Electronic absorption spectra of linear C−6, C6, and C6H have been identified in neon matrices at 5 K. The species were produced by codepositing mass selected cations and anions with neon. The ions were generated in a hot cathode discharge source using diacetylene. The spectra of C−6 and C6 could also be observed using a pure carbon anion source or laser vaporization of graphite. The assignment is based on the mass selection, experimental and spectroscopic evidence, leading to the location of the 000 transitions of C−6: 2Πg←X 2Πu, C6: 3Σ−u←X 3Σ−g, and C6H: 2Π←X 2Π at 16 458, 19 558, and 18 854 cm−1, respectively. The frequencies of the symmetric carbon stretching vibrations have been obtained for these species in their excited electronic states.

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Double-resonance spectroscopy, using collisional energy transfer to populate high-lying vibrational states, reveals the location of 27 previously undetected levels in the ground electronic state of HCN. Three laserpumped overtones, (0,0 0 ,4), (0,0 0 ,5), and (1,0 0 ,5) provide the doorway through which to collisionally populate vibrational levels lying between 10330 and 17650 cm -1 . This greatly increases the energy range over previous studies using collision-assisted spectroscopy and provides access to states which have internal energy above the predicted isomerization barrier. Levels populated by collisions from (0,0 0 ,4) and (0,0 0 ,5) are detected directly via A ˜state fluorescence, while those from (1,0 0 ,5) are detected through resonant multiphoton dissociation resulting in CN (BfX) fluorescence. Although potentially, collision-assisted spectroscopy above the isomerization barrier could access HNC localized states, such states have not been identified in the present experiments.