Infrared diode laser kinetic spectroscopy of the CCH radical ν3 band

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Abstract
Infrared diode laser kinetic spectroscopy was applied to the photodecomposition of acetylene at 193 nm to observe the ν3 (C–C stretching) band of the CCH radical. More than one hundred absorption lines were observed between 1750 and 1900 cm−1, but none of them could have been assigned to the ν3 band. This is probably due to CCH molecules being excited to high‐lying vibrational states by the excess energy of the photolysis. In fact, when either hydrogen or deuterium was added to acetylene as a buffer gas, new absorption lines appeared and were assigned to the ν3 fundamental band. The band origin which was derived from the observed spectrum is 1840.5711(4) cm−1, in agreement with the result of a previous matrix study. The spin–rotation interaction constant was found to decrease by as much as 30% upon ν3 excitation, presumably because of mixing with the lowest excited electronic state.