Experimental Study and Modelling of Formation and Decay of Active Species in an Oxygen Discharge

A microwave (2.45 GHz) oxygen discharge (3 hPa, 150 W, 50 mL.min –1 ) is studied by optical emission spectroscopy of O ( 5 P) (line 777.4 nm) and of the atmospheric system of O 2 (head‐line 759.4 nm). Calibration of the spectral response of the optical setup is used to determine the concentrations of O ( 5 P) and O 2 (b). The concentration of the O ( 5 P) atoms is in the range 108–109 cm –3 and the concentration of the O 2 (b) molecules is in the range 10 14 – 2 × 10 14 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 O 2 – , fundamental electronic neutral species O ( 3 P), O 2 , O 2 (X,v), O 3 and excited neutral species O 2 (a), O 2 (b), O 2 (A), O ( 1 D), O ( 1 S), O ( 5 P), O (4d 5 D o ), O (5s 5 S o ), O (3d 5 D o ) and O (4s 5 S o )]. A fair agreement between experimental results and modelling is obtained with the following set of fitting values: – heterogeneous deactivation coefficient for O 2 (b) γ = 2.6 × 10 –2 ; – rate constant of reaction [ O ( 1 D) + O ( 3 P) → 2 O ( 3 P)] k 34 = 1.4 × 10 –11 cm 3 .s –1 ; – electron concentration in the range 10 10 – 10 11 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 [ O 2 ]initial) is about 33–50%. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)