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. / text

The aim of thesis was to establish a methodology for 210Pb measurements by Accelerator Mass Spectrometry (AMS). The potential application is to measure 210Pb in people who have been exposed to radon. This will better our understanding of radon toxicity, which is not possible now with current radiometric and mass spectrometry techniques. The determination of 210Pb by AMS was done in two major studies 1) Studying Pb chemistry in a Cs+ sputter source used in AMS and 2) Evaluating 204,205 & 208Pb spikes for the quantification of 210Pb by isotope dilution. Pb chemistry was investigated using an 834 SIMS-type and a SO-110 Cs+ sputter source at the IsoTrace Laboratory and A.E Lalonde AMS facility, respectively. Different molecular anions of Pb were studied with the 834 SIMS-type Cs+ sputter source and the strongest molecular anion current of Pb and thus greatest ionization efficiency was achieved form the superhalogen PbF3-. The average 208PbF3- current was unaffected by varying the ratio of the fluorinating compounds (AgF2 and CsF) packed into a target. The average current of 208PbF3- was reproducibly increased by chemically mixing the targets of AgF2, CsF and PbF2 in concentrated HF rather than mechanically mixing them the powders with a stir rod. The count rate of 210Pb reproducibly increased by a factor of 20 when μg quantities of PbF2 were present in mg AMS targets compared to AMS targets that had pg quantities of PbF2. The average current of 208PbF3- for pure PbF2 targets in an SO-100 Cs+ sputter source was reproducibly increased when the Cs+ flux was decreased by a factor of 10. This phase of my work maximized the overall efficiency of PbF3-, to a value of 1.8x10-10 ±8x10-11s-, which was a key first step in the measurement by AMS. Then isotope dilution was tested to quantify 210Pb and the next stage of my work evaluated the use of 204,205 & 208Pb spikes. 210Pb was measured in the +3 charge state by isotope dilution assays using 204,205 & 208Pb spikes. 204Pb+3 reproducibly suffered from the molecular interference from 68Zn3+3, which could not be easily removed without negatively impacting the detection limit for 210Pb. 205Pb+3 continually suffered from 205Tl+3 interference which could be readily be removed but not without negatively affecting the II detection limit for 210Pb. 208Pb+3 suffers from no molecular interferences but if a large amount of 208Pb is needed to swamp the Pb in a sample, this could limit the detection limit for 210Pb as the abundance sensitivity is 210Pb/208Pb=1.3×10-12. A calibration curve is required when 208Pb is used as a spike due to a difference in collection efficiency of a Faraday cup, where 208Pb+3 is detected and the gas ionization chamber, where 210Pb+3 is detected. The quantification of 210Pb with 208Pb as a spike yielded a detection limit of 4.4mBq at the IsoTrace facility. A theoretical detection limit of ≤0.11mBq is expected at the A.E Lalonde AMS facility. The expected detection limit at the A.E Lalonde AMS facility is on par with α-spectroscopy but AMS samples can be counted in less than 1 hour whereas alpha spectrometry samples must be counted for about 1 day.

Depuis leur découverte dans divers environnements astrophysiques, les anions polyyne CxH¯ (x = 2, 4, 6) et les anions cyanopolyyne CxN¯ (x = 1, 3, 5) ont reçu une attention considérable. Ces anions semblent jouer des rôles importants dans leur environnement. Cependant, les données à basse température sur les voies chimiques menant à leur formation et à leur destruction sont encore rares, en particulier ce qui concerne l'identité du produit et les ratios de ramification. Pour résoudre ce problème, nous nous sommes engagés dans la recherche de la réactivité de ces anions moléculaires en utilisant des instruments dédiés couplant des jets subsonique et supersonique avec des méthodes de spectrométrie de masse. De cette façon, nous avons étudié la réactivité des anions C3N avec le cyanoacétylène (HC3N) ainsi que la réactivité de CN¯, C3N¯, et C5N¯ avec l'acide formique (HCOOH) de 298 K à des températures aussi basses que 36 K. Nous rapportons dans ce travail le taux de vitesse, les produits, et les ratios de ramification de ces réactions. Ce travail aborde également la source prototype d'ions sélectionnée, qui a récemment été mis en place dans notre laboratoire afin d'étendre notre recherche à d'autres anions d'intérêt astrophysique (e.g. les anions CxH¯ et Cx¯). Une description de cet instrument ainsi que des résultats préliminaires sont présentés dans ce travail. Cette thèse, «Reactivity of Molecular Anions at Low Temperature: Implications for the Chemistry of the Interstellar medium and Titan’s atmosphere», a été réalisée au sein de l'Institut de physique de Rennes et de l'Ecole Nationale Supérieure de Chimie de Rennes. Mots-clés: astrochimie, atmosphère de Titan, anions moléculaires, cinétique en phase gazeuse, jet supersonique, spectrométrie de masse, source d’ions sélectionnée / Ever since their discovery in various astrophysical environments, polyyne anions CxH¯ (x = 2, 4, 6) and cyanopolyyne anions CxN¯ (x = 1, 3, 5) have received a considerable attention. These anions appear to be playing important roles in their environments. However, low temperature data on the chemical pathways leading to their formation and destruction is still scarce, especially regarding product identity and branching ratios. To address this issue, we have engaged in the investigation of the reactivity of these molecular anions by employing dedicated instruments coupling subsonic and supersonic flows with mass spectrometry methods. In this fashion, we have investigated the reactivity of C3N¯ anions with cyanoacetylene (HC3N) as well as the reactivity of CN¯, C3N¯, and C5N¯ with formic acid (HCOOH) from 298 K down to temperatures as low as 36 K. We report in this work the rate coefficient, the nature of the products, and the branching ratios of these reactions.This work also addresses the prototype selected ion source in our laboratory, which was recently implemented in order to extend our investigation to other anions of astrophysical interest (e.g. CxH¯ and Cx¯ anions). A description of this instrument as well as some preliminary results are presented in this work. This thesis, «Reactivity of Molecular Anions at Low Temperature: Implications for the Chemistry of the Interstellar medium and Titan’s atmosphere», was carried out at the Institut de Physique de Rennes and the Ecole Nationale Supérieure de Chimie de Rennes.Keywords: astrochemistry, Titan’s atmosphere, molecular anions, gas phase kinetics, supersonic flow, mass spectrometry, selected ion source