The millimeter wave spectrum of iodoethyne,HCCI

The rotational spectrum of iodoethyne, HCCI, has been observed in the frequency range from 105 to 305 GHz. A sensitive spectrometer system employing source modulation, a harmonic generator millimeter wave source and a liquid helium cooled InSb-detector has been used. The rotational constants and nuclear quadrupole coupling parameters of HCCI have been obtained for the ground state and the first and second excited vibrational states of ν₄. The rotational spectrum of the 2ν₅ state has also been identified. The very weak spectrum shows unusual relative intensities which are interpreted by assuming a dipole moment change with ν₄ excitation of 0.07 D. The ground state dipole moment is then estimated to be 0.020(10) D.