Based on molecular orbital theory, He 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. The existence of this molecule was proposed as early as 1930.
He is the largest known molecule of two atoms when in its ground state, due to its extremely long bond length. The He molecule has a large separation distance between the atoms of about 5200 pm (= 52 ångström). This is the largest for a diatomic molecule without ro-vibronic excitation. The binding energy is only about 1.3 mK, 10eV or 1.1×10 kcal/mol. According to high-level theoretical calculations, the bond is 5000 times weaker than the covalent bond in the hydrogen molecule.
Both helium atoms in the dimer can be ionized by a single photon with energy 63.86 eV. The proposed mechanism for this double ionization is that the photon ejects an electron from one atom, and then that electron hits the other helium atom and ionizes that as well. The dimer then explodes as two helium cations ions repel each other with the same speed but opposite directions.
A dihelium molecule bound by Van der Waals forces was first proposed by John Clarke Slater in 1928.