Chemistry Reference
In-Depth Information
excited atomic levels. Hence in non-equilibrium plasmas the concept of temperature
may have a certain physical meaning, but often it is rather artificial.
6.3.3.3.1 Translational Temperature
The gas (kinetic) temperature
T
is a very important plasma parameter, since the rate
coefficients of chemical reactions usually show an exponential increase with T. On
the other hand, even in a monoatomic gas the plasma density distribution cannot
be calculated without knowledge of the temperature distribution. Determination of
the gas temperature or attempts to do that by emission and absorption spectroscopy
are rather common in plasma diagnostics [174-196]. A measurement of the Doppler
broadening of spectral lines is a well known and widely used method of temperature
determination in gas discharge plasmas [128,129]. For a single spectral line the full
width half maximum (FWHM) of the Doppler profile is
8
kT
ln2
mc
2
λ
d
=
λ
m
,
(6.20)
where
λ
d
[nm] is the FWHM of the Gaussian line profile
T
[K] is the neutral gas temperature
m
[kg] is the particle mass
λ
m
[nm] is the wavelength at line center
Onecanseethatforplasmadiagnosticsitisbettertouselineswithlongerwavelengths
emitted by lighter species. It should be noted that implementation of the method is
not that simple in practice. The main problems are
1. The determination of the instrumental function of the spectrometer
2. Taking into account the multiplet and fine structure of lines
3. Extraction of the Doppler broadening contribution from the measured line
profile (so-called deconvolution)
Moreover, in non-equilibrium molecular plasmas the velocity spectra of excited and
ground state species can be different. Nevertheless, the determination of translational
temperatures from Doppler broadening of atomic and molecular hydrogen lines is
often used in non-equilibrium plasmas [176,180,182,186,189,191]. In the case of
H
alpha
and D
alpha
spectral lines, which are of special importance, it is necessary to take
into account their fine structure [194].
6.3.3.3.2 Rotational Temperature
When the populations of rotational levels in excited vibronic states
n
,
v
are close
to a Boltzmann distribution, the rotational temperature
T
rot
(
n
,
v
)
for this state is
given by
exp
,
hcE
n
ν
N
kT
rot
(
N
n
ν
N
=
c
n
ν
g
a
,
s
(
2
N
+
1
)
−
(6.21)
n
, ν
)
