Cavity ringdown spectroscopy of the linear carbon chains HC7H, HC9H, HC11H, and HC13H

Optical bands from the (3)Sigma(u)(-) <-- X (3)Sigma(g)(-) transition of th e linear carbon chains HC7H, HC9H, HC11H, and HC13H have been measured in t he gas phase by cavity ringdown spectroscopy. The bands exhibit well resolv ed P and R branches, which for HC7H are partially rotationally resolved. Co mparisons between observed and simulated spectra indicate broadening in exc ess of that expected from the laser linewidth and Doppler width, suggesting rapid radiationless transitions in roughly 0.1-0.01 ns, a time scale consi stent with the absence of observed fluorescence from these molecules. The H C7H and HC9H bands are of nearly equal strength, but those of the shorter c hain HC5H are too weak to detect at the 1 ppm level. None of the bands obse rved here lies sufficiently close to any of the optical diffuse interstella r bands to provide positive identification. (C) 2000 American Institute of Physics. [S0021-9606(00)00423-2].