STEVE P. DEMPSTER AND WOLFGANG JÄGER, Department of Chemistry, University of Alberta, Edmonton, Canada T6G 2G2.
In a recent experimental and theoretical study by Li et al ., direct evidence for molecular superfluidity in small clusters of para -hydrogen molecules ( p H2) seeded with CO2 was discovered. The authors also found that the anisotropy of the CO2-- p H2 interaction potential promotes localization of the p H2 molecules at larger cluster sizes ( N ~ 17 para -hydrogen molecules), with a simultaneous reduction of the superfluid p H2 fraction. It has been suggested that light rotors with a more isotropic interaction potential, such as CO and HCN, are potentially more subtle probes of p H2 superfluidity that would allow superfluidity to persist, especially at larger values of N . In the current study, the hyperfine structures of the end-over-end rotational transitions of ( p H2) N -HCN clusters were measured using our chirped-pulse Fourier transform microwave spectrometer. Based on tentative assignments, the evolution of effective rotational constants, Beff , as a function of N shows evidence for superfluid behaviour by a clear ``turn-around'' point at a low N value. The trend of Beff vs. N and the results from the hyperfine structure analysis will be compared to the recent studies on He N -HCN and ( p H2) N -CO.