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A sensitive and generally applicable technique for direct absorption spectroscopy on electronic transitions of transient species in the gas phase is presented. The method is based on cavity ring down spectroscopy in a pulsed slit nozzle, incorporating a discharge in a high pressure supersonic expansion. The performance is demonstrated with spectra of the 0 0 0 origin band of the 2 ⌸←X 2 ⌸ electronic transition of the isoelectronic linear carbon chain radicals C 6 H and C 6 H 2 ϩ. Rotationally resolved and rotationally cold spectra (T rot Ͻ15 K͒ have been obtained. The sensitivity of the technique is demonstrated for anions with a detection limit as low as 10 7 C 2 Ϫ molecules cm Ϫ3 for rovibrational transitions of the B 2 ⌺ u ϩ ←X 2 ⌺ g ϩ system.

The 2Π ← X 2Π electronic transition of linear C8H/C8D and C10H/C10D has been detected in the gas phase. The carbon radical chains were produced at low temperatures in a pulsed slit nozzle, incorporating a discharge in a high pressure expansion. Cavity ring down spectroscopy is used as a sensitive technique to observe the band systems in absorption. The 000 band of the 2Π3/2 ← X 2Π3/2 electronic transition of C8H in the gas phase has its origin near 15 973.5 cm−1, whereas that of C10H is around 14 000 cm−1. Some transitions involving vibrational excitation in the upper 2Π electronic state have been also detected. These measurements were undertaken because carbon chains are among the appealing candidates as carriers of diffuse interstellar bands; the observed origin bands do not show matches with the hitherto reported wavelengths. However, these gas phase data now provide a firm basis for a specific astronomical search.

The origin and three vibronic bands of the 1Πu←X 1Σg+ electronic transition of linear C5 have been observed in the gas phase. The carbon chain is produced in a slit nozzle employing both discharge and ablation techniques. Cavity ring down spectroscopy is used to measure the electronic transition. The origin band is found at 510.94(1) nm, shifted 29 cm−1 to the red of the value in a neon matrix. Intramolecular processes lead to broadening and irregularities in the rotational structure. The relation to astronomical observations is discussed.