Microsolvation and vibrational shifts of OCS in helium clusters

We present a theoretical study of the solvation structure around an OCS mol ecule embedded in helium clusters containing 1-100 He-4 atoms, obtained fro m diffusion Monte Carlo calculations employing an ab initio, vibrational-st ate dependent internuclear potential and incorporating the molecular rotati onal degrees of freedom. The effect of the molecular rotation is to make th e local helium density around the molecule considerably more delocalized in the ellipsoidal coordinates than that seen around a nonrotating OCS molecu le. We find an unexpectedly sharp energy signature associated with completi on of the first solvation shell at N similar to 20, suggesting that strongl y bound molecules like OCS could have a "magic" quantum solvation number wh ich is not apparent from the structural quantifiers of the solvating adatom s of that shell. The frequency shifts of the asymmetric stretch transition of the OCS molecule are computed as a function of cluster size via a dynami cally adiabatic decoupling scheme. The vibrational frequency shows a monoto nically increasing red shift with cluster size up to the completed first so lvation shell at N similar to 20, where it saturates to a value in good agr eement with experimental measurements made for OCS in much larger clusters. (C) 2001 American Institute of Physics.