EGAS41 - Swansea University

41 st EGAS CP 73 Gdańsk 2009
Perturbed molecular lines of N + 2 for plasma diagnostics
E. Pawelec
Institute of Physics, Opole University, ul. Oleska 48, 45-023 Opole, Poland
E-mail: ewap@uni.opole.pl
Plasma diagnostics is an important tool for any plasma sources applications, both scientific
as technical. In temperatures higher than ˜8000K the diagnostics can be performed by
using intensities of the atomic lines, but in lower temperatures already the existence
of temperature can be doubtful as the equilibrium between the energy subsets starts
to break down. In such a plasma the temperature that can be still relatively safely
assumed to exist is the one derived from Maxwell translational energy distribution for
the heavy particles. Measuring this kind of energy distribution can be done by using the
atomic lines’ profiles, but this requires using either lasers, or very, very precise Fabry-
Pèrot spectrometers. The other method of calculating this parameter is by using the
Boltzmann plot of the rotationally resolved molecular lines, as the Boltzmann distribution
of the rotational energy can be assumed to be in equilibrium with the translational energy
distribution. Unfortunately, in many cases the rotational spectra are difficult to resolve,
and the lines that overlap originate from very different energy levels, making the classical
method of modeling the molecular spectra to fit the observed ones [1] very imprecise, as
the molecular spectrum shape can change very slightly for higher temperatures.
One of the very widely used molecule is N + 2 , as this molecular ion can be found in most
of the plasmas, even without nitrogen added explicitly, as it is an often found impurity.
This spectrum, for example the 0-0 band of B→X transition around 390 nm, can be used
with very good precision for temperatures from hundreds to about 3000 K, even unresolved,
as the profile of this spectrum changes very strongly in this region. Unfortunately,
for temperatures 4000K and higher the fit starts to exhibit big uncertainties, because the
P and R branches of the spectrum begin to overlap, masking the temperature dependence.
There is, nevertheless, one interesting point in the spectrum where they do not, because
the rotational levels for the upper level are disturbed by high rotational sublevels of the
A N + 2 level [2]. Perturbation of the levels shifts the lines in this region enough so the P
and R branches are not overlapping and the lines’ intensities can be measured precisely,
and used for determining the temperature.
Unfortunately, for determining the temperature, the transition probabilities of the
observed lines have to be determined. Theory gives the transition probabilities (or at
least their dependence on the rotational number) as the Hönl-London factors [3]:
S R J = (J” + 1) − 1 4
J” + 1
S Q J = 2J” + 1
4J”(J” + 1)
S P J = J”2 − 1 4
J”
which in case of the perturbed lines is not true. In this report the measured transition
probabilities are given, and the dependence of line intensities on plasma parameters are
presented and discussed.
References
[1] H. Nassar, S. Pellerin, K. Musiol, O. Martinie, N. Pellerin, J.-M. Cormier, J. Phys. D:
Appl. Phys. 37 1904 (2004)
[2] F. Michaud, F. Roux, S.P. Davis, A.-D. Nguyen, C.O. Laux, J. Molec. Spectr. 203, 1
(2000)
[3] R.S. Mulliken, Rev. Mod. Phys. 3, 89 (1931)
(1)
133