THE TRANS-STILBENE AR VANDERWAALS COMPLEX - VIBRATIONALLY AVERAGED SUBSTITUTION STRUCTURE IN ITS S0 AND S1 ELECTRONIC STATES

Argon atom substitution coordinates, accurate to +/- 0.02 angstrom, in the S0 and S1 states of the trans-stilbene-Ar van der Waals complex have been determined by comparing the inertial constants obtained from fits of two bands in the rotationally resolved electronic spectrum of the complex with those obtained from fits of the corresponding spectrum of the bare molecule (Champagne et al., J. Phys. Chem. 94 (1990) 6). The argon atom is found to be localized above (or below) the plane of one of the aromatic rings by approximately 3 angstrom in both electronic states but is displaced off the ring center by more than approximately 1 angstrom in both transverse directions. This geometry is not the one predicted by simple atom-atom pair potentials. Model calculations suggest that this is a consequence of the vibrational averaging that occurs on the S0 and S1 surfaces, and that these surfaces are significantly influenced by the anisotropy of the van der Waals interaction potential, in both electronic states.