Gas phase electronic spectrum of propadienylidene C3H2

The rotationally resolved vibronic bands in the forbidden A˜1A2-X˜1A1 electronic transition of the cumulene carbene C3H2 have been observed in the gas phase by cavity ring down absorption spectroscopy through a supersonic planar plasma with allene as precursor. The band detected in the 16 223 cm−1 region is a result of vibronic interaction and is assigned to a combination of a1 and b2 vibrations with a frequency around 2250 cm−1. Another vibronic band near 15 810 cm−1 has an unusual rotational structure because the Ka = 0-1 subband is absent. It is assigned to a combination of a1 and b1 vibrations, ∼1850 cm−1, which borrow intensity from the near lying B˜1B1 state due to a-type Coriolis coupling. A rotational analysis using a conventional Hamiltonian for an asymmetric top molecule yields molecular constants for the vibrational excited levels of the Ã1A2 state, which were used for the determination of the geometry. The stronger B˜1B1-X˜1A1 transition of C3H2, measured in a neon matrix in the 16 161-24 802 cm−1 range, was not detected. The reason for this is a short lifetime of the B˜ state, leading to line broadening.