Une étude expérimentale des décharges de pure O2 DC : rôle des atomes O³P et des molécules O2 fondamentales (X³∑ -g) et états métastables (a1∆ g, b1∑ +g)

L'Oxygène est très présent dans les plasmas naturels et a été utilisé dans de nombreux processus industriels (gravure, cendrage, etc.). Même s'ils ont été étudiés depuis de nombreuses années, il est difficile de trouver dans la littérature des mesures expérimentales précises des plasmas d'oxygène. C'est pour cela que la compréhension et la modélisation de ces plasmas demeure complexe. Dans ce travail, nous avons réalisé des mesures expérimentales de la recombinaison cinétique de l'atome d'oxygène et des densités, avec une température aux murs constante, une pression de 0,3 à 10 Torr et un courant de décharge de 10 à 40 mA. La température du gaz a aussi été mesurée dans le plasma. Les densités des molécules métastables (états a,b) et leur cinétique ont aussi été étudiées dans des conditions similaires. Toutes ces expériences ont été réalisées avec une décharge DC dans un tube de borosilicate, pour faciliter la simulation. La spectroscopie VUV d'absorption, la spectroscopie d'émission, la méthode HR-TALIF et la méthode CRDS ont été utilisées comme techniques de diagnostics pour différents objectifs. Ce travail a pour but de répondre à certaines questions encore sans réponse sur les plasmas d'oxygène et de fournir une vue détaillée sur la précision des multiples diagnostics utilisés.Oxygen contributes significantly in natural plasmas and have been applied in several industries like etching, ashing etc. Even though oxygen plasma have been studied for long time, there is still a lack of some accurate experimental measurements. That is why understanding and modelling oxygen plasma system still remains difficult. In this work we did experimental studies of oxygen atom recombination kinetics and oxygen atomic densities with a constant wall temperature at 0.3-10 Torr pressure and 10-40 mA discharge direct current . The gas temperature was also measured in the plasma. The densities of metastable a, b state molecules and their kinetics has been studied in similar plasma conditions. All these experiments were done in a DC discharge contained in a borosilicate tube for the ease of modelling. VUV absorption spectroscopy, emission spectroscopy, HR-TALIF and CRDS were used as diagnostic techniques for different purposes. This work will answer some of the remaining questions and will provide a detailed view on the accuracy of the diagnostics for several measurements

Oxygen 3P atom and O- ion density and atomic temperature in O<SUB>2</SUB> DC discharge obtained by Cavity Ringdown spectroscopy

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Oxygen 3P atom density and temperature determined by Cavity Ringdown spectroscopy of the forbidden 1D2 <-3P2 transition

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Oxygen metastable molecule densities in inductively-coupled plasmas in pure O<SUB>2</SUB> measured by VUV absorption

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Metastable Molecules in O<SUB>2</SUB> Plasmas probed by High Resolution Fourier Transform Absorption Spectroscopy

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O<SUB>2</SUB> dissociation in plasmas and problems of the O<SUB>2</SUB> cross section set

International audienceDC glow discharges in pure O2 in a Pyrex tube were studied to determine dissociation rate constant over a wide range of E/N and thereby to probe O2 dissociation cross section close to threshold. Electric field, E, was found from probe measurements while the gas density, N, from the gas temperature derived from the O2(b1&#61523;g ) &#61614; O2(X3&#61523;g-) emission spectrum. O atom density (as well ratio O/N ratio) was measured by HR TALIF while O/N ratio was also determined by Ar actinometry. Time-resolved actinometry of partially-modulated discharges was used to probe the O loss rate. The O2 dissociation rate constant was determined as a function of E/N, and compared to calculations from different O2 cross section sets. This comparison allowed validation of a the self-consistent cross section set for O2

High&#8722;resolution VUV absorption diagnostics of discharges in oxygen

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New methods to determine density, temperature and thermal accommodation coefficient of oxygen atoms

International audienceOxygen atoms are a key reactive species in many plasma applications. Its absolute density is often determined by two-photon laser-induced fluorescence (TALIF) by comparing the signal amplitude with that obtained from a known density of Xe atoms, as proposed by Niemi et al.[1] However, this method has limited accuracy due to uncertainty in the relative excitation cross-sections and other detector calibration difficulties. Our recent measurements in a DC glow discharge yielded unphysically high values, motivating a search for a new method. Absorption spectroscopy is the most direct and unambiguous method for determining absolute densities. However, under most practical situations the O atoms resonance lines at 130nm are heavily saturated (OD of 100 or more) and cannot be used. We measured the vacuum ultraviolet absorption spectrum of O2 discharges at high (106) resolution using the Fourier-Transform Spectrometer[2] on the DESIRS beamline at Synchrotron Soleil, and succeeded in observing the forbidden 3P2&#61614;5S2 transition at 135.56nm. This transition is not saturated, allowing the O atom density to be determined with high accuracy and the TALIF method to be tested. Another key parameter in plasmas is the gas translational temperature, and the thermal accommodation coefficient at reactor walls. We have developed a method to determine the temperature of O atoms from the Doppler width of the TALIF transition, using a home-built narrow bandwidth (0.01cm-1 @ 226nm) pulsed laser system [3], and will use this to determine the temperature profiles in an O2 ICP, and hence the surface thermal accommodation coefficients. Work supported by LABEX Plas@par / ANR-11-IDEX-0004-02

Vacuum ultraviolet absorption spectroscopy of oxygen discharges

International audienceElectrical discharges in oxygen gas are widespread in nature and occur in many plasma applications, including surface treatment and plasma medicine. They are also an ideal archetype for the understanding of molecular plasmas, showing the effects of dissociation/surface recombination, electron attachment, high densities of molecular and atomic metastable states, vibrational excitation and gas heating. Nevertheless, many uncertainties remain about the cross-section and reaction sets, as well as the surface processes, all essential for reliable modelling. We used the unique DESIRS vacuum ultraviolet beamline at synchrotron SOLEIL to perform absorption spectroscopy in the 120-200nm range. Measurements were made in a DC positive column in pure O2. This provides a uniform plasma column with constant reduced field over a wide range of gas pressure and electron density (pressure 0.2-10 Torr, current 5-40 mA), ideal for model validation. The Fourier-Transform branch was used to record absorption spectra with a resolution of 106, giving complete high-resolution spectra of O2 in the X, a and b states. The absorption cross-sections of O2 X and a are known [1] (although the a state absolute cross-section was only measured once in 1973 and at low resolution), allowing absolute densities to be determined. We analysed the complex rotational structure of the (4p&#960;1&#931;u &#61612;b1&#61523;g ).band at 131.3nm , and derived the first absolute absorption cross-sections using a combination of ab-initio calculations and absolute emission spectrometry, thus allowing the first determination of the absolute density of this state. Using the monochromatic branch of the VUV beamline we performed time-resolved measurements of O2 X and a during full and partial modulation of the discharge current, probing their creation and loss kinetics in the gas phase and on the tube surface. The recovery of the O2 X density in the afterglow shows components with distinct time constants, allowing the O atom and O2 a absolute densities to be calibrated