Two electronic bands at similar to 18881 and similar to 18920 cm(-1) -previously assigned to the carbon chain molecule C9H3 - have been recorded, resolving for the first time their K-stack structure. The C9H3 radicals are produced by discharging and expanding a diluted gas mixture of acetylene in helium employing a pulsed pinhole nozzle. Cavity ring-down spectroscopy is used to record spectra in direct absorption. The improved experimental data and spectrum simulations based on new theoretical structure predictions show that the HC4(CH)C4H isomer (with C-2v symmetry) is a likely carrier of the two observed C9H3 bands. (C) 2010 Elsevier B.V. All rights reserved.