Optical diagnostics of active species in N2 microwave flowing post-discharges

ELSEVIER

Surface and Coatings Technology 74 75 (1995) 474 478

Optical diagnostics of active species in N 2 microwave flowing post-discharges S. Bockel a, A.M. Diamy b, A. Ricard a a Laboratoire de Physique des Gazet des Plasmas, Bdtiment 212, CNRS, UniversitO Paris-Sud 91405 Orsay Cedex, France b Laboratoire de Chimie G~n~rale, Universitk Paris 6, 4 place Jussieu, 75 252 Paris Cedex, France

Abstract The nitrogen atom density is determined from NO titration and from the N2 first-positive-band intensity. The main kinetic reactions which produce the N2 first-positive-band emission in the afterglow are the following: N2(X, v) q- N2(A )

or

--+Nz(B, v') + Nz(X)

Nz(X, v) + N2(X, v)

in an early afterglow at times 10-3-10 2 s and N + N + Nz--+N2(B, v') + N 2 in the full afterglow period (10-3-10 1 s). From the Nz(B, v'-A, v") first-positive-band intensity, parts of the above reactions have been established along a 2.45 GHz, 120 W N 2 flowing post-discharge at 5-130 hPa gas pressure and at 0.2-1 standard l min 1 flow rate. It has been found that the NO titration method is only available in the late afterglow where the second reaction is dominant. By discriminating the second reaction from the first reaction, the first-positive-band intensity allows the N atom density determination in the post-discharge, along a larger domain than with NO titration including the end of the early afterglow. Furthermore, the first reactions contribute to produce highly excited levels up to Nz(B, v' = 17, 18) whose vibrational distribution is given in the present paper. These levels, whose Nz(B, v' > 13 15) states are autodissociative states are not created by the second reaction. Consequently, this is also a second method for specifying the afterglow parts where the above reactions are the dominant kinetic processes. Keywords: Nitriding post-discharge; N-atom density; Afterglow spectroscopy; Pink and late afterglow; Autodissociative vibra-

tional states

1. Introduction P r o d u c t i o n of nitrogen active species in flowing discharges is studied in connection with iron surface nitriding. In a review paper at the International Conference on Plasma Surface Engineering in 1992 [ 1 ], correlations were established between the density of nitrogen atoms measured in an Ar N 2 - H 2 microwave flowing postdischarge and the thickness and the chemical composition of the iron nitrided layers. Recently, a m a x i m u m in the microwave post-discharge reactivity has been found as the H2-to-N 2 density ratio is in the 10 -3 10 2 range [ 2 ] . The nitrogen a t o m density is usually determined from 0257-8972/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0257-8972(94)08205-7

the N O titration and from the N 2 first-positive-band intensity in the afterglow [ 1 ] . This diagnostics m e t h o d gives accurate results in the late afterglow ( 1 0 - 2 _ 10-1 s) as has been ascertained from the laser spectroscopy on N atoms [ 3 ] . At shorter times in the post-discharges (10 3 1 0 - 2 S ) , other active species such as N2(A) and N 2 ( X , v) metastable molecules react to produce emissions of an early afterglow and a pink afterglow [ 4 ] . It is the purpose of the present paper to analyse the N 2 first-positive-band emissions in the early and late afterglows of N 2 flowing discharges. The determination of active species density such as N atoms in the flowing post-discharge is then deduced. A discussion is given on the use of such optical

S. Bockel et al./Surface and Coatings Technology 74-75 (1995) 474-478

diagnostics process.

to

control

a

post-discharge

nitriding

2. The experimental set-up The experimental set-up of the microwave 2.45 GHz flowing discharge is reproduced in Fig. 1. A microwave generator (Raytek; adjustable power up to 2kW) is connected to a resonant cavity. The plasma is produced in a quartz tube (12mm outside diameter and 10mm inside diameter) crossing the microwave cavity. The gas pressure and the flow rate are measured by means of a baratron gauge and a mass flowmeter. Optical emission is analysed with a monochromator (Jobin-Yvon, HRS1; 1220 grooves mm -1) and a photomultiplier (Hamamatsu R928) with x - y recorder. The spectra are recorded along the z axial distance (cf. Fig. 1) from the discharge end 1 to the early afterglow 2, the pink afterglow 3 and the late afterglow 4. The early afterglow and the pink afterglow can be separated at low N2 gas pressure (6.6 hPa) for residence times of 10 -2 and 10-x s. They have been characterized by strong N2 first-positiveband and N~ first-negative-band emissions respectively [4]. The late afterglow is well characterized by the N 2 first-positive-band emission with a peak in intensity for the v' = 11 vibrational level [ 1].

3. Kinetic reactions in the afterglows The emission spectra in the discharge condition are the results of electron excitations as written for the N2(B, v') state, at the origin of the first-positive-band emission: e + N2--* e + N2(B, v')

(1)

In post-discharge conditions, the electron density strongly decreases by recombination with N f ions. The heating of electrons by the electric field disappears and only the superelastic collisions on N2 excited

~M.

/~

states remain to give some energy to electrons [5]. Consequently, collisions between neutral excited states are the main excitation processes in the post-discharge. The early afterglow is mainly produced by the following reaction: Nz(X, v > 5) + Nz(A)~ N2(B, v') + N2(X)

Nz(X, v

> 32) + N z ( X , v > 32)~Nz + e + N~-

N2(X, v > 11) + N~- ~ N 2 + N~-(B, v)

~N"

I

(3) (4)

with the reaction rates k3=3.5 x 10-16cm3s-1 and k4 = 5 x 10 11 c m 3 s - 1 at T = 600 K [7]. Finally, the late afterglow comes from the following recombination reaction: N + N + N2--*N2(B, v'= 11) + N2

(5)

with the three-body reaction rate ks= 10-33cm3s -1

[8]. By comparing reactions (2) and (3) in the early afterglow and in the pink afterglow, it can be deduced that the pink afterglow is produced as the N2(X, v) vibrational excitation strongly increases. Such a phenomenon can be explained by a drop in the Nz(X, v) quenching by neutrals (V-T process) as the gas temperature decreases. The gas temperature is effectively strong in the N2 microwave discharge (1800 K at 170 W; pressure 25-130 hPa) but decreases quickly to 600 K in the pink afterglow and to 400 K in the late afterglow [9].

4. Vibrational distributions of the Nz(B) state in the post-discharge The Nz(B,v') vibrational distributions have been determined in the early, pink and late afterglows from

~ Antenna I I

(2)

where the reaction rate k2=(3_+ 1.5) x 10-11cm3s -1 [6]. The pink afterglow is produced by the following Penning processes:

icrowave~ 45

475

z

optical fiber

( Pump exlmus 9

Fig. 1. The set-up for a microwave discharge in flowing N 2 gas: P.M., photomultiplier.

S. Bockel et al./Surjace and Coatings Technology 74 75 (1995) 474 478

476

0.1 0.1

~, 0.01

0.01 6

7

8

9

10

11

12

13

0.001

V'

6

Fig. 2. Normalized vibrational distribution of N2(B, v') for t,' = 7 12 in a 390 MHz, 100 W N2 flowing discharge at 6.6 hPa: curve (a), from Nz(X, o)+N2(A) reaction [6]; curve (b), pink afterglow; curve (c), late afterglow.

the first-positive-band intensities as follows:

hc

IN2 (first positive) = K(21) z- Ai[B, v'] Ai

(6)

where K(2/) is the spectral response of the optical system at the wavelength 2;, Ai is the radiative emission probability [10] and [B, v'] is the Nz(B, v') density. The normalized vibrational distributions rB,~,, = N2(B, v'

N2(B, v')

are reproduced in Fig. 2; curve (a) represents, following

10

12

14

16

18

20

yV

Fig. 3. N2(B, v'} vibrational distribution for v ' = 7 18 in an N2 2.45GHz, 510W flowing discharge at 4 h P a [12] (a.u. arbitrary units); curve (a), discharge end; curve {b), pink afterglow.

[-6], the early afterglow and curves (b) and (c) the pink afterglow and the late afterglow respectively of a 390 MHz, 100 W N2 flowing discharge at 6.6 hPa. It can be seen in Fig. 2 that the vibrational distribution is more expanded in the pink afterglow (curve (b)) than in the early afterglow (curve (a)). The Nz(B, v'= 11) peak is clearly enhanced in the late afterglow (curve (c)). The Nz(B, v'= 13) state is autodissociative, following the reaction N z ( X , tl' =

/t/=7

8

13)--* N2(A '5 Zg+)--* N + N

(7)

with a rate constant of 3.1 × 108s t [11].

i

0,1

0.01 8

9

10

11

12

VQ

f

l |

L

"-'~'~ Rv'N+N -RI

' - - O - " - R v ' p = R2

~0.2RI+0.SR2

-.--'(~-- 0.4R 1 "+0.6R2,

+

~

0.6Rl+0.4R2

0 . S R I +0.2R2

Fig. 4. ra, ~, normalized vibrational distributions of N2IB, v') for v ' = 7 12 calculated by adding the rB,t,'(N + N ) and r~.,,(pink) contributions as given in Fig. 2, curves (c) and (b) respectively.

S. Bockel et al./Surface and Coatings Technology 74-75 (1995) 474-478

477

1-1 -

1 I

0.1 t_

0.01 7

8

i

!

!

i

9

10

11

12

V'

"

(a)

--

Rv'N+N -RI

+

----

0.2RI'+'O.BR2

"--O"---0.4111+0.6112

R v ' p - R2

.K

0.6RI '~0.41R2

-"'O---

O.SR1 -~'0.2R.2

- - " P-~33hPa z ' = l S 0 m m Q~'0.5 ~ m n - I

'TJ -).

m"

o.1

L

0.01 7

8

I

t

I

I

9

10

11

12

V'

(b)

t

"

Rv'N+N -RI

"-"0-"-

R v ' p - IR2

--"

02R1+0.SR2

~

0.4RI+0.6"R2

A

0.6RI+O.4R.2

" - - ' [ ~ - - " 0.SRI +0.,2 R2

- - " P='-26.5 hiP= z ' ~ 7 f i m m Q~--0.2 . ~ m n - I

Fig. 5. rB,,,, n o r m a l i z e d v i b r a t i o n a l d i s t r i b u t i o n s o f N 2 ( B , v') for v' = 7 12 ( - - , I min

calculations; ....

, e x p e r i m e n t s ) (a) p = 53 h P a , Q = 0.5 s t a n d a r d

1, z = 150 m m ; (b) p = 26.5 h P a , Q = 0.2 s t a n d a r d 1 m i n - 1, z = 75 m m .

The autodissociative rate constants decrease for v' > 13, with a value of 5 x 10 6 s - 1 for N2(B, v' = 17-18) [ 11]. As a consequence the first-positive-band emission from these high vibrational levels can be observed, especially the Nz(B, v' = 17-A, v' = 12), 2 = 520.4 nm band and N2(B, v' = 18-A, v' = 13), 2 = 517.4 nm band. Such high vibrational level densities are reproduced in Fig. 3 for a high frequency 2.45 G H z N2 flowing discharge at 4 h P a , 510W: curve (a) represents the discharge end and curve (b) pink afterglow (spectra from [12]). The autodissociative mechanism is clearly

observed for N2(B, v ' = 13-15). It is also observed in Fig. 3 that the high Nz(B, v') levels are more excited in the early afterglow than in the discharge. This observation is also reported in [13]. In the late afterglow, the emission of N2(B, v' = 17-18) levels has not been detected. It is estimated that the N2(B, 18-A, 13)-to-N2(B, l l - A , 6) intensity ratio is lower than one order of magnitude, indicating that the density ratio of Nz(B, 18 ) to Nz(B, 11) is less than 2 x 10 - 2 . The emission from the Nz(B, 17-18) levels in the post-

478

S. Bockel et al./Surface and Coatings Technology 74 75 (1995) 474-478

discharge is thus a signature of the pink afterglow where the vibrational N2(X, v) population is very high. The absence of emission from the N=(B, 17-18) level is a second signature of the late afterglow (the first signature being the peak of N2(B, 11) density) where the N + N reassociation which cannot populate Nz(B, v'> 12) is the dominant process.

5. Determination of N atom density in the post-discharge

6. Concluding remarks The active species in a nitrogen flowing microwave post-discharge have been analysed by emission spectroscopy. In the late post-discharge (10 -2 10 -1 s), the afterglow is mainly produced by N + N atom recombination on the N/(B, v' < 12) vibrational states. At shorter times in the post-discharge (10-3-10 -2 s), the dominant kinetic reactions to produce the N~- (B, v') and Nz(B, v') states are collisions between N z ( X , v) "+"N z ( X , v) and/or N2(X, v)+ N2(A) as suggested by Polak and Slovetsky

[14]. The N atom density is usually determined in flowing post-discharge from the intensity of the Nz(B,v'= 11) level after calibration by NO titration [1]. It is at present considered the situation where the pink afterglow (Nz(X, v) vibrational population) and the late afterglow (N atom density) cannot be clearly separated. Then the ratio 12

rB,~, = Nz(B, v')

~

Nz(B, v')

v'='7

is the addiction of rBj(N + N) as given by Fig. 2, line (c), and rB,~,, (pink) of Fig. 2, line (b). The ra,~,' = a r B , v , ( N + N) + brB, v, (pink) normalized distribution in the v'= 7 12 range is reproduced in Fig. 4 vs. v' for a = 0, 0.2, 0.4, 0.6, 0.8 and 1. In Fig. 5, the calculated ra.v, distributions (full lines) are compared with the experimental values, obtained with the experimental set-up in Fig. 1; for Fig. 5(a) the parameters are p = 5 3 h P a , Q = 0 . 5 standard lmin -x and z = 150mm and, for Fig. 5(b), p = 26.5 hPa, Q = 0.2 standard 1 min and z = 75 mm. It is concluded in Fig. 5(a) that a = 0.7 and in Fig. 5(b), that a = 0.2. Then, the N atom density is determined at different times in the afterglow, including the pink afterglow at shorter times. By NO titration the N atom density has been measured in the same experimental conditions as for emission spectroscopy. The ratio of N atom absolute density obtained from the NO titration to the N atom relative density deduced from optical spectroscopy has a constant value as a/> 0.5, i.e. in the condition where the N + N recombination is the dominant production of N2(B,v'). At shorter times in the afterglow, the NO titration gives too high a N atom density, probably as a result of competing reactions.

A method is given to separate clearly the part of N2(B, v') excitation from N + N recombination and then to determine the N atom density by NO titration in the late afterglow and to follow the density variation in going from the pink afterglow to the late afterglow. Also relative densities of Nz(B, v' -- 15-18) high vibrational levels, resulting probably from collisions between N z ( X , v) levels, have been determined in pink afterglow conditions. The appearance or the absence of such highly excited Nz(B , v') levels is also a method to separate the N + N recombination process.

References [1] A. Ricard, Surf. Coat. Technol., 59 (1993) 67. [2] H. Malvos, H. Michel and A. Ricard, J. Phys. D, 27 {1994) 1328. [3] G. Baravian and A. Ricard, in J. Harry (ed.), Proc. l l t h Int. Syrup. on Plasma Chemistry, Loughborough, 1993, Vol. 4, 1993, 1511. [4] A. Ricard and A.R. de Souza, in G. Ecker (ed.), Proc. 21st Int. Cot!l~ on Phenomena in Ionized Gases, Bochum, 1993, Vol. 2, 1993, p. 21. [5] G. Colonna, C. Gorse, M. Capitelli, R. Winkler and J. Wilhelm, Chem. Phys. Lett., 213 (1993) 5. [6] L.G. Piper, J. Chem. Phys., 9I (1989) 864. [7] L.S. Polak, High Temp. (Engl. Transl.), 15 (1977) 13. [8] A. Ricard, J. Tetreault and J. Hubert, 3. Phys. B, 24 (1991) 1115. [9] J. Alandari, A.M. Diamy, L. Hochard, J.C. Legrand and A. Ricard, in J. Harry (ed.), Proc. l lth Int. Symp. on Plasma Chemistry, Loughborough, 1993, Vol. 4, 1993, p. 1344. [10] F.R. Gilmore, R.R. Laher and P.J. Espy, J. Chem. Phys. Ref. Data, 21 (1992) 1028. [11] H. Geisen, D. Neusch~ifer and C. Ottinger, J. Chem. Phys., 92 (1990) 104. [ 12] C. Normand-Chave, Thdse, Orsay, 1991. [13] P. Soupiot, Th~se, Lille, 1993. [14] L.S. Polak and D.I. Slovetsky, Proc. Int. Conf. on Phenomena in Ionized Gases, Berlin, 1977, 1977, pp. 51 56.