The submillimeter-wave spectrum of the HCCO radical

The pure rotational spectra of the HCCO and DCCO radicals were observed for the first time in the submillimeter-wave region. The HCCO radical was generated by either the C2H2+O reaction or the H2CCO+F reaction. The former was found to be about twice as efficient as the latter, and was employed for obtaining DCCO using C2D2 as a precursor. The observed spectra were fitted to the pattern of a near prolate symmetric top with a large A rotational constant. The observed spin-rotation splittings result in an extremely large εaa constant and exhibit an anomalous K dependence, indicating that there is a low-lying excited electronic state which interacts with the ground state.