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 ant hracene . He-n (n=1,2) clusters in the ground (S-0) and in the first spin-a llowed 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 potent ials, while the potential in the S-1 state also included changes in dispers ive energy and in repulsive interactions. Variational calculations were con ducted for anthracene . He-1. For anthracene . He-2 we carried out configur ation interaction calculations with the wave functions consisting of Hartre e products, accounting for boson permutation symmetry. Extensive, anisotrop ic, one-dimensional spatial delocalization of the He atoms on the anthracen e microsurface, which originates from large-scale confinement by the aromat ic 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 fr om mutually canceling dispersive and repulsive contributions which, togethe r with the electronic-vibrational level structure, manifest quantum effects of anisotropic spatial delocalization, confinement and He-He interaction i n nonrigid clusters. (C) 2001 American Institute of Physics.