1,721,215 research outputs found
Publisher's Note: Communication: Saturated CO<inf>2</inf> absorption near 1.6 μ m for kilohertz-accuracy transition frequencies [J. Chem. Phys. 142, 191103 (2015)]
Full text linkWater Vapor Self-continuum By Cavity Ring Down Spectroscopy In The 1.6 Micron Transparency Window
Full text linkSince its discovery one century ago, a deep and unresolved controversy remains on the nature of the water vapor continuum. Several interpretations are proposed: accumulated effect of the distant wings of many individual spectral lines, metastable or true bound water dimers, collision-induced absorption. The atmospheric science community has largely sidestepped this controversy, and has adopted a pragmatic approach: most radiative transfer codes used in climate modelling, numerical weather prediction and remote sensing use the MT\_CKD model which is a semi-empirical formulation of the continuum\footnote{Mlawer, E.J., V.H. Payne, J.L. Moncet, et al. (2012), Phil. Trans. R. Soc. A, 370, 2520–2556.}. The MT\_CKD cross-sections were tuned to available observations in the mid-infrared but in the absence of experimental constraints, the extrapolated near infrared (NIR) values are much more hazardous.
Due to the weakness of the broadband absorption signal to be measured, very few measurements of the water vapor continuum are available in the NIR windows especially for temperature conditions relevant for our atmosphere. This is in particular the case for the 1.6 m window where the very few available measurements show a large disagreement.
Here we present the first measurements of the water vapor self-continuum cross-sections in the 1.6 m window\footnote{Mondelain, D., A. Aradj, S. Kassi, et al. (2013), JQSRT, 130, 381–391.} by cavity ring down spectroscopy (CRDS). The pressure dependence of the absorption continuum was investigated during pressure cycles up to 12 Torr for selected wavenumber values. The continuum level is observed to deviate from the expected quadratic dependence with pressure. This deviation is interpreted as due to a significant contribution of water adsorbed on the super mirrors to the cavity loss rate. The pressure dependence is well reproduced by a second order polynomial. We interpret the linear and quadratic terms as the adsorbed water and vapour water contribution, respectively. The derived self-continuum cross sections, measured between 5875 and 6450 \wn, shows a minimum value around 6300 \wn.
These cross sections will be compared to the existing experimental data and models, especially to recent FTS measurements and to the last version of the MT\_CKD 2.5 model.Made available in DSpace on 2014-09-17T16:55:20Z (GMT). No. of bitstreams: 3
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Previous issue date: 2014-06-17Made available in DSpace on 2015-04-14T18:38:47Z (GMT). No. of bitstreams: 4
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Previous issue date: 2014-06-1
COLLISION INDUCED ABSORPTION OF THE a1∆g-X3Σ−g BAND OF OXYGEN NEAR 1.27 μM BY CAVITY RING DOWN SPECTROSCOPY
No full textCollision induced absorption (CIA) coefficients of the a-X(v=0-0) band of oxygen have been measured using cavity ring down spectroscopy (CRDS) technique at room temperature. More precisely, the B, B and B coefficients have been determined with a reduced uncertainty from series of low density spectra (from 0.36 to 0.85 amagat) of pure oxygen and N+O mixture with O=20.95\%. For that 12 distributed feed-back laser diodes were used below 7920 \wn together with an external cavity diode laser above this wavenumber. We particularly paid attention to the base line stability ( \wn) during the entire measurements. CIA was obtained from the difference between the absorbing samples spectra and argon spectra recorded for the same densities after removal of the local contribution of the absorption lines. The low densities at which the spectra were recorded were very useful to reliably remove this local contribution. The retrieved coefficients were compared to the CIA reported in HITRAN2016. A good overall agreement is found but differences between 5 and 8\% for B coefficients are observed below 7850 \wn
High performance optical spectroscopy for the study of isotopic CO2 anomalies in natural carbonates.
No full textL'étude de la composition isotopique des matériaux naturels ('traceurs isotopiques') est une des principales sources d'information pour l'étude des environnements et des grands cycles physico-chimiques dans la nature (ex : cycle du carbone, ou celui de l'eau). En géosciences, la plupart des mesures isotopiques se font par spectrométrie de masse et requièrent des niveaux de sensibilité́ et de précision extrêmes. En collaboration avec des géochimistes du Laboratoire des Sciences du Climat et de l'Environnement (LSCE, UMR 8212), le LIPhy développe depuis plusieurs années des instruments de mesure reposant sur la spectroscopie infra-rouge (« VCOF-CRDS »), dédies à l'analyse des isotopes et isotopologues dans diverses molécules (CO2, H2O, H2S ...). L'objectif de cette thèse est, dans un premier temps, d'optimiser la dernière génération de ces prototypes pour atteindre des mesures ultra-précises sur des petits échantillons de CO2, correspondant à des gains considérables en terme de rapidité et de simplicité de mesure pour plusieurs importants traceurs isotopiques (Δ17O, Δ16O13C18O), et permettant d'accéder pour la première fois à de nouveaux traceurs isotopiques (Δ18O12C18O) auparavant inaccessibles. Dans un second temps, le/la doctorant(e) mettra en œuvre cette technique, en collaboration étroite avec les géochimistes du LSCE, pour valider de nouvelles approches permettant de reconstruire les climats du passé (paléotempératures enregistrées dans les spéléothèmes) et de mieux comprendre certains mécanismes de biocalcification sur lesquels repose en partie la régulation naturelle du carbone atmosphérique. A plus long terme, les résultats issus de cette thèse contribueront à la montée en puissance d'une nouvelle génération de spectromètres laser ultra-précis, adaptée à l'étude de nombreuses autres molécules (H2O , CH4, N2O, H2S...), avec une très large gamme d'applications scientifiques en spectroscopie moléculaire comme en géosciences.The study of the isotopic composition of natural materials ('isotopic tracers') is one of the main sources of information for the study of environments and major physico-chemical cycles in nature (e. g. carbon or water cycles). In geosciences, most isotope measurements are made by mass spectrometry and require extreme levels of sensitivity and accuracy. In collaboration with geochemists from the Laboratoire des Sciences du Climat et de l'environnement (LSCE, UMR 8212), LIPhy has been developing for several years measurement instruments based on infrared spectroscopy ('VCOF-CRDS'), dedicated to the analysis of isotopes and isotopologues in various molecules (CO2, H2O, H2S, etc.). The objective of this thesis is, in a first step, to optimize the last generation of these prototypes to achieve ultra-precise measurements on small CO2 samples, corresponding to considerable gains in terms of measurement speed and simplicity for several important isotopic tracers (Δ17O, Δ16O13C18O), allowing access - for the first time - to new isotopic tracers (Δ18O12C18O) previously inaccessible. In a second step, the doctoral student will apply this technique, in close collaboration with LSCE geochemists, to validate new approaches to reconstruct past climates (paleo-temperatures recorded in speleothems) and to better understand certain bio-calcification mechanisms on which the natural regulation of atmospheric carbon is partly based. In the longer term, the results of this thesis will contribute to the development of a new generation of ultra-precise laser spectrometers, suitable for the study of many other molecules (H2O, CH4, N2O, H2S...), with a very wide range of scientific applications in molecular spectroscopy and geoscience
Collision induced absorption of the a1∆g-X3Σ−g band of oxygen near 1.27 μM by cavity ring down spectroscopy
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Previous issue date: 6Collision induced absorption (CIA) coefficients of the a1∆g-X3Σ − g(v=0-0) band of oxygen have been measured using cavity ring down spectroscopy (CRDS) technique at room temperature. More precisely, the BO2−O2, BO2−N2 and BO2−Air coefficients have been determined with a reduced uncertainty from series of low density spectra (from 0.36 to 0.85 amagat) of pure oxygen and N2+O2 mixture with O2=20.95%. For that 12 distributed feed-back laser diodes were used below 7920 cm−1 together with an external cavity diode laser above this wavenumber. We particularly paid attention to the base line stability (2×10−10 cm−1 ) during the entire measurements. CIA was obtained from the difference between the absorbing samples spectra and argon spectra recorded for the same densities after removal of the local contribution of the absorption lines. The low densities at which the spectra were recorded were very useful to reliably remove this local contribution. The retrieved coefficients were compared to the CIA reported in HITRAN2016. A good overall agreement is found but differences between 5 and 8% for BO2−Air coefficients are observed below 7850 cm−1
high resolution optical-feedback cavity ring-down spectroscopy at 8.45 µm
No full textLa spectroscopie d’absorption s’est imposée comme un outil de diagnostic d’une grande efficacité, couvrant un large éventail d’applications, des études physico-chimiques de l’atmosphère jusqu’à la physique fondamentale. Au cours des dernières décennies, le développement de techniques basées sur des lasers couplés à des cavités optiques résonantes a permis d’atteindre des niveaux records en termes de sensibilité d’absorption et de résolution fréquentielle, notamment dans le proche infrarouge. Cependant, les instruments performants dans le moyen-infrarouge restent encore trop rares bien que cette gamme spectrale présente un intérêt majeur pour des applications spectroscopiques. Cette limitation est en grande partie liée au manque de composants optiques adaptés et à leurs performances souvent réduites à ces longueurs d’onde, ce qui rend complexe une simple adaptation des techniques de spectroscopie en cavités existantes.Le travail présenté dans ce mémoire de thèse est en grande partie consacré au développement d’un spectromètre moyen-infrarouge répondant à ces objectifs de sensibilité et de résolution fréquentielle. Deux prototypes ont été développés, basés sur un laser à cascade quantique (QCL) émettant à 8.45 µm et une cavité optique linéaire de haute finesse. Ces instruments s’appuient sur une stratégie qui permet de réaliser simultanément des mesures par spectroscopie à temps de déclin (CRDS, pour Cavity Ring-Down Spectroscopy en anglais) et de stabiliser le laser grâce à la rétroaction optique (LOF, pour Linear Optical Feedback) issue de cette même cavité. Cette combinaison permet une injection optimale des photons dans la cavité.Le premier dispositif développé, appelé LOF-CRDS, permet l’acquisition de spectres sur une gamme de fréquences de l’ordre de la centaine de gigahertz, avec une résolution fréquentielle de 300 MHz. La très haute sensibilité d’absorption atteinte, de l’ordre de 4×10⁻¹⁰ cm⁻¹, a permis d’évaluer la contribution du self-continuum de la vapeur d’eau, une mesure d’un grand intérêt pour la physique atmosphérique, car essentielle pour modéliser le transfert radiatif terrestre.Un second dispositif que nous avons nommé LOFS-CRDS exploite les mêmes principes mais a été adapté à la détection de signatures d’absorption très étroites en fréquence (de l’ordre du MHz). Il intègre pour cela un contrôle actif de la longueur de la cavité résonante. Il nous a permis d’enregistrer des spectres avec une résolution spectrale de 20 kHz, soit une amélioration de quatre ordres de grandeur par rapport au système précédent, tout en améliorant la limite de détection à 2×10⁻¹⁰ cm⁻¹. Les performances et les limites de cette technique ont été évaluées à travers deux études: l’une sur les profils de raies de deux transitions de H₂O et l’autre sur un Lamb dip de l’eau, une signature sub-Doppler d’une largeur proche du MHz, associée à un mécanisme de saturation optique.Nos résultats ouvrent de nombreuses perspectives pour l’instrument LOFS-CRDS, notamment pour des applications de spectroscopie à très haute résolution et sensibilité telles que l’étude des transitions rovibrationnelles du C60 froid d’intérêt capital en astrophysique. Notre instrument, comme brique instrumentale, permettrait alors la mesure quantitative de sa concentration en jet supersonique, un préalable à l’étude de son spectre dans d’autres gammes de longueur d’onde. Mais l’instrument LOF-CRDS pourrait aussi s’intégrer dans des applications de terrain grâce à sa simplicité de développement et d’implémentation ou encore être au centre d’études plus fondamentales une fois son axe des fréquences doté d’une calibration absolue.Absorption spectroscopy has proven to be a very effective tool for a wide range of applications, from the physico-chemical study of the atmosphere to fundamental physics. In recent decades, advances in combining lasers with resonant optical cavities have led to unprecedented levels of absorption sensitivity and frequency resolution, especially in the near-infrared. However, similar instruments for the mid-infrared are still rare, even though this spectral range is highly valuable for spectroscopic applications. This is mainly because suitable optical components are lacking at these wavelengths and their performance is generally lower, making it difficult to directly adapt cavity-based spectroscopy techniques.The work presented in this thesis is primarily dedicated to the development of a spectrometer addressing these requirements for sensitivity and frequency resolution in the mid-infrared. Two prototypes have been developed, based on combining a quantum cascade laser (QCL) emitting at 8.45 µm and a high-finesse linear optical cavity. These instruments use a strategy that allows both cavity ring-down spectroscopy (CRDS) and laser stabilization through optical feedback from the same linear cavity (LOF), ensuring optimal photon injection.The first system, referred to as LOF-CRDS, enables the acquisition of spectra over a frequency range of approximately 100 GHz, with a frequency resolution of 300 MHz. The very high absorption sensitivity of about 4×10⁻¹⁰ cm⁻¹, made it possible to measure the water vapor self-continuum, which is critical in atmospheric physics for modeling Earth’s radiative transfer.A second system, named LOFS-CRDS, uses the same principles but is designed to detect ultra-narrow absorption features (sub-MHz). It achieves this by actively controlling the resonant cavity length. This system allowed the recording of spectra with a spectral resolution of 20 kHz, a four-order-of-magnitude improvement over the previous one, while reaching a limit of detection of 2×10⁻¹⁰ cm⁻¹. The technique’s performance and limitations were assessed through two case studies: one on the line profiles of two H₂O transitions, and another on a water Lamb dip, a sub-Doppler feature with a width near one megahertz, originating from an optical saturation effect.Our results open many possibilities for the LOFS-CRDS instrument, especially for high-resolution and high-sensitivity spectroscopic applications, such as studying the rovibrational transitions of cold C60, which is crucial in astrophysics. As a key component, our instrument could allow for the quantitative measurement of its concentration in a supersonic jet, a prerequisite for studying its spectrum in other wavelength ranges. Furthermore, the LOF-CRDS technique could be integrated into field instruments because it is easy to implement, or serve as the core of more fundamental studies once its frequency axis is absolutely calibrated
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
HYPERSONIC POST-SHOCK CAVITY RING-DOWN SPECTROSCOPY
Full text linkA highly sensitive experimental set-up ( = 10 cm) has been developed to produce high-temperature infrared spectra of methane in the Tetradecad polyad region (1.67 m) using cw-CRDS. A continuous flow of methane admixed to argon is initially heated at 1000 � 1500 K and then accelerated to hypersonic speeds in a vacuum chamber before being abruptly stopped by the impact on a planar screen set perpendicular to the flow axis, forming a stationary shock wave detached from the screen (bow shock). The CRD optical beam probes the very hot subsonic zone behind the shock where the gas temperature is close to the stagnation one. Computational Fluid Dynamics calculations have been performed to characterize the post-shock structure of the flow. Spectra reveal a series of new hot bands of fundamental interest for the modeling of highly excited levels of methane.Made available in DSpace on 2016-01-05T20:02:47Z (GMT). No. of bitstreams: 4
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Previous issue date: 2
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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