Discovery of the rotational spectrum of the weakly bound complex CO–H2

Rotational transitions of the CO–H2 van der Waals complex have been measured between 108 and 125 GHz in a supersonic jet. Three absorption lines were recorded and assigned as belonging to CO–paraH2 with a new highly sensitive intracavity spectrometer based on the millimeter wave generator, called OROTRON. The results provide confirmation of the recent infrared data of McKellar at 4.7 μm, enabling a more precise determination of the energies of the rotational levels with different parity in the vibrational ground state. The measured millimeter wave transitions provide precise frequencies for an astronomical search of CO–paraH2.