Spectroscopic investigation of OCS (p-H(2))(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 (4)He and mixed (4)He(3)He 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>or=11, indicating that the axial rotations are no longer active.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>or=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.