Quasimolecular

Quasimolecular refers to transient, short‑lived states in which two or more atoms or ions form a collision complex that resembles a molecule but does not persist as a stable species. These states occur during atomic or ionic encounters, or in excited gas‑phase conditions, and they exhibit molecular‑like electronic structure and potential‑energy surfaces for a limited lifetime.

In collision theory, quasimolecules arise when atomic orbitals interact strongly as the atoms approach, creating quasi‑bound or resonant states that can decay by dissociation or radiative processes. They contribute to the interpretation of spectra from high‑temperature plasmas, flames, and gas discharges, and to collision‑induced absorption and line shapes.

In quantum chemistry, the concept is used to describe the early stages of bond formation, where two center orbitals mix into a quasimolecular orbital and the potential energy surface connects separated atoms to a bound molecule. The term is often used for transient diatomic or polyatomic systems and for metastable states that are short‑lived.

Difference from a true molecule: stability and lifetime; treatment as a transient complex; sometimes used interchangeably with "collision complex" or "resonance state" in specific contexts.