Localized helium excitations in 4HeN-benzene clusters

We compute ground and excited state properties of small helium clusters 4HeN containing a single benzene impurity molecule. Ground-state structures and energies are obtained for N=1, 2, 3, 14 from an importance-sampled, rigid-body diffusion Monte Carlo method. Excited state energies due to helium vibrational motion near the molecule surface are evaluated using the projection operator, imaginary time spectral evolution method. We find excitation energies of up to ∼23 K above the ground state. These states all possess vibrational character of helium atoms in a highly anisotropic potential due to the aromatic molecule, and can be categorized in terms of localized and collective vibrational modes. These results appear to provide precursors for a transition from localized to collective helium excitations at molecular nanosubstrates of increasing size. We discuss the implications of these results for analysis of anomalous spectral features in recent spectroscopic studies of large aromatic molecules in helium clusters