spectroscopic and computational studies of intermolecular interactions

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Intermolecular interactions are of importance in many areas, including processes such as condensation and evaporation, properties of condensed phase matter, biological processes such as protein folding and molecular recognition, and self assembly. This proposal describes study of intermolecular interactions by the characterization of molecular aggregates, from small complexes that contain a few molecules to large nano-clusters that consist of several thousand atoms, using state-of-the-art high resolution spectroscopic techniques. The two major innovative thrusts are: a) to use cluster spectroscopy to study paraHydrogen clusters doped with a probe molecule and b) to apply helium nanodroplet spectroscopy to the study chemical reactions on the microscopic level. Rotational spectroscopy of paraHydrogen molecules containing clusters will be facilitated by the construction of a chirped pulse broadband Fourier transform microwave spectrometer. The resulting spectroscopic parameters will be analyzed to detect, and to follow the evolution of, a bulk property, namely superfluidity, from the molecular, microscopic regime. These studies will be complemented by high level ab initio calculations of He - molecule and H2 - molecule dimers and subsequent bound state calculations. The theoretical results on the dimers will provide an important starting point for the experimental investigations of the larger clusters. We will use our newly constructed helium nanodroplet spectrometer to investigate photo-induced chemical reaction of species embedded in helium droplets that contain several thousand helium atoms. The helium nanodroplets serve here as microscopic reactor and calorimeter. Further spectroscopic studies on doped helium nanodroplets in the infrared, Terahertz, and microwave regions will contribute to our understanding of the dopant - helium droplet interaction and also of the interesting properties of the helium droplets themselves.