Free rotation of dopants in superfluid helium clusters

We present quantum Monte Carlo calculations of the effective rotational constant B of several cromophore molecules embedded in He clusters, as a function of the cluster size. The predictive power of the computed B values is demonstrated not only by their agreement with available measurements, but also by their use in the assignment of several lines in both infrared and microwave spectra. The simulation results complement and extend the experimental information, offering insight into the relationship between structural and dynamical properties and the onset of superfluidity. The range of cluster sizes studied in our simulations includes systems of several tens particles, intermediate between the small-cluster and the nanodroplet regimes. In this size range we find unexpected trends for the evolution of B towards its asymptotic nano-droplet value.