text: In a normal helium atom, two electrons are found in the 1s orbital. However, if sufficient energy is added, one electron can be elevated to a higher energy level. This high energy electron can become a valence electron, and the electron that remains in the 1s orbital is a core electron. Two excited helium atoms can react with a covalent bond to form a molecule called dihelium that lasts for short times of the order of a microsecond up to second or so. Excited helium atoms in the 2S state can last for up to an hour, and react like alkali metal atoms.
  The first clues that dihelium exists were noticed in 1900 when W. Heuse observed a band spectrum in a helium discharge. However, no information about the nature of the spectrum was published. Independently E. Goldstein from Germany and W. E. Curtis from London published details of the spectrum in 1913. 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.
  The emission band spectrum shows a number of bands that degrade towards the red, meaning that the lines thin out and the spectrum weakens towards the longer wavelengths. Only one band with a green band head at 5732 Å degrades towards the violet. Other strong band heads are at 6400 (red), 4649, 4626, 4546, 4157.8, 3777, 3677, 3665, 3356.5, and 3348.5 Å. There are also some headless bands and extra lines in the spectrum. Weak bands are found with heads at 5133 and 5108.
  If the valence electron is in a 2s 3s, or 3d orbital, a Σ state results; if it is in 2p 3p or 4p, a Σ state results. The ground state is XΣ.
  The three lowest triplet states of He have designations aΣ, bΠ and cΣ. The aΣ state with no vibration (v=0) has a long metastable lifetime of 18 s, much longer than the lifetime for other states or inert gas excimers. The explanation is that the aΣ state has no electron orbital angular momentum, as all the electrons are in S orbitals for the helium state.
  The lower lying singlet states of He are AΣ, BΠ and CΣ. The excimer molecules are much smaller and more tightly bound than the van der Waals bonded helium dimer. For the AΣ state the binding energy is around 2.5 eV, with a separation of the atoms of 103.9 pm. The CΣ state has a binding energy 0.643 eV and the separation between atoms is 109.1 pm. 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. The singlet state AΣ is very unstable with a lifetime only nanoseconds long.
  The spectrum of the He excimer contains bands due to a great number of lines due to transitions between different rotation rates and vibrational states, combined with different electronic transitions. The lines can be grouped into P, Q and R branches. But the even numbered rotational levels do not have Q branch lines, due to both nuclei being spin 0. Numerous electronic states of the molecule have been studied, including Rydberg states with the number of the shell up to 25.
  Helium discharge lamps produce vacuum ultraviolet radiation from helium molecules. When high energy protons hit helium gas it also produces UV emission at around 600 Å by the decay of excited highly vibrating molecules of He in the AΣ state to the ground state. 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.
  The Hopfield continuum is a band of ultraviolet light between 600 and 1000 Å in wavelength formed by photodissociation of helium molecules.
  One mechanism for formation of the helium molecules is firstly a helium atom becomes excited with one electron in the 2S orbital. This excited atom meets two other non excited helium atoms in a three body association and reacts to form a AΣ state molecule with maximum vibration and a helium atom.
  Helium molecules in the quintet state Σ can be formed by the reaction of two spin polarised helium atoms in He(2S) states. This molecule has a high energy level of 20 eV. The highest vibration level allowed is v=14.
  In liquid helium the excimer forms a solvation bubble. In a d state a He molecule is surrounded by a bubble 12.7 Å in radius at atmospheric pressure.  When pressure is increased to 24 atmospheres the bubble radius shrinks to 10.8 Å. This changing bubble size causes a shift in the fluorescence bands.