VAN-DER-WAALS VIBRATIONS AND ISOMERS OF 2,3-DIMETHYLNAPHTHALENE-CENTER-DOT-NE - EXPERIMENT AND QUANTUM 3-DIMENSIONAL CALCULATIONS

We report a combined experimental and theoretical study of the van der Waals isomers and intermolecular vibrations of the 2,3-dimethylnaphthalene . Ne complex in the S-1 electronic state. The two-color resonant two-photon ionization spectrum exhibits eight bands within approximate to 40 cm(-1) of the electronic origin. Theoretical considerations in combination with hole-burning spectroscopic measurements show that the transition closest to the electronic origin (at 0(0)(0)+5 cm(-1)) arises from an isomer which is different from that responsible for the other seven bands in the spectrum. The latter involve excitations of the intermolecular vibrations of the main isomer of 2,3-dimethylnaphthalene . Ne. Accurate three-dimensional quantum calculations of the van der Waals vibrational levels of the complex were performed using a discrete variable representation method. Combination of theory and experiment led to a complete assignment as well as to a quantitative theoretical reproduction of the experimental intermolecular vibrational level structure, and a parametrization of the intermolecular potential energy surface, modeled as sum of atom-atom Lennard-Jones pair potentials. This potential surface exhibits a global minimum above (and below) the aromatic ring plane of 2,3-dimethylnaphthalene and a shallower local minimum at C-2 nu geometry, on the C-2 axis of the molecule, adjacent to the two methyl groups. The main and minor isomers identified experimentally are associated with the global and the local minimum, respectively. The quantum calculations were extended to approximate to 1000 van der Waals vibrational states, i.e., to energies up to 78% of D-0. These include levels localized either in the global or local minima, as well as highly excited vibrational states delocalized over all three potential minima, providing comprehensive insight into the quantum dynamics of the high-lying van der Waals states of an atom-large aromatic,molecule complex. (C) 1995 American Institute of Physics.