VibrationtoElectronic energy transfers in the Nitrogen Afterglow

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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_at_ist.utl.pt

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

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Why???
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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
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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
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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
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Second hint the V-V up-pumping (cont.)
The V-V up-pumping mechanism
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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
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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!

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The kinetic model discharge

• Electron kinetics (Boltzmann equation)
• Vibrational kinetics of N2(X,v0,...,45)
• Chemical kinetics N2(A, B, C, a, a, w, a)
  N(4S, 2D, 2P)
• Ion kinetics N2, N4
• Input w/2p433 MHz p3.3 Torr R1.9 cm
  ne3?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

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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
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Results EEDF
The EEDF is quickly depleted in the first
instants but attains a quasi-stationary state
at t 10-6 s.
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Results EEDF
Measurements from Dias and Popov, Vacuum (2002)
69 159
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Results electron processes in the afterglow
(cont.)
De Benedictis et al Chem. Phys. (1995) 192
149 J. Chem. Phys. (1999) 110 2947
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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
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Results the V-V pumping-up effect (cont.)

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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
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Results population of N2(A)
Measurements from Sadeghi et al, J. Phys. D
Appl. Phys. 34 (2001) 1779
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Results population of N2(B)
Measurements from Sadeghi et al, J. Phys. D
Appl. Phys. 34 (2001) 1779
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Results population of N2(B)
Measurements from Blois et al, J. Phys. D Appl.
Phys. 28 (1998) 2521
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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 (?)
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(No Transcript)
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Results population of N(2P)
Measurements from Eslami et al, ESCAMPIG 2004,
Constanta, Romania
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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!

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Results population of N2(a)
Measurements from Eslami et al, ESCAMPIG 2004,
Constanta, Romania
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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
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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,vgt32) N2(X,vgt32) ? N4 e ???
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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.

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Conclusions (continued)

• Probable mechanism