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ng the tetrahedral sites however the solid c 60 3he is not stable and loses helium on a timescale of 340 hours when not under a helium atmosphere when the helium intercalated fullerite is cooled it has an orientational phase transition that is 10 k higher than for pure solid c 60 the actual discontinuous change in volume at that point is smaller but there are more rapid changes near the transition temperature perhaps due to varying occupancy of the voids by helium 29 30 endohedral edit helium atoms can be trapped inside molecular cages such as the fullerenes he c 60 he c 70 he 2 c 60 and he 2 c 70 have all been made using compressed helium and fullerenes 31 when using only pressure and heat the yield is quite low under 1 however by breaking and reforming the carbon ball much higher concentrations of he c 60 or he c 70 can be made high performance liquid chromatography can concentrate the helium containing material hen c 60 and hen c 70 have also been made these have a lower symmetry due to the two atoms being trapped together in the same cavity this also causes esr line broadening 32 dodecahedrane can trap helium from a helium ion beam to yield he c 20 h 20 other cage like inorganic or organic molecules may also trap helium for example c 8 he with he inside a cube 33 or he mo 6 cl 8 f 6 34 impurity helium condensates edit impurity helium condensates ihcs or impurity helium gels 35 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 36 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 37 these condensates were first investigated as a possible rocket fuel 38 the mixtures are given a notation involving square brackets so that n he represents a nitrogen atom impurity in helium citation needed 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 39 the substance resembles crumbly snow and condenses and settles from the liquid helium 39 it also contains variable proportions of n 2 molecules 39 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 39 this substance is not a true compound but more like a solid solution 36 e b gordon et al suggested that this material may exist in 1974 39 the localised helium shells around an individual atom are termed van der waals spheres 39 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 40 other inert gas impurity helium condensates can also be made from a gas beam into superfluid helium 41 ne he decomposes at 8 5 k with release of heat and formation of solid neon its composition approximates nehe 16 ar he contains 40 60 helium atoms per argon atom 42 kr he contains 40 60 helium atoms per krypton atom 42 and is stable up to 20 k 37 xe he contains 40 60 helium atoms per xenon atom 42 n 2 he contains 12 17 he atoms per n 2 molecule 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 additional inert gas stabilises more nitrogen atoms it decomposes around 7 k with a blue green light flash 41 excited nitrogen atoms in the n 2 d state can be relative long lasting up to hours and give off a green luminescence 41 h 2 he or d 2 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 43 using h 2 results in the product floating and stopping further production but with deuterium or a half half mixture it can sink and accumulate 37 atomic hydrogen in impurity helium decays fairly rapidly due to quantum tunneling h h h 2 atomic deuterium dimerises slower d d d 2 but reacts very quickly with any diprotium present d h 2 hd h 37 atomic hydrogen solids are further stabilised by other noble gases such as krypton 44 45 46 lowering temperatures into the millikelvin range can prolong the lifetime of atomic hydrogen condensates 38 condensates containing heavy water or deuterium are under investigation for the production of ultracold neutrons 35 other impurity gels have been investigated for producing ultracold neutrons include cd 4 deuterated methane and c 2 d 5 od deuterated ethanol 47 the water helium condensate h 2 o he contains water clusters of several nanometers in diameter and pores from 8 to 800 nm 48 oxygen o 2 impurity helium contains solid oxygen clusters from 1 to 100 nm 49 impurity solid helium edit introducing impurities into solid helium yields a blue solid that melts at a higher temperature than pure he 50 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 50 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 51 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 52 solid solution edit 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 53 in nickel there can be 1 in 10 10 atoms and in gold 1 in 10 7 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 54 nanowires edit gold copper rubidium caesium or barium atoms evaporated into liquid helium form spiderweb like structures 55 rhenium produces nano flakes molybdenum tungsten and niobium produce thin nanowires with diameters of 20 25 and 40 å 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 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 57 indium tin lead and nickel produce nanowires about 80 å in diameter 58 these same four metals also produce smooth spheres about 2 μm across that explode when examined with an electron microscope 59 copper permalloy and bismuth also make nanowires 60 two dimensional ionic crystal edit 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 61 known van der waals molecules edit lihe 62 dihelium trihelium ag 3 he 63 heco is weakly bound by van der waals forces it is potentially important in cold interstellar media as both co and he are common 64 cf 4 he and ccl 4 he both exist 65 hei 2 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 hei 2 has a similar optical spectrum to i 2 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 66 na 2 he molecules can form on the surface of helium nanodroplets 67 nohe 68 known ions edit 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 sf 6 to yield sf 6 or sf 5 and atomic fluorine 69 ionized clusters edit 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 70 the diionized dihelium he 2 2 1 σ g is in a singlet state it breaks up he 2 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 70 the trihelium cation he 3 71 is in equilibrium with he 2 between 135 and 200k 72 helium hydride edit the helium hydride ion heh has been known since 1925 70 the protonated dihelium ion he 2 h can be formed when the dihelium cation reacts with dihydrogen he 2 h 2 he 2 h h this is believed to be a linear molecule 70 larger protonated helium cluster ions exist he n h with n from 3 to 14 he 6 h and he 13 h appear to be more common these can be made by reacting h 2 or h 3 with gaseous helium 70 heh 2 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 70 h 2 he has been made and could occur in nature via h 2 he h 2 he 70 h 3 he n exists for n from 1 to over 30 and there are also clusters with more hydrogen atoms and helium 73 noble gas edit noble gas cluster ions exist for different noble gases singly charged cluster ions containing xenon exist with the formula he n xe m where n and m 1 74 many different he n kr exist with n between 1 and 17 with higher values possible he n kr 2 and he n kr 3 also exist for many values of n he 12 kr 2 and he 12 kr 3 ions are common these singly charged cluster ions can be made from krypton in helium nanodroplets subject to vacuum ultraviolet radiation 74 the ar argon ion can form many different sized clusters with helium ranging from hear to he 50 ar but the most common clusters are he 12 ar 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 75 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 74 doubly charged ions of helium with noble gas atoms also exist including arhe 2 krhe 2 and xehe 2 76 metals edit 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 x 1 σ 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 70 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 he 2 is very close in abundance na he 8 is much more abundant than clusters with more helium but na 2 he n with n from 1 to 20 also appears na 3 he n with small n is also made for potassium k he n with n up to 28 and k 2 he n for n from 1 to 20 is formed k he and k he 2 are both common and k he 12 is a bit more commonly formed than other similar sized clusters 77 caesium and rubidium cations also form clusters with helium 77 other known metal helium ions include cr he co he co he 3 ni he and ni he 3 70 pthe 2 78 79 formed by high electric field off platinum surface in helium 76 vhe 2 76 herh 2 is decomposed in high strength electric field 80 81 ta 2 he mo 2 he w 2 he re 2 he ir 2 he pt 2 he 2 w 3 he 2 w 3 he 3 and w 3 he 4 70 nonmetals edit hen 2 can form at around 4 k from an ion beam of n 2 into cold helium gas 82 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 83 he n n 2 with n from 2 to 6 have been made by shooting electrons at a supersonically expanding mix of nitrogen and helium 70 c 60 he is formed by irradiating c 60 with 50ev electrons and then steering ions into cold helium gas c 60 he 2 is also known 84 he oh has been detected although it is not produced when hto tritiated water decays 70 he n co has been detected for values of n from 1 to 12 also ch 3 he ochhe and nh 2 he have been detected 70 young and coggiola claimed to make hec by an electric discharge off graphite into helium 85 when tritium substituted methane ch 3 t decays ch 3 he is produced in a very small amount 86 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 83 hehco can be produced by a supersonic expansion of a gas mixture of he co and h 2 which is hit by a cross beam of ...
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