Electronic spectrum of C6H: (2)Pi-X (2)Pi in the gas-phase detected by cavity ringdown

The (2) Pi-X (2) Pi electronic transition of the carbon chain C6H has been detected in the gas-phase. The radical is generated in a hollow cathode discharge of acetylene in helium, and cavity ringdown spectroscopy is used as a sensitive means to observe the band system in absorption. This is the first result aimed at locating the electronic transitions of the neutral carbon chains, of astrophysical interest, in the gas-phase. The search was based on the measurements made previously on C6H in neon matrices. The origin band (Omega = 3/2) in the gas phase at 18996.4 cm(-1) is 142 cm(-1) to the blue of the matrix value.