Spectroscopic investigation of OCS (p-H2)n (n=1-16) complexes inside helium droplets: Evidence for superfluid behavior

Up to 16 parahydrogen and orthodeuterium molecules have been assembled around an OCS carbonyl sulfide chromophore molecule inside the pure He-4 and mixed He-4/He-3 droplets at temperatures of 0.38 and 0.15 K, respectively. The infrared spectra of the resulting complexes exhibit a sequence of rotationally resolved vibrational nu(3) bands in the vicinity of 2060 cm(-1), which are sufficiently separated to assign them to clusters with specific numbers of attached molecules for n=1-16. The present article contains the first complete analysis of the spectra for n=2-8 and a full documentation of the results for n=8-15 briefly described in a short report [Europhys. Lett. 83, 66008 (2008)]. Distinct rotational Q-branches are observed for all OCS-(o-D-2)(n) clusters at the He droplet temperatures of 0.38 K and 0.15 K, indicating that the (o-D-2)(n) shell rotates nearly freely about the molecular OCS axis. In the case of OCS-(p-H-2)(n) at 0.38 K, the Q-branch is seen for most n, with the exception of n=5, 6 and n=12. At 0.15 K, the Q-branch has disappeared for all n >= 11, indicating that the axial rotations are no longer active. Previously, the absence of a Q-branch for n=5 and 6 was explained by the high group symmetry of the bosonic p-H-2 rigid (donut) rings around the OCS molecule. This model, however, fails in explaining the disappearance of the Q-branch for n >= 11. In essential agreement with recent path-integral Monte Carlo calculations, the observed phenomenon is attributed to the onset of superfluidity in the multiring p-H-2 shell and the related permutations of bosonic p-H-2 molecules. A floppy shell model, which accounts for the effect of tunneling and exchange of molecules within the clusters, is able to explain the postulated superfluid behavior of the p-H-2 shell at low temperatures. Within this model the activation of states of low axial symmetry is responsible for the appearance of the Q-branch at higher temperatures.