Simultaneous estimation of N and O atom density in N2 – O2 discharges
P. Vašina, V. Kudrle, A. Tálský
M. Mrázková, P. Botoš
Department of Physical Electronics, Masaryk University, Brno,
 The Czech Republic

Titrating NO into N2 afterglow containing nitrogen atoms,
these reaction take part :
quick    N + NO à N2 + O                                                       [1]
                                                k1 = 1.6 10-10 cm3s-1
                         A.N. Wright, C.A. Nelson : Active nitrogen Acad. Press, 1968
slow     N + O + N2  à NO* + N2  (NO beta system)               [2]
                            k2 = 9.1 10-33 cm6s-1 à 9.1 10-16 cm3s-1 (400 Pa)
                         C.B. Kretschmer, H.L. Peterson : J. Chem. Phys., 1963
slow     NO + O à NO2* (green continuum)                           [3]
                                                k3 = 6.4 10-17 cm3s-1
                         A. Fontijn, C.B. Meyer, H.I. Schiff : J. Chem. Phys., 1964




I = -k [NO]02 +k[NO]0[N]0
INO = k [N][O]








Experiment was done at following conditions :
                        N2 flow = 200 sccm
                                   corresponding pressure = 400 Pa
                                   power output = 50 W










Conclusion 1
N atom concentration is correctly estimated by NO titration if
-dark point corresponds to simultaneous zero intensity of NO* and NO2*
                                                                                   or
-NO* has  parabolic shape
                                                        or
-NO2* is zero until a N extinction point and then increases linearly
Parabolic fit of NO* enable us
-to determine dark point better than somewhere between the last positive
  value and the first zero value of NO*
Linear fit of NO2* enable us
-to determine dark point better than somewhere between the last zero
  value and the first positive value of NO2*

Density of N and O atoms in N2 and O2 gas mixtures
 determined by NO titration
There are two ways how to determine density of N and O atoms in
 N2 – O2 discharges
A. Ricard, V. Monna, M. Mozetic : Production of O atoms in Ar-O2 and N2-O2
                               microwave flowing post-discharges
                 Surface and Coating Technology, 174, (2003), 905-908
A.Ricard, V Monna : Reactive molecular plasmas
                 Plasma Sources Sci. Technol. 11, (2002), A150-A153
- from NO2*
- from NO*
P. Vašina, V. Kudrle, A. Tálský, P. Botoš, M. Mrázková, M. Meško
 Plasma Sources Sci. Technol. 13(4), (2004) 668-674

Determining of N and O atom density from NO2* - A. Ricard’s method
1.Starting with a N2 (or Ar – N2) post-discharge, N atoms density [N]1 is
       determined from the extinction point between NO* and NO2*.
2.  After the extinction point, the NO2* intensity is linearly increasing with NO with
      a slope r1
3.Then N2 is replaced by N2 – O2 gas mixture, N atoms are transformed into O
       atoms by reaction [1]. N atoms density [N]2 is deduced from dark point.
       After the dark point, the NO2* intensity is linearly increasing with a slope r2.
4.  O atom density is given by the equation
[O] = [N]1 r2/r1 – [N]2

Determination of N and O atom density from NO* - our results
Assuming that reaction [1] is finished and taking into account that in afterglow
  there are not only oxygen atoms produced by discharge, but also oxygen atoms
  from reaction [1]
                             [O] = [O]0 + [O]tr = [O]0 +  [NO]0
which is equation of parabola
                  a    =   - k
                  b    =     k ( [N]0 - [O]0 )
                  c    =     k [N]0 [O]0

I = -k [NO]02 + k ( [N]0 - [O]0 )[NO]0 + k [N]0 [O]0
it is possible to write for NO* intensity
I =         k         [N]                   [O]
=         k ( [N0] – [NO]0 ) ( [O]0 + [NO]0 )




INO = [N] [O]






How to determine N and O atom density from NO* - our results
1.Perform a measurement, slowly increase concentration of NO added into
        N2 – O2 afterglow. Measure NO* intensity.
2.Plot NO* intensity as a function of NO concentration added into afterglow.
       Fit this dependence with a parabola to obtain parameters a, b and c and
       its errors da, db and dc.
3.Determine N atom density using equation

                                [N]0 = (-b – (b2 – 4ac)1/2 ) / 2a
4.Determine O atom density using equation

                               [O]0 = -(-b + (b2 – 4ac)1/2 ) / 2a

Experiment was done at following conditions
                    N2 flow = 100 sccm
                        O2 flow = 0 - 2.5 sccm
                        corresponding pressure = 460 Pa
                        power output = 50 W










How  reaction time influences correctness of
                                       N and O atom density estimation?


[NO]t = k2[N]t[O]t / (k1[N]+k3[O]t)
[NO]t = k2[O]t / k1
 N + NO à N2 + O
    N + O à NO
 NO + O à NO2
N + NO à N2 + O

[N] = [N0] exp (-2k2[O]t)
[O] = [O]0 + [NO]0
              N + NO à N2 + O
                 N + O à NO
N + N + NO + O à N2 + NO + O

Conclusion 2
Sensitivity
        - estimation of N and O atom density from NO* is more sensitive for lower
          O atom densities ( O < N ). For high O atom densities, it is better to
          estimate O atom density from NO2*.
Accuracy
        - to estimate correct value of O atom density from NO*, it is necessary to
          measure at places corresponding to reaction times in range 10-1-10-2 s.
Calibration
        -  to estimate O atom density from NO2*, it is necessary to calibrate titration
           probe by pure N2 before measurement. Estimation from NO* does not
           need any calibration.