On the use of actinometry to measure the dissociation in O₂DC glow discharges: determination of the wall recombination probability

Abstract

A study has been performed to re-investigate the actinometric technique used to determine the absolute concentration of O atoms in DC O2 flowing glow discharges for pressures ranging from 0.36 to 2 Torr and discharge currents ranging from 5 to 80 mA in Pyrex tubes of three different diameters (16, 7 and 4 mm). Actinometric measurements using O(3P-3S) 844 nm, O(5P-5S) 777 nm and Ar(2p1-1S2) 750 nm transitions are compared to VUV absorption spectrometry. The choice of the excitation cross sections for the calculations of atomic excitation rates as a function of the reduced electric field using a Boltzmann code and the contribution of the quenching processes of the excited states are discussed. The dissociation ratio (O)/(O2) can be determined from the ratio of intensities I844/I750 by the relation (O)/(O2)=C3P2p1 I844/I750. We have found that C3P2p1 remains constant (C3P2p1=2.6*10-3) throughout the range of experimental conditions investigated. The recombination probability gamma of the O atoms at the wall is calculated and correlated to the wall temperature of the Pyrex tubes. The variation of the recombination probability as a function of the wall temperature is fitted by the relation gamma =0.94exp(-1780/Twall) for 300<Twall<500 K.