Infrared Q-branch anomalies on cooling small OCS (pH2)n clusters from 0.38 K to 0.15 K

Infrared spectra of OCS molecules with 8≤n≤16 attached para-hydrogen (pH2) molecules inside pure 4He droplets at 0.38 K and in mixed 4He/3He droplets at 0.15 K are compared with the same-sized oD2 clusters. In the pH2 clusters the Q-branch is observed at 0.38 K, except for n=12, but disappears for n≥11 on cooling to 0.15 K, whereas it is present in all the oD2 clusters. The high axial symmetry of the pH2 clusters at 0.15 K is consistent with the onset of microscopic superfluidity resulting predominantly from exchanges accompanying the hindered rotations of two or more rings.