Internal dynamics and "superfluidity" in small helium and hydrogen clusters

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Small clusters counting a few to several tens helium atoms or hydrogen molecules doped with a probe molecule will be investigated by millimeter-wave spectroscopy in order to characterize the properties of these clusters in dependence on their size. This study is intended to address fundamental questions related to the microscopic nature of superfluidity, namely, what number of helium atoms is required for the onset of superfluidity and whether hydrogen can be made superfluid? Rotational spectra of cold clusters in a supersonic jet will be measured with the ultrahigh sensitivity intracavity spectrometer, OROTRON. Activities will be constrained to clusters doped with carbon monoxide, HeN-CO and (H2)N-CO, since the properties of these systems are very similar to those of pure helium or hydrogen clusters and in contrast to other systems to date no pure rotational spectra have been recorded. Furthermore, establishment of a relation of rotational dynamics of the complex to its superfluid response in the small to intermediate cluster size regime are the goals of present proposal.