Knowledge Bank

In a recent experimental and theoretical study by Li {\em et al}. {\bf 105}, 133401 (2010).}, direct evidence for molecular superfluidity in small clusters of {\em para}-hydrogen molecules ({\em p}H$2$) seeded with CO$2$ was discovered. The authors also found that the anisotropy of the CO$2$--{\em p}H$2$ interaction potential promotes localization of the {\em p}H$2$ molecules at larger cluster sizes ({\em N} $\approx$ 17 {\em para}-hydrogen molecules), with a simultaneous reduction of the superfluid {\em p}H$2$ 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 {\em p}H$2$ superfluidity that would allow superfluidity to persist, especially at larger values of {\em N}. In the current study, the hyperfine structures of the end-over-end rotational transitions of ({\em p}H$2$){\em }-HCN clusters were measured using our chirped-pulse Fourier transform microwave spectrometer. Based on tentative assignments, the evolution of effective rotational constants, {\em B}, as a function of {\em N} shows evidence for superfluid behaviour by a clear "turn-around'' point at a low {\em N} value. The trend of {\em B} vs. {\em N} and the results from the hyperfine structure analysis will be compared to the recent studies on He{\em }-HCN and ({\em p}H$2$){\em }-CO.