HHe

HHe refers to the diatomic molecule formed by a hydrogen atom and a helium atom. In its neutral ground electronic state, HHe is generally considered a van der Waals molecule, bound by very weak dispersion interactions rather than by conventional covalent bonding. The molecule is expected to be extremely weakly bound, with an equilibrium separation much larger than typical covalent bonds and a binding energy that is very small by chemical standards.

Stability and theoretical predictions are the primary means by which HHe is studied. High-level ab initio calculations suggest a very shallow potential energy surface for the neutral HHe, and some studies indicate it may not support a bound vibrational state at all. Others predict the possibility of at most one extremely weakly bound vibrational level if a bound state exists. Because of the tiny binding energy, any such state would be difficult to observe experimentally.

Spectroscopic properties, should a bound state exist, would be challenging to detect. Vibrational transitions would occur at very low frequencies, and rotational-vibrational lines would be exceedingly weak. Electronic transitions are unlikely to be accessible with standard spectroscopy due to the weak bonding and large energy gaps involved.

Formation and occurrence of HHe would be restricted to specific environments where hydrogen and helium interact before separating, such as certain high-temperature plasmas or astrophysical contexts. In practice, it remains uncertain and uncommonly observed, especially when contrasted with the more stable helium hydride ion, HeH+, which has broader relevance in theory and astrophysics.

See also: HeH+, He2, H2, van der Waals molecule, intermolecular forces.