The bond that holds this dimer together is so weak that it will break if the molecule rotates, or vibrates too much.

Based on molecular orbital theory, He2 should not exist, and a chemical bond cannot form between the atoms.

So a molecule composed of two helium atoms bound by the van der Waals force can exist.

[5] He2 is the largest known molecule of two atoms when in its ground state, due to its extremely long bond length.

[9] Both helium atoms in the dimer can be ionized by a single photon with energy 63.86 eV.

[10] The dimer then explodes as two helium cations repel each other, moving with the same speed but in opposite directions.

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

He2+ reacts with N2, Ar, Xe, O2, and CO2 to form cations and neutral helium atoms.

[14][18] The negative helium dimer He2− is metastable and was discovered by Bae, Coggiola and Peterson in 1984 by passing He2+ through cesium vapor.

Two excited helium atoms can form a covalent bond, creating a molecule called dihelium that lasts for times from the order of a microsecond up to second or so.

[23]) The first clues that dihelium exists were noticed in 1900 when W. Heuse observed a band spectrum in a helium discharge.

[24][25] 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.

[30] The excimer molecules are much smaller and more tightly bound than the van der Waals bonded helium dimer.

[27] 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.

But the even numbered rotational levels do not have Q branch lines, due to both nuclei being spin 0.

[33] 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.

In a 3d state a He*2 molecule is surrounded by a bubble 12.7 Å in radius at atmospheric pressure.

[36] In very strong magnetic fields, (around 750,000 Tesla) and low enough temperatures, helium atoms attract, and can even form linear chains.

The plasma containing the helium dimer can be as low as 30 °C in temperature, and this reduces heat damage to samples.

These can be detected by the nuclear magnetic resonance of 3He having a small shift, and by mass spectrometry.

[44] Even when the two helium atoms are placed closely to each other in a small cage, there is no chemical bond between them.