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 orig
in 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 lifeti
me of the upper state of the origin band are determined to be 0.1 ns for HC
7H 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 t
he series in the gas phase.