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Vibration-to-Electronic energy transfers in the Nitrogen Afterglow

Vibration-to-Electronic energy transfers in the Nitrogen Afterglow. Vasco Guerra Centro de Física de Plasmas, Instituto Superior Técnico, 1049-001 Lisboa, Portugal email contact: vguerra@ist.utl.pt. The pink afterglow:. The nitrogen afterglow:. Supiot et al: J. Phys. D: Appl. Phys .

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Vibration-to-Electronic energy transfers in the Nitrogen Afterglow

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  1. Vibration-to-Electronic energy transfers in the Nitrogen Afterglow Vasco Guerra Centro de Física de Plasmas, Instituto Superior Técnico, 1049-001 Lisboa, Portugal email contact: vguerra@ist.utl.pt

  2. The pink afterglow:

  3. The nitrogen afterglow: Supiot et al: J. Phys. D: Appl. Phys. 28 (1995) 1826 31 (1998) 2521 32 (1999) 1887

  4. The nitrogen afterglow : Enhancement of the emissions after a dark zone: • First positive system N2 (BA) N2 (B) • First negative system N2+ (B X) N2+(B) Is this behavior found in other species? • Yes! [Sadeghi et al 2001-2005] • ne • N2(A) • N(2P) • N2(a) • N2+(X) • N2(C)

  5. Why???

  6. First clues to the solution of the puzzle: (1) N2(X,v12) + N2+(X) N2(X) + N2+(B) (2) N2(A) + N2(a’) N2(X) + N2+(X) + e (3) N2(a’) + N2(a’) N2(X) + N2+(X) + e (4) N2(A) + N2(X, 5v14) N2(X) + N2(B) (2) and (3) also produce electrons ne  N2(A) and N2(a’) have to be produced in the afterglow

  7. First clues to the solution of the puzzle (continued): N2+(X), N2(a), N(2P) and N2(C) (5) N2(a’) + N2 N2 + N2(a) (6) N2(A) + N(4S) N2(X, 6v9) + N(2P) (7) N2(A) + N2(A) N2(X) + N2(C) (8) N2(A) + N2(X, v20) N2(X) + N2(C)  N2(A) and N2(a’) have to be produced in the afterglow

  8. Second hint: the V-V up-pumping N2(X,v30) are not populated in the discharge… but can be strongly populated in the afterglow!  N2(X,v30) are involved in the formation of the pink afterglow

  9. N2(X,v) + N2(X,w)  N2(X,v-1) + N2(X,w+1)  Second hint: the V-V up-pumping (cont.) The V-V up-pumping mechanism:

  10. The pink afterglow: production of N2(A) & N2(a’) Local production of N2(A) and N2(a’): • N2(X,v39) + N(4S)  N2(A) + N(2D) • N2(X,v38) + N(4S)  N2(a’) + N(4S) and/or • N2(X,v25) + e  N2(A) + e • N2(X,v38) + e  N2(a’) + e

  11. Self-consistent modeling: • Input: • collisional data • discharge operating parameters (p, R, I or ne, w) • Wall temperature • Output: • Electron energy distribution function (EEDF) • Vibration distribution function (VDF) • Concentration of N2(A, B, C, a, a’, w, a’’), N(4S, 2D, 2P), N2+(X, B) and N4+ • Gas temperature • Wave number and attenuation coefficient • Physical insight!

  12. The kinetic model: discharge • Electron kinetics (Boltzmann equation) • Vibrational kinetics of N2(X,v=0,...,45) • Chemical kinetics: N2(A, B, C, a’, a, w, a’’); N(4S, 2D, 2P) • Ion kinetics: N2+, N4+ • Input: w/2p=433 MHz; p=3.3 Torr; R=1.9 cm; ne~31010 cm-3 [Blois et al, J. Phys. D: Appl. Phys. (1998) 32 1887] [Sadeghi et al, J. Phys. D: Appl. Phys. (2001) 38 1779] [Mazouffre et al, Plasma Sources Sci. Technol. (2001) 10 168]

  13. The kinetic model: post-discharge Heavy-particle kinetics: • Relaxation of the set of coupled kinetic master equations for N2(X,v); N2(A, B, C, a’, a, w, a’’); N(4S, 2D, 2P); N2+ and N4+ [Guerra et al, Eur. Phys. J. Appl. Phys. (2004) 28 125] • N2(X,v39) + N(4S)  N2(A) + N(2D) • N2(X,v38) + N(4S)  N2(a’) + N(4S) Electron kinetics: • Time-dependent electron Boltzmann equation [Guerra et al, Phys. Rev. E (2001) 63 046404-1] N2(A) + N2(a’) N2(X) + N2++ e N2(a’) + N2(a’) N2(X) + N2++ e

  14. Results: EEDF The EEDF is quickly depleted in the first instants… … but attains a quasi-stationary state at t ~ 10-6 s.

  15. Results: EEDF Measurements from [Dias and Popov, Vacuum (2002) 69 159]

  16. Results: electron processes in the afterglow (cont.) De Benedictis et al: Chem. Phys. (1995) 192 149 J. Chem. Phys. (1999) 110 2947

  17. Results: the V-V pumping-up effect (again) oSupiot et al J. Phys. D: Appl. Phys. 32 (1999) 1887  Macko et al J. Phys. D: Appl. Phys. 34 (2001) 1807

  18. Results: the V-V pumping-up effect (cont.)

  19. The pink afterglow: population of N(4S) atoms Measurements from [Mazouffre et al, Plasma Sources Sci. Technol.10 (2001) 168] N recombination cannot explain the pink afterglow! [Loureiro et al, J. Phys. D: Appl. Phys.39 (2006) 122]

  20. Results: population of N2(A) Measurements from [Sadeghi et al, J. Phys. D: Appl. Phys.34 (2001) 1779]

  21. Results: population of N2(B) Measurements from [Sadeghi et al, J. Phys. D: Appl. Phys.34 (2001) 1779]

  22. Results: population of N2+(B) Measurements from [Blois et al, J. Phys. D: Appl. Phys.28 (1998) 2521]

  23. Results: electron density Measurements from [Sadeghi et al, J. Phys. D: Appl. Phys.34 (2001) 1779] (o) and [Guerra et al, IEEE Trans. Plasma Sci. 31 (2003) 542] ()

  24. Results: population of N(2P) Measurements from [Eslami et al, ESCAMPIG 2004, Constanţa, Romania]

  25. Reminder: The population of N(2P) atoms is strongly coupled with the kinetics of N(4S) and N2(A), via vibrationally excited N2(X,v) N2(A) +N(4S) N2(X, 6v9) + N(2P) N2(X, v10) + N(2P)  N2(A) + N(4S) • We have a good description of the elementary processes ruling the atomic, vibrational, and triplet kinetics!

  26. Tests to N2(X,v≥38) + N(4S) → N2(a’) + N(4S) Results: population of N2(a) Measurements from [Eslami et al, ESCAMPIG 2004, Constanţa, Romania]

  27. Results: kinetics of N2(a) and ionization But N2(a’), strongly coupled to N2(a), is very important for ionization… N2(a’) + N2 N2 + N2(a) N2(A) + N2(a’) N2(X) + N2++ e N2(a’) + N2(a’) N2(X) + N2++ e

  28. Results: kinetics of N2(a) and ionization The kinetics of N2(a) appears to be well described: • Overestimation of creation? X • Underestimation of destruction? X Ionization sources in the post-discharge are missing! N2(X,v>32) + N2(X,v>32)  N4++ e ???

  29. Conclusions: • The VDF strongly determines the shape of the EEDF, which is quickly depleted in the first instants of the afterglow • Direct excitation by electron impact is not effective • Low-threshold electron excitation processes may occur! • N2(A) and N2(a’) are created locally in the afterglow, in V-E mechanisms involving highly vibrationally excited N2 molecules. • These V-E processes can be mediated both by N atoms and • electrons.

  30. N2(X,v39) + N(4S)  N2(A) + N(2D) • N2(X,v38) + N(4S)  N2(a’) + N(4S) N2(A) + N2(X, 5v14) N2(X) + N2(B) N2(A) + N2(a’) N2(X) + N2++ e N2(a’) + N2(a’) N2(X) + N2++ e N2(X,v12) + N2+(X) N2(X) + N2+(B) N2(a’) + N2 N2 + N2(a) N2(A) + N(4S) N2(X, 6v9) + N(2P) N2(A) + N2(A) N2(X) + N2(C) N2(A) + N2(X, v20) N2(X) + N2(C) Conclusions (continued): • Probable mechanism:

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