Molecular beam investigations of open-shell Van der Waals molecules
Van der Waals molecules are complexes of two or more atoms or molecules bound by weak intermolecular forces rather than a conventional chemical bond. This thesis contains a high resolution molecular beam study of the rotational spectra of two such species, ArNO and NeNO. A new hamiltonian is presented to account for the spectra of these open-shell complexes. The theory is similar to that already known for the uncomplexed diatom, nitric oxide, but is modified to an asymmetric top form appropriate for these non-linear species. Two novel interactions require inclusion. The first models a slight quenching of the NO orbital angular momentum by the rare gas atoms. The second is a large centrifugal distortion associated with the effects of the bending motion on the projection of the electronic angular momentum on the complex's a-inertial axis. A comparatively rich, though incomplete, spectrum is obtained for ArNO. Most of the transitions that originate from the lower rotational states are assigned and fitted to within a few MHz using the modified open-shell hamiltonian. Possible sources are suggested for the origins of the small residual errors between the observed and calculated transition frequencies. A vibrationally-averaged structure is derived from the fitted molecular parameters indicating an equilibrium geometry which is quite close to a T-shape. The hyperfine structure in the spectrum which is associated with the 14 N nucleus is well fitted by the hamiltonian; however, the magnetic hyperfine parameters are significantly perturbed from the values that best fit the lambda-doubling spectrum of the uncomplexed diatom. The implications of this with respect to the nature of the rare gas-nitric oxide intermolecular interaction are discussed. Greater experimental difficulties are encountered in trying to record the radiofrequency and microwave spectrum of NeNO and so fewer transitions have been measured than for ArNO. A partial analysis yields approximate values for the structural parameters. The complex exhibits a larger deviation from the T-shaped geometry than is observed for ArNO. The spectrum also provides an estimate of the magnitude of the quenching of the orbital angular momentum. The quenching interaction is somewhat larger in the neon complex.