Negative molecular ions in the laboratory and in space

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Gupta, Harshal Dilip

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This dissertation describes the theoretical, laboratory, and astronomical spectroscopy of negative molecular ions (anions), starting with the laboratory detection of the large carbon chain anion C₆H⁻ in the radio band and its identification in the molecular envelope of the carbon star IRC+10216 and in the cold dark molecular cloud TMC-1. In IRC+10216 the identification solved the long standing problem of the unidentified series of lines with rotational constant 1377 MHz first observed by K. Kawaguchi et al. Rotational spectra of the structurally similar anions---CCH⁻, C₄H⁻, C₈H⁻, CN⁻, and C₃N⁻---have been detected in the laboratory, and three more anions---C₄H⁻, C₈H⁻, and C₃N⁻---have now been identified in space. Molecular structure calculations using the CCSD(T) method and large basis sets predicted accurately the rotational constants (B₀) and centrifugal distortion constants (D[subscript J]) of all six anions and their isotopomers, guiding laboratory searches for these species. Reported here are the radio spectra of C₄H⁻, C₆H⁻, C₈H⁻, and C₃N⁻, measured to within 0.1 ppm in the centimeter-wave band by Fourier transform microwave spectroscopy of supersonic molecular beams and in the millimeter-wave band by absorption spectroscopy of low-pressure DC discharges. The spectroscopic constants derived from these measurements are so accurate, that the rotational frequencies of the anions can be calculated to within 1 km s⁻¹, adequate for radio searches in essentially all astronomical molecular sources. Radio astronomical observations with the 100 m Green Bank Telescope (GBT) toward TMC-1 yielded detection of C₈H⁻, as well as an improved estimate of the column density of C₆H⁻. The two anions are surprisingly abundant relative to their neutral radicals: a C₆H⁻/C₆H ratio of 1.6% and a C₈H⁻/C₈H ratio of 5% was derived. Upper limits were obtained for C₄H⁻/C₄H (< 0.004%) and C₃N⁻/C₃N (< 0.8%). A survey of C₆H⁻ and the related radicals C₄H and C₆H was done with the GBT toward 24 galactic molecular sources. The C₆H⁻ ion was newly detected in two dark clouds: L1544 and L1521F; C₄H was detected in nearly all dark clouds surveyed (in six for the first time); and C₆H was newly detected in five dark clouds and a translucent cloud. The observed C₆H⁻/C₆H (1%-4%) and C₆H/C₄H (0.2%-1%) ratios suggest that C₆H⁻ may be close to detection in many other dark clouds.



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