Other impurity gels have been investigated for producing ultracold neutrons include CD₄ (deuterated methane) and C₂D₅OD. (deuterated ethanol)

The water-helium condensate [H₂O]/[He] contains water clusters of several nanometers in diameter, and pores from 8 to 800 nm.

Oxygen O₂ impurity helium contains solid oxygen clusters from 1 to 100 nm.

Impurity solid helium

Introducing impurities into solid helium yields a blue solid that melts at a higher temperature than pure He. For caesium, the absorption has a peak at 750 nm, and for rubidium, maximal absorption is at 640 nm. These are due to metal clusters with diameters of 10 nm or so. However, the low concentration of clusters in this substance should not be sufficient to solidify helium, as the amount of metal in the solid is less than billionth that of the impurity helium condensate solids, and liquid helium does not "wet" caesium metal. The solid is possibly due to helium "snowballs" attached to Cs⁺ (or Rb⁺) ions. The snowball is a shell that contains helium atoms solidified in particular positions around the ion. The helium atoms are immobilized in the snowball by polarization. Neutral metallic atoms in liquid helium are also surrounded by a bubble caused by electron repulsion. They have typical sizes ranging from 10 to 14 Å diameter. Free electrons in liquid helium are enclosed in a bubble 17 Å in diameter. Under 25 atmosphere pressure an electron bubble reduces to 11 Å.

Solid solution

Helium can dissolve to a limited extent in hot metal, with concentration proportional to pressure. At atmospheric pressure, 500 °C bismuth can absorb 1 part in a billion; at 649 °C lithium can take 5 parts per billion; and at 482 °C potassium can take 2.9 parts per million (all atom fractions). In nickel there can be 1 in 10¹⁰ atoms, and in gold 1 in 10⁷. The supposition is that the higher the melting point the less helium can be dissolved. However, when a liquid metal is quenched, higher concentrations of helium can be left dissolved. So cooled liquid steel can have one part per million of helium. In order to get a helium atom into a metal lattice, a hole has to be formed. The energy to make that hole in the metal is basically the heat of solution.

Nanowires

Gold, copper, rubidium, caesium, or barium atoms evaporated into liquid helium form spiderweb-like structures. Rhenium produces nano flakes. Molybdenum, tungsten, and niobium produce thin nanowires with diameters of 20, 25 and 40 Å. When platinum, molybdenum or tungsten is evaporated into liquid helium, nanoclusters are first formed, accompanied by high temperature thermal emission pulse, above the melting point of the metals. In superfluid helium, these clusters migrate to the vortices and weld together to yield nanowires once the clusters are mostly solid. In higher temperature liquid helium, larger clusters of metal are formed instead of wires. The metal vapours can only penetrate about 0.5 mm into liquid helium. Indium, tin, lead and nickel produce nanowires about 80 Å in diameter. These same four metals also produce smooth spheres about 2 μm across that explode when examined with an electron microscope. Copper, permalloy, and bismuth also make nanowires.

Two-dimensional ionic crystal

Helium II ions (He⁺) in liquid helium when attracted by an electric field can form a two-dimensional crystal at temperatures below 100 mK. There are about half a trillion ions per square meter just below the surface of the helium. Free electrons float above the helium surface.

Known van der Waals molecules

• LiHe
• Dihelium
• Trihelium
• Ag₃He
• HeCO is weakly bound by van der Waals forces. It is potentially important in cold interstellar media as both CO and He are common.
• CF₄He and CCl₄He both exist.
• HeI₂ can be formed by supersonic expansion of high pressure helium with a trace of iodine into a vacuum. It was the first known triatomic helium van der Waals molecule. It can be detected by fluorescence. HeI₂ has a similar optical spectrum to I₂, except that the bands and lines are shifted to form two extra series. One series is blueshifted by between 2.4 and 4.0 cm⁻¹, and the other between 9.4 and 9.9 cm⁻¹. The two series may be due to different amounts of vibration in the He–I bond. The lines are narrow indicating that the molecules in their excited vibrational state have a long lifetime.
• Na₂He molecules can form on the surface of helium nanodroplets.
• NOHe

Known ions

Helium has the highest ionization energy, so a He⁺ ion will strip electrons off any other neutral atom or molecule. However, it can also then bind to the ion produced. The He⁺ ion can be studied in gas, or in liquid helium; Its chemistry is not completely trivial. For example, He⁺ can react with SF₆ to yield SF⁺
₆ or SF⁺
₅ and atomic fluorine.

Ionized clusters

He⁺
₂ was predicted to exist by Linus Pauling in 1933. It was discovered when doing mass spectroscopy on ionized helium. The dihelium cation is formed by an ionized helium atom combining with a helium atom: He⁺ + He → He⁺
₂.

The diionized dihelium He²⁺
₂ (¹Σ⁺
_g) is in a singlet state. It breaks up He²⁺
₂ → He⁺ + He⁺ releasing 200 kcal/mol of energy. It has a barrier to decomposition of 35 kcal/mol and a bond length of 0.70 Å.

The trihelium cation He⁺
₃ is in equilibrium with He⁺
₂ between 135 and 200K.

Helium hydride

The helium hydride ion HeH⁺ has been known since 1925. The protonated dihelium ion He₂H⁺ can be formed when the dihelium cation reacts with dihydrogen: He⁺
₂ + H₂ → He₂H⁺ + H. This is believed to be a linear molecule. Larger protonated helium cluster ions exist He_nH⁺ with n from 3 to 14. He₆H⁺ and He₁₃H⁺ appear to be more common. These can be made by reacting H⁺
₂ or H⁺
₃ with gaseous helium.

HeH²⁺ is unstable in its ground state, but when it is excited to the 2pσ state, the molecule is bound with an energy of 20 kcal/mol. This doubly charged ion has been made by accelerating the helium hydride ion to 900 keV, and firing it into argon. It has a short life of 4 ns.

H₂He⁺ has been made and could occur in nature via H₂ + He⁺ → H₂He⁺.

H₃He⁺
_n exists for n from 1 to over 30, and there are also clusters with more hydrogen atoms and helium.

Noble gas

Noble gas cluster ions exist for different noble gases. Singly charged cluster ions containing xenon exist with the formula He_nXe⁺
_m, where n and m ≥ 1.

Many different He_nKr⁺ exist with n between 1 and 17, with higher values possible. He_nKr⁺
₂ and He_nKr⁺
₃ also exist for many values of n. He₁₂Kr⁺
₂ and He₁₂Kr⁺
₃ ions are common. These singly charged cluster ions can be made from krypton in helium nanodroplets subject to vacuum ultraviolet radiation.

The Ar⁺ argon ion can form many different sized clusters with helium ranging from HeAr⁺ to He₅₀Ar⁺, but the most common clusters are He₁₂Ar⁺ and smaller. These clusters are made by capturing an argon atom in a liquid helium nanodroplet, and then ionizing with high speed electrons. He⁺ is formed, which can transfer charge to argon and then form a cluster ion when the rest of the droplet evaporates.

NeHe⁺
_n can be made by ultraviolet photoionization. Clusters only contain one neon atom. The number of helium atoms can vary from 1 to 23, but NeHe⁺
₄ and NeHe⁺
₈ are more likely to be observed.

Doubly charged ions of helium with noble gas atoms also exist including ArHe²⁺, KrHe²⁺, and XeHe²⁺.

Metals

Various metal-helium ions are known.

Alkali metal helide ions are known for all the alkalis. The molecule ground state for the diatomic ions is in the X¹Σ⁺ state. The bond length gets bigger as the periodic table is descended with lengths of 1.96, 2.41, 2.90, 3.10, and 3.38 Å for Li⁺He, Na⁺He, K⁺He, Rb⁺He, and Cs⁺He. The dissociation energies are 1.9, 0.9, 0.5, 0.4 and 0.3 kcal/mol, showing bond energy decreases. When the molecule breaks up, the positive charge is never on the helium atom.

When there are many helium atoms around, alkali metal ions can attract shells of helium atoms. Clusters can be formed from absorbing metal into helium droplets. The doped droplets are ionized with high speed electrons. For sodium, clusters appear with the formula Na⁺He_n with n from 1 to 26. Na⁺He is the most common, but Na⁺He₂ is very close in abundance. Na⁺He₈ is much more abundant than clusters with more helium, but Na⁺
₂He_n with n from 1 to 20 also appears. Na⁺
₃He_n with small n is also made. For potassium, K⁺He_n with n up to 28, and K⁺
₂He_n for n from 1 to 20 is formed. K⁺He and K⁺He₂ are both common, and K⁺He₁₂ is a bit more commonly formed than other similar sized clusters. Caesium and rubidium cations also form clusters with helium.

Other known metal-helium ions include Cr⁺He, Co⁺He, Co⁺He₃, Ni⁺He, and Ni⁺He₃. PtHe²⁺; formed by high electric field off platinum surface in helium, VHe²⁺, HeRh²⁺ is decomposed in high strength electric field, Ta²⁺He, Mo²⁺He, W²⁺He, Re²⁺He, Ir²⁺He, Pt²⁺He₂, W³⁺He₂, W³⁺He₃, and W³⁺He₄.

Nonmetals

HeN⁺
₂ can form at around 4 K from an ion beam of N⁺
₂ into cold helium gas. The energy needed to break up the molecule is 140 cm⁻¹ which is quite a bit stronger than the van der Waals neutral molecules. HeN⁺
₂ is tough enough to have several vibrational, bending and rotational states. He_nN⁺
₂ with n from 2 to 6 have been made by shooting electrons at a supersonically expanding mix of nitrogen and helium.

C₆₀He⁺ is formed by irradiating C₆₀ with 50eV electrons and then steering ions into cold helium gas. C₆₀He⁺
₂ is also known.

He(OH)⁺ has been detected, although it is not produced when HTO (tritiated water) decays.

He
_n(CO)⁺
has been detected for values of n from 1 to 12. Also CH₃He⁺, OCHHe⁺ and NH₂He⁺ have been detected.

Young and Coggiola claimed to make HeC⁺ by an electric discharge off graphite into helium.

When tritium substituted methane (CH₃T) decays, CH₃He⁺ is produced in a very small amount.

The helium formyl cation, HeHCO⁺ is a linear molecule. It has a vibrational frequency red shifted 12.4 cm⁻¹ compared to HCO⁺. It can be considered as a deenergized protonation reaction intermediate for the HeH⁺ + CO → HCO⁺ + He. HeHCO⁺ can be produced by a supersonic expansion of a gas mixture of He, CO, and H₂, which is hit by a cross beam of electrons. CO and H₂ are only supplied at 1% of the helium.

The HeHN⁺
₂ molecule is linear. The He-H bondlength is 1.72 Å. It has an infrared band, due to B-H stretching, with a base at 3158.42 cm⁻¹. The binding energy is 378 cm⁻¹ in the 000 vibrational state, and 431 cm⁻¹ in the 100 vibrational state. He₂HN⁺
₂ is also known. One helium atom is linked to a hydrogen, and the other is less tightly bound.

H₂O⁺, H₂OSF₅⁺, SF₅⁺ and SF₆⁺ can form clusters with varying numbers of Helium atoms.

Excimers

The He^*
₂ excimer is responsible for the Hopfield continuum. Helium also forms an excimer with barium, Ba⁺He^*.

Predicted compounds

Predicted solids

He(H₂O)₂ is predicted to form a solid with orthorhombic structure Ibam.

Iron helide (FeHe) was early on claimed to have been found, but the discovery was classified as an alloy. Early studies predicted the FeHe exists as an interstitial compound under high pressure, perhaps in dense planetary cores, or, as suggested by Freeman Dyson, in neutron star crust material. Recent density functional theory calculations predict the formation of FeHe compounds at pressures above about 4 TPa, suggesting indeed that these compounds could be found inside giant planets, white dwarf stars, or neutron stars.

Na₂HeO is predicted to have a similar structure to Na₂He, but with oxygen atoms in the same position as the electron pair, so that it becomes O²⁻. It would be stable from 13 to 106 GPa. This substance could be a way to store helium in a solid.

La_2/3-xLi_3xTiO₃He is a porous lithium ion conduction perovskite that can contain helium like a clathrate.

Helium is predicted to be included under pressure in ionic compounds of the form A₂B or AB₂. These compounds could include Na₂OHe, MgF₂He (over 107 GPa) and CaF₂He (30-110 GPa). Stabilisation occurs by the helium atom positioning itself between the two like charged ions, and partially shielding them from each other.

Helium is predicted to form an inclusion compound with silicon, Si₂He. This has a hexagonal lattice of silicon atoms with helium atoms lined up in the channels. It should be formed when liquid silicon is injected with helium at over 1GPa and cooled.

Helium is predicted to form a fluoride He₃F₂ under extreme pressures well over 1 Tpa.

Predicted van der Waals molecules

The beryllium oxide helium adduct, HeBeO is believed to be bonded much more strongly than a normal van der Waals molecule with about 5 kcal/mol of binding energy. The bond is enhanced by a dipole induced positive charge on beryllium, and a vacancy in the σ orbital on beryllium where it faces the helium.

Variations on the beryllium oxide adduct include HeBe₂O₂, RNBeHe including HNBeHe, CH₃NBeHe, CH_4−xNBeHe_x, SiH_4−xNBeHe_x, NH_3−xNBeHe_x, PH_3−xNBeHe_x, OH_2−xNBeHe_x, SH_2−xNBeHe_x, and HeBe(C₅H₅)⁺.

Hydridohelium fluoride (HHeF) is predicted to have a lifetime of 157 femtoseconds with a 5 kcal/mol barrier. The lifetime of the deuterium isotopomer is predicted to be much longer due to a greater difficulty of tunneling for deuterium. This molecule's metastability is slated due to electrostatic attraction between HHe⁺ and F⁻ which increases the barrier to an exothermic breakup. Under pressures over 23 GPa HHeF should be stable.

Calculations for coinage metal fluorides include HeCuF as stable, HeAgF is unstable, HeAuF is predicted, and Ag³He with binding energy 1.4 cm⁻¹, Ag⁴He binding energy 1.85 cm⁻¹, Au³He binding energy 4.91 cm⁻¹, and Au⁴He binding energy 5.87 cm⁻¹

HeNaO is predicted.

Calculation for binary van der Waals helium molecules include HeNe, Li⁴He binding energy 0.008 cm⁻¹, the Li³He is not stable. Na⁴He binding energy 0.03 cm⁻¹, the Na³He is not stable. Cu³He binding energy 0.90 cm⁻¹, O⁴He binding energy 5.83 cm⁻¹, S⁴He binding energy 6.34 cm⁻¹, Se⁴He binding energy 6.50 cm⁻¹, F⁴He binding energy 3.85 cm⁻¹, Cl⁴He binding energy 7.48 cm⁻¹, Br⁴He binding energy 7.75 cm⁻¹, I⁴He binding energy 8.40 cm⁻¹, N⁴He binding energy 2.85 cm⁻¹, P⁴He binding energy 3.42 cm⁻¹, As⁴He binding energy 3.49 cm⁻¹, Bi⁴He binding energy 33.26 cm⁻¹, Si⁴He binding energy 1.95 cm⁻¹, Ge⁴He binding energy 2.08 cm⁻¹, CaH⁴He binding energy 0.96 cm⁻¹, NH⁴He binding energy 4.42 cm⁻¹, MnH⁴He binding energy 1.01 cm⁻¹, YbF⁴He binding energy 5.57 cm⁻¹ I⁴
₂He or I³
₂He,

Bonds are predicted to form to nickel with helium as a weak ligand in HeNiCO and HeNiN₂.

(HeO)(LiF)₂ is predicted to form a planar metastable molecule. 1-Tris(pyrazolyl)borate beryllium and 1-tris(pyrazolyl)borate magnesium are predicted to bind helium at low temperatures. There is also a prediction of a He-O bond in a molecule with caesium fluoride or tetramethyl ammonium fluoride.

LiHe₂ is predicted to be in an Efimov state when excited.

Predicted ions

Many ions have been investigated theoretically to see if they could exist. Just about every diatomic cation with helium has been studied. For the diatomic dications, for stability the second ionization level of the partner atom has to be below the first ionization level of helium, 24.6 eV. For Li, F, and Ne the ground state is repulsive, so molecules will not form. For N and O the molecule would break up to release He⁺. However HeBe²⁺, HeB²⁺ and HeC²⁺ are predicted to be stable. Also second row elements from Na to Cl are predicted to have a stable HeX²⁺ ion.

HeY³⁺ is predicted to be the lightest stable diatomic triply charged ion. Other possibly thermochemically stable ions include HeZr³⁺, HeHf³⁺, HeLa³⁺, HeNd³⁺, HeCe³⁺, HePr³⁺, HePm³⁺, HeSm³⁺, HeGa³⁺, HeTb³⁺, HeDy³⁺, HeHo³⁺, HeEr³⁺, HeTm³⁺, and HeLu³⁺ where the third ionization point is below that of helium.

The positronium helide ion PsHe⁺ should be formed when positrons encounter helium.

The Fluoroheliate FHeO⁻ ion should be stable but salts like LiFHeO are not stable.

• HHeCO⁺ theoretical
• FHeS⁻ is predicted to be stable.
• FHeBN⁻
• HHeN²⁺ is unlikely to exist.
• (HHe⁺)(OH₂) is probably unstable.

The lithium hydrohelide cation HLiHe⁺ is linear in theory. This molecular ion could exist with big bang nucleosynthesis elements. Other hydrohelide cations that exist in theory are HNaHe⁺ sodium hydrohelide cation, HKHe⁺ potassium hydrohelide cation, HBeHe²⁺ beryllium hydrohelide cation, HMgHe²⁺ magnesium hydrohelide cation, and HCaHe²⁺ calcium hydrohelide cation.

HeBeO⁺ is predicted to have a relatively high binding energy of 25 kcal mol⁻¹. Similarly the ethynylhelium cation [HeC≡CH]⁺ is predicted to be bound (with respect to ethynyl cation and helium) by 22 kcal/mol. However, vinylhelium ([HeCH=CH₂]⁺, 0.03 kcal/mol) and methylhelium ([HeCH₃]⁺, 1.5 kcal/mol) cations are predicted to be bound much more weakly.

• HCHe⁺
• HCHeHe⁺

For negative ions the adduct is very weakly bound. Those studied include HeCl⁻, HeBr⁻, HeF⁻, HeO⁻ and HeS⁻.

• FHeS⁻
• FHeSe⁻
• C₇H₆He²⁺
• C₇H₆HeHe²⁺
• FHeCC⁻
• HHeOH⁺
  ₂
• HHeBF⁺
• HeNC⁺
• HeNN⁺
• HHeNN⁺ H-He 0.765 Å He-N bond length 2.077 Å. Decomposition barrier of 2.3 kJ/mol.

HHeNH⁺
₃ is predicted to have a C_3v symmetry and a H-He bond length of 0.768 Å and He-N 1.830. The energy barrier against decomposition to ammonium is 19.1 kJ/mol with an energy release of 563.4 kJ/mol. Cleavage to helium hydride ion and ammonia is predicted to be endothermic, requiring 126.2 kJ/mol.

Discredited or unlikely observations

Numerous researchers attempted to create chemical compounds of helium in the early part of the twentieth century. In 1895 L. Troost and L. Ouvrard believed they had witnessed a reaction between magnesium vapour and helium (and also argon) due to the spectrum of helium disappearing from the tube they were passing it through. In 1906, W. Ternant Cooke claimed to have noticed a reaction of helium with cadmium or mercury vapour by observing an increase in the density of the vapour. Zinc vapour did not react with helium.

J. J. Manley claimed to have found gaseous mercury helide HeHg in 1925 HgHe₁₀; publishing the results in Nature, but then had trouble finding a stable composition, and eventually gave up.

Morrison thought that helium, if irradiated so that one electron is pushed to a higher orbit, would behave like hydrogen. Consequently, he predicted that radioactive elements might form helides, and claimed in 1928 to succeed in forming a compound of helium with lead-214, and a compound of helium with bismuth-214.

Between 1925 and 1940 in Buenos Aires, Horacio Damianovich studied various metal–helium combinations including beryllium (BeHe), iron (FeHe), palladium (PdHe), platinum (Pt₃He), bismuth, and uranium. To make these substances, electrical discharges impacted helium into the surface of the metal. Later these were demoted from the status of compounds, to that of alloys.

Platinum helide, Pt₃He was discredited by J. G. Waller in 1960.

Palladium helide, PdHe is formed from tritium decay in palladium tritide, the helium (³He) is retained in the solid as a solution.

Boomer claimed the discovery of tungsten helide WHe₂ as a black solid. It is formed by way of an electric discharge in helium with a heated tungsten filament. When dissolved in nitric acid or potassium hydroxide, tungstic acid forms and helium escapes in bubbles. The electric discharge had a current of 5 mA and 1,000 V at a pressure between 0.05 and 0.5 mmHg for the helium. The process works slowly at 200 V. and 0.02 mmHg of mercury vapour accelerates tungsten evaporation by five times. The search for this was suggested by Ernest Rutherford. It was discredited by J. G. Waller in 1960. Boomer also studied mercury, iodine, sulfur, and phosphorus combinations with helium. Mercury and iodine helium combinations decomposed around −70 °C Sulfur and phosphorus helium combinations decomposed around −120 °C

• Bismuth dihelide, BiHe₂

H. Krefft and R. Rompe claimed reactions between helium and sodium, potassium, zinc, rubidium, indium, and thallium.