MOLECULAR ROTATION IN QUANTUM LIQUIDS

Author Institution: General Physics Institute RAS; Department of Chemistry, University of Southern California; Department of Chemistry, Max-Planck-Institut fur StromungsforschungRotationally resolved IR spectra of molecules and hydrogen clusters trapped in $^{4}He$ and mixed $^{3}He/^{4}He$ droplets are presented. The newest analysis of spectra in terms of effective rotational constants of $OCS-(pH_{2})_{n}$ and $OCS-(oD_{2})_{n}$ complexes with $n=11,{\ldots}16$ is presented. Whereas molecular rotation in $^{4}He$ can be modelled by a free rotor with effective rotational constants the effect of interaction between a molecule and its H2 solvation shell is more pronounced. Analysis of the spectra within the frameworks of a rigid symmetric $H_{2}$ shell proves the inconsistency of the rigid shell model with the experimental observations. Furthermore it suggests existence of some low energy excitations $(E{\sim}0.1 cm^{-1})$ which are tentatively assigned to tunnelling states of the $H_{2}$ cluster