The A2 S+ state of Ar.NO

The ?2S+ state of Ar·NO is studied using (1+1) resonance-enhanced multiphoton ionization (REMPI) spectroscopy. Higher quality spectra than obtained in other studies allow the identification of a number of previously unreported features. The spectrum is analyzed using two models: a rigid van der Waals complex in which NO is weakly bonded to Ar; and a complex in which the free internal rotation of NO is hindered by the anisotropy caused by the presence of the Ar atom. It is concluded that as the intermolecular stretch is excited, then the anisotropy decreases, and the angular motion of the complex becomes more and more like that of a free rotor. Near the origin, the complex has an average geometry approaching linear, whereas when the intermolecular stretch is excited, an average geometry closer to T-shaped occurs; however, when the anisotropy is small, the concept of geometry becomes ill-defined.