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 (4pπ1Σu b1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