6Li2

6Li2 is a diatomic molecule consisting of two lithium-6 atoms. It is one isotopologue of the lithium dimer (Li2), with other isotopologues including 7Li2 and mixed forms such as 6Li7Li. In nature, lithium exists as a mixture of isotopes, but the 6Li2 molecule is primarily produced and studied in laboratory settings, especially in ultracold-atom experiments.

Bonding and structure: 6Li2 is a homonuclear covalent molecule with a singlet electronic ground state, designated X1Σg+. The bond is relatively weak compared with many diatomic molecules. The equilibrium bond length is about 2.67 Å, and the dissociation energy D0 is on the order of 1.0 eV (roughly 100 kJ/mol). The molecule has no permanent dipole moment due to its symmetry.

Spectroscopy and isotopic effects: The observed spectra arise from vibrational and rotational transitions. Isotopic substitution shifts vibrational frequencies because the reduced mass changes; in general, 6Li2 has slightly higher vibrational frequencies than its heavier isotopologues such as 7Li2. Spectroscopic data for Li2 isotopologues support tests of molecular potential energy surfaces and Born–Oppenheimer approximations.

Production and applications: 6Li2 is not common in bulk nature and is largely studied in controlled laboratory environments. In ultracold atomic physics, 6Li atoms can be cooled to ultra-low temperatures and bound into weakly bound molecules via Feshbach resonances or photoassociation. Such 6Li2 molecules are used to explore quantum degeneracy, pairing phenomena, and the crossover between Bose–Einstein condensation and Bardeen–Cooper–Schrieffer-type superfluidity.