Nonrigidity, delocalization, spatial confinement and electronic-vibrational spectroscopy of anthracene-helium clusters

In this paper we present quantum mechanical calculations for the energetics, nuclear dynamics, spectral shifts, and vibrational level structure of anthracene . He-n (n=1,2) clusters in the ground (S-0) and in the first spin-allowed excited (S-1) electronic states. The anthracene-He potential in the S-0 state was described in terms of a sum of Lennard-Jones atom-atom potentials, while the potential in the S-1 state also included changes in dispersive energy and in repulsive interactions. Variational calculations were conducted for anthracene . He-1. For anthracene . He-2 we carried out configuration interaction calculations with the wave functions consisting of Hartree products, accounting for boson permutation symmetry. Extensive, anisotropic, one-dimensional spatial delocalization of the He atoms on the anthracene microsurface, which originates from large-scale confinement by the aromatic molecule, is exhibited, being further enhanced by repulsive interactions in the S-1 state and by the He-He repulsion. The anomalous size-dependence of the (red) spectral shifts for the S-0-->S-1 electronic origin arises from mutually canceling dispersive and repulsive contributions which, together with the electronic-vibrational level structure, manifest quantum effects of anisotropic spatial delocalization, confinement and He-He interaction in nonrigid clusters. (C) 2001 American Institute of Physics.