text: He is a related ion bonded by a half covalent bond. It can be formed in a helium electrical discharge. It recombines with electrons to form an electronically excited He(aΣ) excimer molecule. Both of these molecules are much smaller with more normally sized interatomic distances. He reacts with N, Ar, Xe, O, and CO to form cations and neutral helium atoms.
  The helium dication dimer He is extremely repulsive and releases much energy when it dissociates, around 835 kJ/mol. Dynamical stability of the ion was predicted by Linus Pauling. An energy barrier of 138.91 kJ/mol prevents immediate decay. This ion is isoelectronic with the hydrogen molecule. He is the smallest possible molecule with a double positive charge. It is detectable using mass spectroscopy.
  The negative helium dimer He is metastable and was discovered by Bae, Coggiola and Peterson in 1984 by passing He through cesium vapor.    Subsequently, H. H. Michels theoretically confirmed its existence and concluded that the Π state of He is bound relative to the aΣ state of He. The calculated electron affinity is 0.233 eV compared to 0.077 eV for the He[P] ion. The He decays through the long-lived 5/2g component with τ~350 μsec and the much shorter-lived 3/2g, 1/2g components with τ~10 μsec. The Π state has a 1σ1σ2σ2π electronic configuration, its electron affinity E is 0.18±0.03 eV, and its lifetime is 135±15 μsec; only the v=0 vibrational state is responsible for this long-lived state.
  The molecular helium anion is also found in liquid helium that has been excited by electrons with an energy level higher than 22 eV. This takes place firstly by penetration of liquid He, taking 1.2 eV, followed by excitation of a He atom electron to the P level, which takes 19.8 eV. The electron can then combine with another helium atom and the excited helium atom to form He. He repels helium atoms, and so has a void around it. It will tend to migrate to the surface of liquid helium.