Plasma Diagnostics - Nonthermal Plasma Chemistry and Physics - page 211

expressed in wavelength is equal to

C − 3 / 2 N 2 / e .

λ S = ( N e , T e ) (6.18)

The broadening factor C shows only a weak dependence on electron density and
temperature. The error for the determination of N e is given to be 15%-20%. The
application of atomic hydrogen lines is recommended for the measuring of electron
density. The application of the Stark broadening requires the separation of other line
broadening effects in the plasmas under study as instrumental broadening, Doppler
broadening, and resonance broadening.
The measured, spatially localized line emission intensities allow the calculation
of the population densities of the electronic or rovibronic levels of the species, pro-
vided transition probabilities are known and calibration of the spectrometer functions
has been performed. The calculation of species densities in the ground state from
measured line intensities is often also an inverse problem. It requires a theoretical
model for the excitation and de-excitation processes, and knowledge of all necessary
cross sections, transition probabilities and the electron energy distribution function
(EEDF). These requirements are often nontrivial to achieve and in practice an eas-
ier approach is to use actinometry, described in the following section, in which the
emission intensity is referenced to a known standard.

6.3.3.2 Concentration Measurements
6.3.3.2.1 Actinometry
Actinometry is an emission spectroscopy method, which offers the possibility of
determining relative concentrations of ground state species from emission intensity
measurements [160,161]. This technique requires the introduction of a selected gas
as a minor impurity, an actinomer, into the feed gas stream. Typically a rare gas like
argon is used. The actinomer indicates changes in the electron density or energy and
is used for normalizing the emission intensity shown by the species of interest [162].
If actinometry is valid, then the emission intensity of the species of interest I spe over
the intensity of an emission line of the admixed actinomer I act is proportional to the
electronic ground state relative concentration n spe , provided that the concentration n act
of the actinomer is known

Kn act I spe
I act

n spe =

. (6.19)

The factor K contains the spectral and geometrical factors of the detection system and
the emission cross sections of the observed transitions. Clearly unless the factor K in
(6.19)canbeevaluatedabsolutelythenthecalculated n spe valuesareonlyrelativeones.
The main conditions to be fulfilled to ensure valid actinometric measurements are

1. Excitation of emitting states of the species of interest and of the actinomer
only by direct electron impact from their ground states
2. De-excitation by spontaneous emission
3. Similarity of the threshold U thres act

U thres targ and shape of the electron-
impact cross sections of both actinomer and target species

=