12 Group Equilibrium and Transport in Plasmas Department of Applied Physics P. O. Box 513, 5600 MB Eindhoven The Netherlands Ro-vibrational excitation of hydrogen formed by association in a very dense expanding P. Vankan, D. C. Schram, S. B. S. Heil, and R. Engeln plasma p. j. w. vankan@tue. nl INTRODUCTIO N WALL ASSOCIATION DIAGNOSTICS H 2 potential energy diagram The motivation is the study of the formation of molecules in flowing highly activated plasmas. If a dense plasma with atomic hydrogen radicals expands from a dense plasma source into a low pressure background H 2 (r, v) molecules are formed. The experiment focuses on the measurement of rovibrationally excited H 2(r, v) molecules. These molecules are formed in association at surfaces under conditions of large radical fluxes. recirculation Probe ro-vibrational distribution in the electronic groundstate via X B transition widely tunable VUV photons necessary spectrally resolve transitions narrowband Association at surfaces (Å) Method: Stimulated Anti-Stokes Raman Scattering (SARS) in H 2, down tot 122 nm: • Large Raman shift of 4155. 23 cm-1 • Hydrogen is transparant to generated VUV photons H source spatially resolved, high sensitivity required also for O 2, NH 3, NO … Laser Induced Fluorescence (LIF) A VUV-LIF setup is used to study the H 2(r, v) density EXPERIMENTAL SETUP PLASMA 20 Hz rep. rate SHG of dye laser: 5 -10 m. J @ 230 nm H 2 Vacuum system due to O 2 absorption < 195 nm optical path under vacuum. p~10 -5 mbar RESULTS Part of the VUV-LIF spectrum of H 2(r, v) Plasma cascaded arc source (I=60 A, V=140 V) expanding H 2 plasma ( 3 slm) background pressure 10 -100 Pa CONCLUSION S non-thermal Strong Density of H 2(r, v) per statistical weight • H 2(r, v) rotation/ vibration excitation • Low levels v=0, J=0 -5 in thermal equilibrium • Additional population with high rotation/ vibration excitation • High r, v population in rough agreement with Goldberg-Waage dissociation-association balance • Mechanism: association at surface of H-atoms with H-atoms adsorbed at surface. • Plasma dissociates surface associates • Significant chemical potential! H 2 (r, v) + e H- + H , H 2 (r, v) + mol 1 + H ACKNOWLEDGEMENTS The authors greatly appreciate the skillful technical assistance of M. J. F. van de Sande, A. B. M. Hüsken, and H. M. M. de Jong.
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