Laser spectroscopy of the carbon chains HC7H and HC9H

Strong vibronic bands of the (3)Sigma(u)(-) <-- X(3)Sigma(g)(-) transition of the linear triplet radicals HC7H and HC9H have been measured in the gas phase by cavity ringdown spectroscopy. The origin band of HC7H and the origin and nu(3) bands of HC9H all exhibit well resolved P and R branches. For HC9H, the origin band is within 1 Angstrom of the diffuse interstellar band lambda 5818, yet insufficiently close for positive identification. In both molecules, internal conversion is much more rapid than radiative emission, and fluorescence is not observed. Lower limits on the radiationless lifetime of the upper state of the origin band are determined to be 0.1 ns for HC7H and 0.01 ns for HC9H. Because the origin bands of these two radicals are about equally intense, it should be possible to detect longer members of the series in the gas phase.