Absorption spectra observed between 400 and 2500 nm in 5 K neon matrices have been assigned to electronic transitions of linear C−2n (n=2–10): 2Π←X 2Π, C2n (n=3–7): 3Σ−u←X 3Σ−g, and C2nH (n=3–8): 2Π←X 2Π chains. The species have been produced by mass selected deposition of cations or anions produced in a hot cathode discharge source (C2n, C−2n, C2nH) and by laser vaporization of graphite (C2n, C−2n). In addition to experimental and chemical evidence, the dependence of the absorption wavelength on the number of carbon atoms was used to assign the electronic transitions. Infrared absorptions which were recorded around 2000 cm−1 are attributed to asymmetric stretching frequencies of C8, C10, and C12. This is based on correlation of their intensities with the identified electronic bands. The possible relevance of the electronic spectra of these carbon chains to astrophysical observations of diffuse interstellar bands is discussed.

The electronic absorption spectra of HC2n+1H and HC2n+1H+ (n=2−7) in neon matrices have been obtained. The species were prepared by codeposition of mass selected HC2n+1H+ ions with neon at 5 K. For each species the 000 band of the electronic transition as well as frequencies of fundamental vibrations in the excited state have been determined. The wavelength of the origin band in this homologous series is linearly dependent on the number of carbon atoms. Isotopic labeling supports the vibrational assignment and confirms the presence of two equivalent hydrogen atoms for HC2n+1H and HC2n+1H+. Based on spectroscopic considerations, a linear geometry is assumed with one hydrogen atom located at each end, and that the observed spectra of HC2n+1H are their 3Σu−←X 3Σg− transition.

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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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