THE (2)PI[-X (2)PI ELECTRONIC-SPECTRA OF C8H AND C10H IN THE GAS-PHASE

The (2)Pi<--X (2)Pi electronic transition of linear C8H/C8D and C10H/C 10D 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 absor ption. The 0(0)(0) band of the (2)Pi(3/2)<--X (2)Pi(3/2) electronic tr ansition of C8H in the gas phase has its origin near 15 973.5 cm(-1) w hereas that of C10H is around 14 000 cm(-1). Some transitions involvin g vibrational excitation in the upper (2)Pi electronic stare have been also detected. These measurements were undertaken because carbon chai ns are among the appealing candidates as carriers of diffuse interstel lar bands; the observed origin bands do not show matches with the hith erto reported wavelengths. However, these gas phase data now provide a firm basis for a specific astronomical search. (C) 1998 American Inst itute of Physics. [S0021-9606(98)00934-9].