Abstract
Excitation and fluorescence spectra of benzonitrile-benzene and toluene complexes seeded in helium and expanded in a supersonic free jet reveals two types of transition attributed to the coexistence of two isomeric forms. One consists of narrow bands in both excitation and emission and is characteristic of wealdy perturbed van tier Waals complexes. The other involves broadened absorption and gives rise to an exciplex type emission.