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The principal goal of this research is to understand intermolecular forces through the structure and vibrational properties of small clusters. The intermolecular forces holding these complexes together also largely determine the properties of many important systems such as condensed phases and macromolecules. Clusters involving water are of particular interest in understanding liquid water and ice, aqueous solutions and atmospheric phenomena. Thus this work will help in understanding these systems and the general phenomena of intermolecular forces.

This research project studies the rotational and vibrational spectroscopy of hydrogen bonded complexes and van der Waals molecules and develops new techniques for gathering this information. The purpose is to provide basic information for modelling the effects of these molecules in atmospheric phenomena and to achieve insights concerning the intermolecular forces manifested in the weak bonds holding cluster molecules together. Rotational spectra were investigated the molecular beam electric resonance method. High resolution radiofrequency and microwave spectra of complexes observed by this technique give accurate rotational constants, electric dipole moments, and nuclear hyperfine interaction data which was used to provide structural data. Methods were developed for observing vibrational spectra of complexes in molecular beams using CARS. Raman spectra were obtained for molecular beams of HCN and DCN polymers in the CN and CH stretching regions. A force field model for the axial modes of these molecules was constructed by combining infrared and Raman data. Infrared absorption spectra for molecular beams of rare gas-OCS complexes on the au3 monomer mode region were obtained using tunable diode lasers. The spectra are rotationally resolved and the Doppler limited linewidths (0.004 cm) show no evidence of vibrational predissociation broadening.

Molecular beams of hydrogen bonded water dimer, generated in a supersonic nozzle, have been studied using electric resonance spectroscopy. Radio frequency and microwave transitions have been observed in (H2(16)O)2, (D2(16)O)2, and (H2(18)O)2. Transitions arising from both pure rotation and rotation-tunneling occur. The pure rotational transitions have been fit to a rigid rotor model to obtain structural information. Information on the relative orientation of the two monomer units is also contained in the electric dipole moment component along the A inertial axis, which is obtained from Stark effect measurements. The resultant structure is that of 'translinear' complex with an oxygen-oxygen distance, R(00), of 2.98(1) A, the proton accepting water axis is 58 (6) degrees with respect to R(00), and the proton doning water axis at -51 (6) degrees with respect to R(00). This structure is consistent with a linear hydrogen bond and the proton acceptor tetrahedrally oriented to the hydrogen bond. The limits of uncertainty are wholly model dependent and are believed to cover variations from the zero-point vibrational structure observed to the equilibrium structure. The A component of the electric dipole moment shows strong dependence on J and K and is about 2.6D. Centrifugal distortion constants have been interpreted in terms of the monomer-monomer stretching frequency and give omega = 150/cm. (Author)