We study the characteristics of vibronic eigenfunctions of a multidimensional nonadiabatic system and their consequences in the quantum spectra. As an illustrative example, we investigate the properties of highly excited eigenfunctions of Heller's multimode nonadiabatic system. The system consists of two diabatic states and two-dimensional (two-mode) harmonic potentials that are nonadiabatically coupled with the Condon approximation and with an appropriate magnitude of the Duschinsky angle. "Quantum chaos" thus produced has no classical counterpart. In addition to rather characterless chaotic eigenfunctions that are uniformly widespread in configuration space, we have found highly excited localized eigenfunctions of two extreme types which favor either the diabatic picture or adiabatic picture. As a result, the features of the associated quantum spectra are strongly affected by the initial preparation of a wave packet. This finding suggests that one can control the rate of nonadiabatic transitions such as that for electron transfer by using laser techniques or by choosing appropriate solvents. (C) 2001 American Institute of Physics.