Formation of CN (B2_Sigma+) radicals in the vacuum-ultraviolet photodissociation of pyridine and pyrimidine molecules

Formation of the excited CN(B2?+) free radicals in the photodissociation of pyridine (C5H5N) and pyrimidine (C4H4N2) molecules was investigated over the energy ranges 16-27 and 14.7-25 eV, respectively. Photon-induced fluorescence spectroscopy was applied to detect the vibrationally and rotationally excited CN radicals by recording the B2?+->X2?+ emission bands (violet system). The measured dissociation yield curves demonstrate that the CN(B2?+) formation occurs via excitation of pyridine and pyrimidine molecules into higher-lying superexcited states. This is followed by rearrangement and isomerization of the excited molecules before dissociation. The vertical excitation energies of the superexcited states were determined and the probable dissociation mechanisms of both molecules are discussed.