INTRODUCTIO INTRODUCTIO N N WALL ASSOCIATION WALL ASSOCIATION DIAGNOSTICS DIAGNOSTICS RESULTS RESULTS CONCLUSIONS CONCLUSIONS ACKNOWLEDGEMENTS ACKNOWLEDGEMENTS Ro-vibrational excitation of hydrogen Ro-vibrational excitation of hydrogen formed by association in a very dense formed by association in a very dense expanding plasma expanding plasma Group Equilibrium and Transport in Plasmas Department of Applied Physics P.O. Box 513, 5600 MB Eindhoven The Netherlands P. Vankan , D.C. Schram, S.B.S. Heil, and R. Engeln FO M The authors greatly appreciate the skillful technical assistan of M.J.F. van de Sande, A.B.M. Hüsken, and H.M.M. de Jong. PLASMA PLASMA 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 H 2 , 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) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0 2 4 6 8 10 12 14 16 18 VUV photon B u + X g + P otentialenergy (eV ) Inte rn u cle ar distan ce (Å) H 2 potential energy diagram EXPERIMENTAL SETUP EXPERIMENTAL SETUP PM T PM T H 2 LN 2 VUV m ono L W BS M M plasma W S W M M Nd:YAG THG Dye (C440) BBO M M 220 nm to pum p to pum p ! )! , ( 2 v r H H H adsorbed plasma also for O 2, N 2, NH 3, NO … 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 ro-vibrationally excited H 2 (r,v) molecules. These molecules are formed in association at surfaces under conditions of large radical fluxes. Strong non-thermal • 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 mol 1 + H [email protected] 20 Hz rep.rate SHG of dye laser: 5-10 mJ @ 230 nm Vacuum system due to O 2 absorption < 195 nm optical path under vacuum. p~10 -5 mbar Plasma cascaded arc source (I=60 A, V=140V) expanding H 2 plasma ( 3 slm) background pressure 10-100 Pa III III II I Association at surfaces H source recirculation VUV-LIF setup is used to study the H 2 (r,v) density 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 10 8 10 10 10 12 10 14 10 16 10 18 10 20 10 22 10 8 10 10 10 12 10 14 10 16 10 18 10 20 10 22 B oltzm ann Guldberg-Waage v=6 v=4 v=2 v=1 v=0 n /g H (m -3 ) n /g H r,v 2 (m -3 ) E n e rg y (e V ) VU V-LIF CARS Density of H 2 (r,v) per statistical weight H 2 H 2 43650 43700 43750 43800 43850 43900 0 2 4 6 8 10 2 ,12 6,1 6,1 3,7 1,15 1 ,12 1 ,11 3,5 3,6 2 ,4 /2 ,1 2 0 ,19 F lu o re sce n ce (a .u .) S H fre q u e n cy (cm -1 ) rt of the VUV-LIF spectrum of H 2 (r,v)