Helium compounds
Other ways to make helium reactive are: to convert it into an ion, or to excite an electron to a higher level, allowing it to form excimers.
This requires 1,900 kilojoules (450 kcal) per gram of helium, which can be supplied by electron impact, or electric discharge.
[citation needed] Disodium helide (Na2He) is a compound of helium and sodium that is stable at high pressures above 113 gigapascals (1,130,000 bar).
The mineral melanophlogite is a natural silica clathrate (clathrasil) that normally would contain carbon dioxide, methane or nitrogen.
[11] Chibaite, another natural silica clathrate has its structure penetrated by helium under pressures higher than 2.5 GPa.
Neon requires a higher pressure, 4.5 GPa to penetrate, and unlike helium shows hysteresis.
However, if the pressure on the crystal is too high (13 GPa) helium penetration does not take place, as the gaps between arsenolite molecules become too small.
It affects this by causing a change to a monoclinic ordered state at a lower pressure (around 4 GPa) than if helium were absent.
The first rapid stage takes a couple of days, and expands the lattice by 0.16% (that is 2.2 pm) filling the larger octahedral sites.
The second stage takes thousands of hours to absorb more helium and expands the lattice twice as much again (0.32%) filling the tetrahedral sites.
When the helium intercalated fullerite is cooled, it has an orientational phase transition that is 10 K higher than for pure solid C60.
The impurities form nanoparticle clusters coated with localised helium held by van der Waals force.
[38] The mixtures are given a notation involving square brackets so that [N]/[He] represents a nitrogen atom impurity in helium.
[39] The localised helium shells around an individual atom are termed van der Waals spheres.
[42] It is stable up to 13 K[37] [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.
[41] Excited nitrogen atoms in the N(2D) state can be relative long lasting, up to hours, and give off a green luminescence.
[37] Atomic hydrogen in impurity helium decays fairly rapidly due to quantum tunneling (H + H → H2).
(deuterated ethanol)[47] The water-helium condensate [H2O]/[He] contains water clusters of several nanometers in diameter, and pores from 8 to 800 nm.
Neutral metallic atoms in liquid helium are also surrounded by a bubble caused by electron repulsion.
[54] Gold, copper, rubidium, caesium, or barium atoms evaporated into liquid helium form spiderweb-like structures.
[56] 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.
[61] Helium has the highest ionisation energy, so a He+ ion will strip electrons off any other neutral atom or molecule.
These singly charged cluster ions can be made from krypton in helium nanodroplets subject to vacuum ultraviolet radiation.
These clusters are made by capturing an argon atom in a liquid helium nanodroplet, and then ionising with high speed electrons.
[53] Early studies predicted the FeHe exists as an interstitial compound under high pressure,[93] perhaps in dense planetary cores,[94] or, as suggested by Freeman Dyson, in neutron star crust material.
[99] 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.
Calculation for binary van der Waals helium molecules include HeNe, Li4He binding energy 0.008 cm−1, the Li3He is not stable.
[71] Numerous researchers attempted to create chemical compounds of helium in the early part of the twentieth century.
[124] J. J. Manley claimed to have found gaseous mercury helide HeHg in 1925[125][126][127] HgHe10;[128][129] publishing the results in Nature, but then had trouble finding a stable composition, and eventually gave up.
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.
