Rotationally resolved electronic spectrum of propadienylidene

The rotationally resolved electronic spectrum of a vibrational band in the A˜1A2-X˜1A1 transition of the cumulene carbon chain C3H2 was measured in the 625 nm region in a supersonic discharge using cw cavity ring down spectroscopy. The rotational structure was analysed using a conventional Hamiltonian for an asymmetric top molecule, and the constants A′, B′, and C′ in the upper state were determined. The observed band is assigned to a combination of a1 and b2 vibrations with the frequency around 2000 cm−1. The geometries in the 1A1, 1A2, 1B1 states and the intersection point between the latter two were obtained using ab initio calculations. The effective structure in the measured vibrational level of the 1A2 state was inferred from the determined constants.