7Li2

7Li2 is a diatomic molecule composed of two lithium-7 nuclei. It is the isotopologue of lithium dimer (Li2) that contains two atoms of the isotope 7Li, and it is one of several isotopologues formed from lithium's stable isotopes. Natural lithium is predominantly 7Li, so 7Li2 is the most common Li2 isotopologue encountered in many contexts, though 6Li-containing and mixed isotopologues also occur.

In terms of structure, 7Li2 is a homonuclear diatomic with a single covalent bond. The ground electronic state is typically designated X1Σg+. The bond length for Li2-like species is roughly a few angstroms (on the order of 2.6–2.7 Å for the common Li2 ground state), and the bond dissociation energy lies in the tens to hundreds of kilojoules per mole range, with values for Li2-type bonds generally around the low hundreds of kJ/mol. Because both nuclei are 7Li, the molecule has a specific reduced mass that sets its vibrational and rotational level spacings, and its spectrum shows shifts relative to other isotopologues.

Spectroscopically, 7Li2 exhibits rotational and vibrational transitions characteristic of a light, covalent diatomic. Isotopic substitution alters vibrational frequencies and rotational constants slightly, and the presence of nuclear spin in 7Li can introduce hyperfine structure in high-resolution spectra.

Applications and occurrences: 7Li2 is observed in high-temperature flames and discharge sources, and it serves as a reference and subject of study in molecular spectroscopy. In ultracold-atom experiments, lithium isotopologues, including 7Li2, are used to explore fundamental molecular interactions and quantum chemistry at ultralow temperatures.

See also: Li2, 6Li2, lithium isotopologues.