Electronic spectroscopy of 1:1 van der Waals dimer of naphthalene and acenaphthene has been studied in a supersonic free jet expansion by measuring the laser-induced fluorescence excitation, dispersed fluorescence, and two-color hole-burning spectra. In fluorescence excitation spectrum the dimer exhibits a long progression of an intermolecular vibration, and similar vibronic structures are observed also in emission spectra from the origin region of the S-1 surface. The excimer formation from the locally excited state appears as a barrier crossing process and excess vibronic energy required to cross the barrier is about 420 cm(-1). The equilibrium structure and binding energy of the dimer are computed by the ab initio quantum chemistry method at the MP2/6-31G and MP2/6-31+G*//MP2/6-31G levels. A parallel-displaced structure, in which two molecules are displaced from a fully overlapping geometry by 1.16 and 0.45 Angstrom, respectively, along the long and short molecular axes, and maintains a vertical separation of 3.48 A between two molecular planes, is found to be the most stable in the ground state. The BSSE corrected MP2/6-31+G*//MP2/6-31G binding energy of the dimer is 9.2 kcal/mol. The observed spectral and dynamical characteristics of the mixed dimer are compared to those reported for the naphthalene homodimer, and the differences are interpreted in terms of geometry and exciton resonance interactions. (C) 2003 American Institute of Physics.