15min:

HARSHAL GUPTA, B. J. DROUIN, J. C. PEARSON, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 .

The rotational spectrum of the radical ion HCl⁺ has been detected at high resolution in the laboratory, supporting conclusively the identification of this ion with the Herschel Space Observatory's Heterodyne Instrument for the Far-Infrared (HIFI) in diffuse clouds toward the Galactic star-forming regions W31C and W49N. Three rotational transitions, one in the ground state ² _3/2 ladder and two in the ² _1/2 ladder (643~cm^-1 above ground), were observed in a microwave discharge of He and HCl. Well-resolved chlorine hyperfine structure and -doubling, and the detection of lines of H³⁷Cl⁺ at precisely the expected isotopic shift, provide conclusive evidence for the laboratory identification. The detection of rotational transitions in the ² _1/2 ladder of HCl⁺ for the first time allows an experimental determination of the individual hyperfine coupling constants of chlorine, and yields a precise value of eQq₂. The spectroscopic constants determined by fitting a Hamiltonian simultaneously to our data and more than 8000 optical transitions are so precise, that they allow calculation of the frequencies of ² _3/2~J=5/2-3/2 transition observed by HIFI to within 0.2~km~s^-1, and indeed, those of the strongest rotational transitions of HCl⁺ below 7.5~THz to better than 1~km~s^-1.