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Accueil > A propos du LPP > Communication > Actualités archivées > 2020 > Probing the reaction pathways in electrical discharges in oxygen : Time-resolved Cavity Ringdown Spectroscopy

Probing the reaction pathways in electrical discharges in oxygen : Time-resolved Cavity Ringdown Spectroscopy

Toutes les versions de cet article : [English] [français]

A. Volynets (à gauche) et J.-P. Booth (à droite) et le montage expérimental de spectroscopie à cavité optique résolue en tempsOxygen gas is a major component of many natural plasmas occurring in air, and is essential in many plasma applications, including industrial surface processing, CO2 recycling , and medical applications of plasmas. Electrical discharges convert oxygen molecules into a range of reactive intermediates that do the chemical work, including oxygen atoms, ozone, negative ions and metastable singlet O2. These unstable species, more reactive than the original oxygen molecules, are what enables the various plasma applications. Discharges in oxygen have been used since the 19th century for ozone production in water treatment. Surprisingly, existing models of oxygen discharges are still not mature, due to a lack of accurate data on the chemical processes occurring, both in the gas and at surfaces. Accurate, quantitative, time-resolved measurements of the number densities of the principal reactive intermediates are necessary to improve these models.
Many of these reactive intermediates can be detected, in principle, by their optical absorptions in the visible and near-infrared spectral regions, but these (forbidden) transitions are very weak. In order to make accurate quantitative measurements we must increase the absorption by making multiple passes through the plasma cell. To do this we use the cavity ringdown spectroscopy (CRDS ) technique, where a tuneable laser beam is injected into an optical cavity composed of high-reflectivity mirrors, increasing the absorption by a factor of around 10,000. Using a small tuneable diode laser at the visible wavelength of 630nm we are able to simultaneously measure the density of oxygen atoms, ozone and O- negative ions, as well as the gas temperature (from the Doppler effect). We have developed automated data acquisition to allow time-resolved measurements in pulsed discharges, showing the temporal evolution during both the plasma breakdown and the afterglow. Such measurements provide highly valuable information on the reactions taking place in the gas phase and on surfaces.

The team of Jean-Paul Booth, with help from Olivier Guaitella, are making measurements on a DC positive column discharge in pure oxygen. This simple discharge is ideal for experiment-model comparison, due to its excellent spatial uniformity and temporal stability. The experimental work was performed by Abhyuday Chatterjee (PhD 2018) and Andrey Volynets (Post-doc financed by Ecole Polytechnique). This project is at the centre of a consortium of several partners, including Synchrotron Soleil, the University of Oxford, Moscow State University (through LIA Kappa) and IST Lisbon. The powerful diagnostic techniques we have developed on the DC discharge are now being transferred to a capacitive radiofrequency plasma reactor, similar to those used for surface processing in the semiconductor industry, in collaboration with the Silicon Valley company Applied Materials Corporation.

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Exploitant du site : Laboratoire de Physique des Plasmas, Ecole Polytechnique route de Saclay F-91128 PALAISEAU CEDEX
Hébergeur : Laboratoire de Physique des Plasmas, Ecole Polytechnique route de Saclay F-91128 PALAISEAU CEDEX
Directeur de la publication : Anne Bourdon (Directrice)

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