Experimental Study and Modelling of Formation and Decay of Active Species in an Oxygen Discharge
Résumé
A microwave (2.45 GHz) oxygen discharge (3 hPa, 150 W, 50 mL.min-1) is studied by optical emission spectroscopy of O(5P) (line 777.4 nm) and of the atmospheric system of O2(head-line 759.4 nm). Calibration of the spectral response of the optical setup is used to determine the concentrations of O(5P) and O2(b). The concentration of the O(5P) atoms is in the range 108-109 cm-3 and the concentration of the O2(b) molecules is in the range 1014 - 2 × 1014 cm-3 along the discharge tube. An attempt is made to simulate the experimental results by using coupling the Boltzmann equation, homogeneous energy transfer V-V and V-T, heterogeneous reactions on the walls (energy transfer and recombination of atoms) and a kinetic scheme (electronic transfer and chemical reactions). The Boltzmann equation includes momentum transfer, inelastic and superelastic processes and e-e collisions. V-V and V-T transfer equations are obtained from the SSH theory and the kinetic scheme includes 65 reactions with 17 species [electrons e, ions O- and O2-, fundamental electronic neutral species O(3P), O2, O2(X,v), O3 and excited neutral species O2(a), O2(b), O2(A), O(1D), O(1S), O(5P), O(4d 5Do), O(5s 5So), O(3d 5Do) and O(4s 5So)]. A fair agreement between experimental results and modelling is obtained with the following set of fitting values: - heterogeneous deactivation coefficient for O2(b) = 2.6 × 10-2; - rate constant of reaction [O(1D) + O(3P) 2 O(3P)] k34 = 1.4 × 10-11 cm3.s-1; - electron concentration in the range 1010 - 1011 cm-3. Modelling shows that the recombination coefficient for oxygen atoms on the silica wall (range 1.4 × 10-3 - 0.2 × 10-3) is of the same order as the values obtained in a previous paper and that the ratio ([O] / 2 [O2]initial) is about 33-50%. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)