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Spectroscopic measurement of atomic hydrogen level populations and hydrogen dissociation degree in expanding cascaded arc plasmas

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Spectroscopic measurement of atomic hydrogen level populations and hydrogen dissociation degree in expanding cascaded arc plasmas Spectroscopic measurement of atomic hydrogen level populations and hydrogen dissociation degree In expanding cascaded arc plasmas D. K. Otorbaev, A. J. M. Buuron, N. T. Guerassimov, M. C. M. van de Sanden,a) and D. C. Schram Department of Physics, Eindhoven University of Technology, PO. Box 513, 5600 MB Eindhoven; The Netherlands (Received 20 April 1994; accepted for publication 30 June 1994) Optical absorption spectroscopy has been applied to measure the absolute population densities of the first excited levels of atomic hydrogen H*(n = 2) and argon Ar”(4s) in an expanding cascaded arc plasma in hydrogen-argon mixture. It is demonstrated that the method allows us to determine both H*(n =2) and Ar”(4sj absolute density radial profiles for H, admixtures in Ar ranging from 0.7% to 10% with good accuracy. The measured H*(n =2) densities are in the 1014-1016 mm3 range, -and Ar*(4s) densities are in the range of 1015-1018 rnw3. It has been shown, that the density of hydrogen excited atoms H*(n =2) serves as an indicator of the presence of argon ions and hydrogen molecules in the expanding plasma. A kinetic model is used to understand evolution of H*(n=2) density in the expansion, and to estimate the total atomic hydrogen population density and hydrogen dissociation degree in sub- and supersonic regions of the plasma. I. INTRODUCTION Hydrogen plasmas have important applications in many areas of modern science and technology, in particular in sur- face modification techniques, such as deposition, etching, implantation, and the desorption of boundary layers of solids and liquids. In connection with these topics, a special interest arises in the development and characterization of high inten- sity sources of reactive hydrogen atoms. For example, it has been shown that hydrogen atom beams are of principal im- portance in the variou

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