Spectroscopic Studies of Clusters and Aggregates

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The research proposal describes a program to study matter in size regimes from the microscopic, molecular world to the nano-scale by the characterization of molecular aggregates. These studies are largely enabled by a revolution in high resolution spectroscopy both in terms of the development of new, more powerful spectroscopic techniques and its successful application to ever larger molecular systems. In the intermediate size regime, we propose to study clusters of helium atoms, and those of hydrogen molecules, doped with molecular chromophores, such as carbon monoxide or hydrogen cyanide. The results will shed further light on how a bulk phase property, namely superfluidity, evolves from the microscopic scale. Superfluidity is a consequence of the bosonic (integer composite spin) character of the constituent particles. We are particularly interested in using the fermionic 3-helium and HD as solvent species. The detection of superfluid effects in these cases would challenge current interpretations. Our newly constructed chirped pulse Fourier transform microwave spectrometer will make these studies possible. Much larger helium nanodroplets, which can consist of about a thousand to millions of helium atoms will serve as nano-cryostats and nano-reactors for the study of low temperature reactions of atmospheric and astrophysical importance. Emphasis will be on barrier-less reactions, such as carbon chain lengthening reactions involving carbon atoms and hydroxyl radical reactions. The reactive species will be produced by laser ablation or laser induced photodissociation of a precursor, such as hydrogen peroxide. A third research area concerns the formation of even larger aggregates, namely secondary organic aerosols ('smog'). These aerosols can have significant effects on climate and human health. Our studies will contribute to the improvement of atmospheric chemistry models.