The existence of isomeric forms of donor-acceptor complexes formed in the supersonic expansion was evidenced by their fluorescence, fluorescence excitation, and hole-burning spectra. For each complex, we observed two types of isomers : one E-isomer with broad structureless excitation spectra yielding diffuse, strongly red-shifted exciplex emission and a number of (one to five) R-isomers with narrow band excitation spectra yielding a resonant emission upon the excitation of the lowest vibrational levels of the excited electronic state and exciplex emission from higher levels. A detailed analysis of their excitation spectra (spectral shifts, vibrational structures, bandwidths) is given. The shape of the ground-state potential energy surface (depths and positions of the energy minima, heights of energy barriers, etc.) was determined by calculations involving the "exchange perturbation" treatment and simulated annealing method. The relation between the properties of observed species and calculated configurations is discussed.