Electronic spectra of the C2n+1H (n=2–4) radicals in the gas phase

The visible electronic spectra of the linear l-C2n+1H (n=2–4) radicals have been measured in the gas phase. These have been obtained by means of a mass-selective resonant two-color two-photon ionization technique coupled to a supersonic plasma source. The observed spectra are assigned to the A 2Δ←X 2Π, B 2Σ−←X 2Π, and C 2Σ+←X 2Π electronic transitions arising from σ→π electron promotion. The assignments are based on ab initio calculations, wavelength dependence of the 000 transition on size, and isotopic substitution. The lifetime broadening of the bands and effects due to vibronic coupling are associated with the carbon skeleton bending modes. The detection of these carbon chains in the diffuse interstellar medium appears to be more favorable by radio astronomy rather than by electronic spectroscopy.