Vibration-to-Electronic Vibration-to-Electronic energy transfers in the energy transfers in the

Nitrogen AfterglowNitrogen 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 pink afterglow:

The nitrogen afterglow:The nitrogen afterglow:

Discharge

Dark zone

Short Lived Afterglow

Microwave cavity (433 MHz)

N2

Emission Intensity

Supiot et al:

J. Phys. D: Appl. Phys.

28 (1995) 1826

31 (1998) 2521

32 (1999) 1887

The nitrogen afterglow :The nitrogen afterglow :

Enhancement of the emissions after a dark zone:

• First positive system N2 (BA) N2 (B)

• First negative system N2+ (B X) N2

+ (B)

Is this behavior found in other species?

Yes!Yes! [Sadeghi et al 2001-

2005]• ne

• N2(A)

• N(2P)• N2(a)

• N2+(X) • N2(C)

Why???

First clues to the solution of the First clues to the solution of the puzzle:puzzle:

(1) N2(X,v12) + 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, 5v14) N2(X) + N2(B)

(2) and (3) also produce electrons ne

N2(A) and N2(a’) have to be produced in the

afterglow

First clues to the solution of the puzzle First clues to the solution of the puzzle (continued):(continued):

(5) N2(a’) + N2 N2 + N2(a)

(6) N2(A) + N (4S) N2(X, 6v9) + N(2P)

N2+(X), N2(a), N(2P) and N2(C)

(7) N2(A) + N2(A) N2(X) + N2(C)

(8) N2(A) + N2(X, v20) N2(X) + N2(C)

N2(A) and N2(a’) have to be produced in the

afterglow

Second hint: the V-V up-pumpingSecond hint: the V-V up-pumping

N2(X,v30) are not populated in the discharge…

but can be strongly populated in the afterglow!

N2(X,v30) are involved in the formation of the pink afterglow

Second hint: the V-V up-pumping (cont.)Second hint: the V-V up-pumping (cont.)

The V-V up-pumping mechanism:

N2(X,v) + N2(X,w) N2(X,v-1) + N2(X,w+1)

N (4 S) + N (4 S)

POTENTIAL ENERGY

INTERNUCLEAR DISTANCE

N 2 ( X 1 Σ + )g

W +1W

V

V - 1

The pink afterglow: production of NThe pink afterglow: production of N22(A) & (A) & NN22(a’)(a’)

N2(X,v39) + N(4S) N2(A) + N(2D)

N2(X,v38) + N(4S) N2(a’) + N(4S)

N2(X,v25) + e N2(A) + e

N2(X,v38) + e N2(a’) + e

Local production of N2(A) and N2(a’):

and/or

Self-consistent modeling:Self-consistent modeling:

Input:

• collisional data

• discharge operating parameters (p, R, I or

ne, )

• Wall temperatureOutput:

• 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!

The kinetic model: dischargeThe 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: /2=433 MHz; p=3.3 Torr; R=1.9 cm; ne~31010 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]

The kinetic model: post-dischargeThe 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,v39) + N(4S) N2(A) + N(2D)

N2(X,v38) + 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

Results: EEDFResults: EEDF

The EEDF is quickly depleted in the first instants…

… but attains a quasi-stationary state at t ~ 10-6 s.

-10-9

-8-7

-6-5

-4-3

-2 108

64

2

10-4

10-3

10-2

10-1

100

f(u,t) (eV

-3/2

)

u (eV)

log10(t)

Results: EEDFResults: EEDF

Measurements from [Dias and Popov, Vacuum (2002) 69 159]

0 1 2 3 4 5 6 710-5

10-4

10-3

10-2

10-1

100

t = 6.5x10-3 s

f(u) (eV

-3/2

)

u (eV)

Results: electron processes in the Results: electron processes in the afterglow (cont.)afterglow (cont.)

De Benedictis et al: Chem. Phys. (1995) 192 149 J. Chem. Phys. (1999) 110 2947

10-9 10-8 10-7 10-6 10-5 10-4 10-310-13

10-12

10-11

10-10

10-9

10-8

10-7

Y=C 3Πu

=Y B 3Πg

( )Afterglow time s

CAY (cm

3s-1)

Results: the V-V pumping-up effect (again)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

0 10 20 30 4010-6

10-5

10-4

10-3

10-2

10-1

100

t = 0 s

t = 10-4 s

t = 10-3 s

t = 10-2 s

t = 10-1 s

[N2(X,v)]/[N

2]

Vibrational quantum number v0 10 20 30 40

10-6

10-5

10-4

10-3

10-2

10-1

100

t = 0 s

t = 10-4 s

t = 10-3 s

t = 10-2 s

t = 10-1 s

[N2(X,v)]/[N

2]

Vibrational quantum number v

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

10-5 10-4 10-3 10-2 10-1 10010-1

100

101

102

103

104

105

106

v=30

v=20

v=35

v=45

v=40

v=10

[N(X,v)]/[N(X,v)]

t=0

Afterglow time (s)

10-6 10-5 10-4 10-3 10-2 10-1 10010-3

10-2

[N]/N

Afterglow time (s)

The pink afterglow: population of N(The pink afterglow: population of N(44SS) ) atomsatoms

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]

Results: population of NResults: population of N22((AA))

Measurements from [Sadeghi et al, J. Phys. D: Appl. Phys. 34 (2001) 1779]

Results: population of NResults: population of N22((BB))

Measurements from [Sadeghi et al, J. Phys. D: Appl. Phys. 34 (2001) 1779]

Results: population of NResults: population of N22++((BB))

Measurements from [Blois et al, J. Phys. D: Appl. Phys. 28 (1998) 2521]

Results: electron densityResults: 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] ()

Results: population of N(Results: population of N(22PP))

Measurements from [Eslami et al, ESCAMPIG 2004, Constanţa, Romania]

Reminder:Reminder:

N2(A) + N(4S) N2(X, 6v9) + N(2P)

N2(X, v10) + N(2P) N2(A) + N(4S)

The population of N(2P) atoms is strongly coupled with the

kinetics of N(4S) and N2(A), via vibrationally excited N2(X,v)

We have a good description of the elementary processes

ruling the atomic, vibrational, and triplet kinetics!

Results: population of NResults: population of N22(a)(a)

Measurements from [Eslami et al, ESCAMPIG 2004, Constanţa, Romania]

Tests to N2(X,v≥38) + N(4S) → N2(a’) + N(4S)

Results: kinetics of NResults: kinetics of N22(a) and ionization(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

Results: kinetics of NResults: kinetics of N22(a) and ionization(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 ???

Conclusions:Conclusions:

• These V-E processes can be mediated both by N atoms and

electrons.

• N2(A) and N2(a’) are created locally in

the afterglow, in V-E mechanisms involving

highly vibrationally excited N2 molecules.

• 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!

Conclusions (continued):Conclusions (continued):

• Probable mechanism:

N2(X,v39) + N(4S) N2(A) + N(2D)

N2(X,v38) + N(4S) N2(a’) + N(4S)

N2(A) + N2(X, 5v14) N2(X) +

N2(B)N2(A) + N2(a’) N2(X) + N2+ + e

N2(a’) + N2(a’) N2(X) + N2+ + e

N2(X,v12) + N2+(X) N2(X) + N2

+(B)

N2(a’) + N2 N2 + N2(a)

N2(A) + N (4S) N2(X, 6v9) + N(2P)

N2(A) + N2(A) N2(X) + N2(C)

N2(A) + N2(X, v20) N2(X) + N2(C)