The spectroscopy of van der Waals molecules

The recent spectroscopy of van der Waals molecules is reviewed. Examples are presented from radio-frequency, microwave, Raman, infrared, and electronic spectroscopy. Diatomic van der Waals molecules (e.g. Ne2, Ar2, Kr2, Mg2) reveal a manifold of closely spaced vibration–rotation levels consistent with the small dissociation energies which are orders of magnitude less than for ordinary chemically bonded molecules. The (isotropic) interatomic potentials which define these molecules may be evaluated from their energy levels. Polyatomic van der Waals molecules (e.g. H2–Ar, FCl–Ar, (H2)2, (O2)2, (CO2)2) are classified according to the strength of the (anisotropic) intermolecular potential which tends to define their geometry. This classification depends on the nature of the coupling of the rotational angular momenta and leads to a labeling of the complexes as free rotor, weakly coupled, strongly coupled, or semirigid. The spectroscopic constants which are determined from the energy levels of diatomic and polyatomic van der Waals molecules can be used to better understand the intermolecular bonding which holds these molecules together.