he2

he2 refers to the diatomic molecule formed by two helium atoms. In its ground electronic state, it is a weakly bound van der Waals dimer bound by dispersion forces. The binding energy is extremely small—on the order of 0.1 cm^-1—making it one of the weakest chemically bound molecules. The equilibrium internuclear distance is relatively large, about 5.0–5.6 Å, reflecting its loose van der Waals character. Because of the shallow potential well, only a very small number of vibrational levels exist, typically one or two.

Formation and observation occur under cryogenic conditions or in specialized experimental settings such as supersonic jet expansions and helium nanodroplets, where the gas is cooled to near absolute zero. He2 has been detected through spectroscopic methods, including photoassociation and vibration–rotation spectroscopy, which probe its weak energy levels and the shape of its potential energy surface. Computational studies using high-level quantum chemistry methods have been essential for describing the delicate balance of forces that stabilize the molecule, and they agree on the exceptional weakness of the bond and the resulting large bond length.

Metastable states, such as He2* in excited electronic configurations, form under different irradiation and collision conditions and often exhibit a deeper potential well, enabling broader spectroscopic investigation. These excited-state dimers have played a significant role in experiments on Penning ionization and ultracold chemistry.

Significance: he2 serves as a benchmark system for testing theories of weak van der Waals interactions and the quantum behavior of noble-gas dimers, contributing to understanding intermolecular forces and ultra-cold molecular physics.