INTRODUCTION

1. (Å) H2 H2 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 plasma P. Vankan, D.C. Schram, S.B.S. Heil, and R. Engeln p.j.w.vankan@tue.nl INTRODUCTION WALL ASSOCIATION DIAGNOSTICS H2 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 H2 (r,v) molecules are formed. The experiment focuses on the measurement of ro-vibrationally excited H2(r,v) molecules. These molecules are formed in association at surfaces under conditions of large radical fluxes. • Probe ro-vibrational • distribution in • the electronic groundstate • via X  B transition • widely tunable VUV photons necessary • spectrally resolve transitions •  narrowband • Method: • Stimulated Anti-Stokes Raman Scattering (SARS) in H2, down tot 122 nm: • Large Raman shift  of 4155.23 cm-1 • Hydrogen is transparant to generated VUV photons • spatially resolved, high sensitivity required •  Laser Induced Fluorescence (LIF) Association at surfaces recirculation H source also for O2, N2, NH3, NO … A VUV-LIF setup is used to study the H2(r,v) density EXPERIMENTAL SETUP PLASMA 20 Hz rep.rateSHG of dye laser: 5-10 mJ @ 230 nm Plasmacascaded arc source (I=60 A, V=140V)expanding H2 plasma ( 3 slm)background pressure 10-100 Pa Vacuum systemdue to O2 absorption < 195 nm optical path under vacuum. p~10-5 mbar RESULTS CONCLUSIONS • Strong non-thermal • H2(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! • H2 (r,v) + e  H- + H ,H2 (r,v) + mol  mol1 + H Part of the VUV-LIF spectrum of H2(r,v) Density of H2(r,v) per statistical weight 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.