A high-resolution gas-phase electronic spectrum has been detected in the 408 nm region using a cw cavity ring-down spectrometer. The absorption band system is observed using a pulsed slit nozzle discharge with a mixture of acetylene in argon. The rotationally resolved spectrum obtained is that of a symmetric top molecule. The determined constants are ¼ 24491.0500 (108) and B 00 ¼ 0.14165 (44) cm À1 . Consideration of the latter value and ab initio calculations leads to the assignment 3 E X 3 E transition of the linear carbon chain species CCCCH þ 3 in the triplet ground electronic state with C 3v symmetry.