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-dimethylnaphth alene . Ne complex in the S-1 electronic state. The two-color resonant two-photon ionization spectrum exhibits eight bands within approximat e to 40 cm(-1) of the electronic origin. Theoretical considerations in combination with hole-burning spectroscopic measurements show that th e transition closest to the electronic origin (at 0(0)(0)+5 cm(-1)) ar ises from an isomer which is different from that responsible for the o ther seven bands in the spectrum. The latter involve excitations of th e intermolecular vibrations of the main isomer of 2,3-dimethylnaphthal ene . Ne. Accurate three-dimensional quantum calculations of the van d er Waals vibrational levels of the complex were performed using a disc rete variable representation method. Combination of theory and experim ent led to a complete assignment as well as to a quantitative theoreti cal reproduction of the experimental intermolecular vibrational level structure, and a parametrization of the intermolecular potential energ y 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 experi mentally are associated with the global and the local minimum, respect ively. The quantum calculations were extended to approximate to 1000 v an der Waals vibrational states, i.e., to energies up to 78% of D-0. T hese 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 dyn amics of the high-lying van der Waals states of an atom-large aromatic ,molecule complex. (C) 1995 American Institute of Physics.