Lifetime broadening in the gas phase B̃2Π←X̃2Π electronic spectrum of C8H

The origin band of the B̃2Π32←X̃2Π32 electronic transition of linear C8H was recorded in a planar supersonic expansion by a cw cavity ring-down spectrometer. The C8H radical was produced using a discharge through a C2H2/He mixture inside a pulsed slit nozzle. Despite the fact that the resolution of the spectrometer is 350 MHz, which is 3–4 times higher than the separation of rotational lines in this band, the rotational structure was not resolved. It is concluded that the rotational lines are broadened by rapid radiationless transitions from the excited electronic state. Simulations of the spectrum give an estimate of 0.8 cm−1 Lorentzian linewidth which corresponds to ∼7 ps lifetime in the excited B̃2Π32 electronic state.