Rotational level structure of SF6-doped He-4(N) clusters

We present diffusion Monte Carlo calculations of low-lying rotational state s of (HeNSF6)-He-4 which confirm recent experiments suggesting that dopant molecules trapped inside He-4 clusters behave as free rotors. Analysis of t he rotational wave functions leads to a clear physical explanation for this effect based on angular momentum coupling arguments; a fraction of the hel ium density is found to follow the rotation of the SF6 molecule adiabatical ly. This leads to a free-rotor spectrum and to a reduction in the effective rotational constant that is in excellent quantitative agreement with exper iment.