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19 décembre 2017: 1 événement

  • Soutenances de Thèse/HDR

    Mardi 19 décembre 11:00-14:00 - Tim Stoltmann - Lame

    [PhD Defense] Tim Stoltmann, Laser Instrument for Triple Oxygen Spectrometry.

    Résumé : High-precision molecular absorption spectroscopy has become a widely used tool in physics and metrology. More recently, such techniques have gained some favor in the earth sciences and industrial monitoring, mostly for their compactness and robustness. The determination of isotopic ratios of different isotopic systems is nowadays possible with commercially available laser spectrometers. However, in particular for CO2, the full potential of such techniques for highest precision measurements has yet to be exploited. In this thesis, we present a new spectrometer based on optical feedback frequency locking of a distributed feedback laser (DFB) to a highly stable V-shaped reference cell. In such way, we obtain a near infra-red source reaching sub-kHz frequency resolution with a drift of 30 Hz/s. This ultra-narrow, ultra-stable laser source was then combined with a high-stability, high-finesse ring-down cell, using a robust dither lock scheme. We demonstrated a single-spectrum sensitivity of 1.2 x 10-12 cm-1, obtained in 30 seconds, and reported, for a narrow scan, a record-setting minimum detection level 3.8 x 10-14 cm-1, after less than 10 hours of measurement. We applied this instrument to the measurement of isotopic ratios in CO2 and demonstrated the feasibility of direct measurements of Δ170 in CO2. Δ17O is a super-ratio which requires precise measurements three isotopologues, offering information over the hydrological environment of the past, if measured from carbonate rocks. The instrument yielded a precision of 10 ppm in a record-setting measurement time of 10 minutes, demonstrating that laser spectrometers now perform on the same level as state-of-the-art isotopic ratio mass spectrometers currently used in geoscientific studies. We also demonstrated the first laser based measurements of the ratio 16O13C18O/13C16O2 ("clumped isotopes"), demonstrating a precision of 20 ppm with a strong potential to go further. The instrument shows the potential to measure all geoscientifically relevant isotopologue ratios in CO2 in one single measurement. Furthermore, we applied the instrument to Doppler-free saturated absorption spectroscopy. We determined transition frequencies of the 30012_00001 band of 13C16O2 in natural abundance with kHz accuracy by referencing the laser source to a GPS-referenced optical frequency comb (OFC). Using combination differences, we were able to re-determine the B, D and H constant of the upper and lower state, providing evidence for differences between our experimental data and literature. Moreover, we investigated the S (2) transition of D2. The zero-pressure transition frequency was determined with a record-setting precision of 32 kHz, meaning an accuracy of 0.17 ppb. The impact of line profile choices on the retrieval of line specific parameters has been investigated.
    The instrumentation which was built during this thesis fulfills two major tasks : First, we have proven the capability of measuring 17O in CO2 with outstanding precision in record time. Moreover, we demonstrated a successful proof of concept for clumped isotope measurements. While a thorough investigation of memory effects and external reproducibility has yet to be done, it shows the great potential of this technique for use in the geosciences. Secondly, the instrument is a valuable tool for spectroscopy, exhibiting extremely high sensitivity and thus allowing the very precise determination of line-shape parameters and the validation of the most advanced line profiles. Moreover, referencing to an OFC allows for precise and accurate determination of transition frequencies and pressure induced shifts.

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