Electronic spectra of carbon chain anions: C_2nH^- (n=5-12)

The electronic absorption spectra of mono-hydrogenated carbon chain anions C_2nH^- (n=5-10) have been measured in the gas-phase and in 6 K neon matrices (n=8-12). The techniques of resonant two-color electron photodetachment in the gas-phase and absorption spectroscopy of mass-selected anions in neon matrix were used. A homologous series is observed, with band system origins shifting from 304 nm for C₁₀H^- to 590 nm for C₂₀H^-. In conjunction with ab initio calculations the band systems are attributed ¹∑⁺ ← + X ¹∑⁺ transition of linear acetylenic anions. Another near lying electronic transition due to a second isomer is also apparent for C₁₀H^- up to C₂₄H^-. Comparison with tables of the known diffuse interstellar bands indicates possible matches for the origin bands of the C₁₈H^- and C₂₀H^- isomers.