We report a combined experimental and theoretical study of the intermolecular vibrations and van der Waals isomerism of the 2,3-dimethylnaphthalene . He van der Waals complex. Two-color resonant two-photon ionization spectra of the S-0-->S-1 electronic transition of 2,3-dimethylnaphthalene . He exhibit five bands within 30 cm(-1) of the electronic origin. The intermolecular potential energy surface was modeled as a sum of atom-atom Lennard-Jones pair potentials; it exhibits two equivalent global minima on each side of the naphthalene moiety, and a single shallower local minimum adjacent to the two methyl groups. Based on this surface, accurate three-dimensional quantum calculations of the van der Waals vibrational levels using the discrete variable representation method were performed. Careful optimization of the potential parameters lead to a quantitative reproduction of four observed bands as intermolecular vibrational excitations, a vibrationally averaged He atom distance from the aromatic plane [z(0)] = 3.22 Angstrom, and a dissociation energy D-0(S-1) = -60.3 cm(-1), compatible with experiments. The fifth band is assigned as a van der Waals isomer, corresponding to the local minimum. The quantum calculations were extended up to the dissociation limit, yielding approximate to 173 van der Waals vibrational states. Above 70% of D-0, many vibrational states are completely delocalized over the potential surface, with root-mean-square vibrational amplitudes up to 6 Angstrom parallel to and up to 1.5 Angstrom perpendicular to the molecular surface. Calculated tunnelling splittings range from < 10(-4) cm(-1) for localized states, to >3 cm(-1) for highly delocalized ones. (C) 1997 American Institute of Physics.