Quantum solvation and molecular rotations in superfluid helium clusters

Spectroscopic experiments on molecules embedded in free clusters of liquid helium reveal a number of unusual features deriving from the unique quantum behavior of this nanoscale matrix environment. The apparent free rotation of small molecules in bosonic He-4 clusters is one of the experimentally mo st well documented of these features. In this Focus article, we set this ph enomenon in the context of experimental and theoretical advances in this fi eld over the last ten years, and describe the microscopic insight which it has provided into the nature and dynamic consequences of quantum solvation in a superfluid. We provide a comprehensive theoretical analysis which is b ased on a unification of conclusions drawn from diffusion and path integral Monte Carlo calculations. These microscopic quantum calculations elucidate the origin of the empirical free rotor spectrum, and its relation to the b oson character and superfluid nature of the quantum nanosolvent. The free r otor behavior of the molecular rotation is preserved because of inefficient angular momentum coupling between the dopant and its quantum liquid surrou ndings. This is consistent with the superfluid character of the droplet, an d has significant implications for the hydrodynamic response of the local q uantum fluid environment of the embedded molecule. The molecule-helium inte raction appears to induce a local nonsuperfluid density component in the fi rst quantum solvation shell. This can adiabatically follow the molecular ro tation, resulting in a reduction of the rotational constant. The dynamic na ture of this adiabatically following density, its relation to the magnitude of the gas-phase molecular rotational constant and to the anisotropy of th e interaction potential, are characterized with several examples. The role of the local superfluid density is analyzed within a continuum hydrodynamic model which is subject to microscopic quantum constraints. The result is a consistent theoretical framework which unites a zero temperature descripti on based on analysis of cluster rotational energy levels, with a quantum tw o-fluid description based on finite temperature analysis of local quantum s olvation structure in the superfluid. (C) 2000 American Institute of Physic s. [S0021-9606(00)01940-1].