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Journal Articles Plasma Sources Science and Technology Year : 2016

High sensitivity ultra-broad-band absorption spectroscopy of inductively coupled chlorine plasma

Abstract

We propose a method to measure the densities of vibrationally excited Cl 2 (v) molecules in levels up to v = 3 in pure chlorine inductively coupled plasmas. The absorption continuum of Cl 2 in the 250 – 450 nm spectral range is deconvoluted into the individual components originating from the different vibrational levels of the ground state, using a set of ab-initio absorption cross sections. It is shown that gas heating at constant pressure is the major depletion mechanism of the Cl 2 feedstock in the plasma. In these line-integrated absorption measurements, the absorption by the hot (and therefore rarefied) Cl 2 gas in the reactor centre is masked by the cooler (and therefore denser) Cl 2 near the walls. These radial gradients in temperature and density make it difficult to assess the degree of vibrational excitation in the centre of the reactor. The observed line-averaged vibrational distributions, when analyzed taking into account the radial temperature gradient, suggest that vibrational and translational degrees of freedom in the plasma are close to local equilibrium. This can be explained by efficient VT relaxation between Cl 2 and Cl atoms. Besides the Cl 2 (v) absorption band, a weak continuum absorption is observed at shorter wavelengths, and is attributed to photodetachment of Cl ‒ negative ions. Thus, line-integrated densities of negative ions in chlorine plasmas can be directly measured using broad-band absorption spectroscopy.
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Dates and versions

hal-01346016 , version 1 (18-07-2016)

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Daniil Marinov, Mickaël Foucher, Ewen Campbell, Mark Brouard, Pascal Chabert, et al.. High sensitivity ultra-broad-band absorption spectroscopy of inductively coupled chlorine plasma. Plasma Sources Science and Technology, 2016, 25 (3), pp.035019 ⟨10.1088/0963-0252/25/3/035019⟩. ⟨hal-01346016⟩
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