Laboratory and astronomical spectroscopy of C3H2, the first interstellar organic ring

The first laboratory and astronomical observations of the carbene ring molecule C3H2 briefly reported in a recent Letter are described in detail. In the laboratory, 22 millimeter-wave lines have been measured in a He and C2H2 discharge. From these the three rotational constants and the five fourth-order centrifugal distortion constants of C3H2 have been determined to high precision, allowing, in turn, calculation to 0.1 km/s of the most important lines in the radio and far-IR spectrum; 123 lines in the rotational spectrum with E/K less than 100 K are tabulated. Eleven astronomical transitions have been identified, and in Sgr B2 and Orion KL, rotational temperatures and column densities were derived and compared with those of other hydrocarbons with a similar number of C atoms.