Λ-type doublets and lifetime broadening in the B2Π-X2Π electronic origin band of C6H

We present a high-resolution study on the B-2 Pi-X-2 Pi electronic origin band spectrum of the C6H radical with its center at similar to 18990 cm(-1) The C6H radicals are produced in a pulsed supersonic slit-jet discharge source and detected in direct absorption by sensitive cavity ring-down spectroscopy. By using a home-made narrow linewidth nanosecond pulsed laser source, the experimental spectrum allows to resolve the Lambda-type doublet fine structures in high-J rotational lines. A set of improved B-2 Pi state constants, including the Lambda-type doubling parameters of p ' = -1.16(9)x10(-3) cm(-1) q ' = -1.22(7)x10(-4) cm(-1) are determined from analysis of the experimental spectrum. An analysis on the spectral line profile has resulted in a significant linewidth broadening of similar to 0.05 +/- 0.01 cm(-1), corresponding to a B-2 Pi state lifetime of similar to 100 +/- 20 ps. The short lifetime of the excited B-2 Pi state is proposed due to fast internal vibronic couplings to high-lying vibronic levels of lower electronic states.