Quantum solvation of carbonyl sulfide with helium atoms.

High-resolution infrared and microwave spectra of He(N)-carbonyl sulfide (He(N)-OCS) clusters with N ranging from 2 to 8 have been detected and unambiguously assigned. The spectra show the formation of a solvation layer beginning with an equatorial "donut" of five helium atoms around the OCS molecule. The cluster moment of inertia increases as a function of N and overshoots the liquid droplet limit for N > 5, implying that even atoms in the first solvation shell are decoupled from the OCS rotation in helium nanodroplets. To the extent that this is due to superfluidity, the results directly explore the microscopic evolution of a phenomenon that is formally macroscopic in nature.