Knowledge Bank

The microwave and radiofrequency spectra of the $Ar-SO2$ van der Waals molecule were first recorded by DeLeon, Yokozeki and $Muenter1$. These authors showed that this molecule has two nonsuperimposable equilibrium configurations with $Cs$ symmetry, and that it can tunnel between these two framework, resulting in a splitting of about 980 MHz of the rigid rotator energy levels. However, from these measurements, it was neither possible to determine the exact structure of the two equilibrium configurations, nor to elucidate the nature of the tunneling motion. The microwave spectrum of $Ar-SO2$ was therefore reinvestigated at NIST. Sixty-five new transitions were measured with a pulsed beam Fourier transform microwave spectrometer. These new data along with those from DeLeon $etal.1$, were analysed using an IAM-like $treatment2$ developed for multidimensional tunneling. Such a formalism can be used to investigate the complicated motions that the complex may undergo because of the looseness of the van der Waals bond. For instance it is not clear $apriori$ whether the large amplitude motion is a pure $C2$ rotation of the $SO2$ subunit about its b-axis, an inversion-like wagging of this subunit about its a-axis, or something in between. These various possibilities as well as their consequences on the J and K dependence of the tunneling splitting will be discussed. The structural implications of the results obtained in the final analysis of the data will also be presented.