Chemistry Reference
In-Depth Information
light molecules. Subsequently the method of determinating
T
vib
from the relative
intensities of Q-branch lines in the Fulcher-α bands was revised and simplified due
to the changes of the
d
3
u
→
a
3
g
transition probabilities and cross sections of the
g
electron impact excitation [202]. Also the emission of the H
2
radiative
dissociation continuum (
a
3
u
←
d
3
X
1
u
) has been employed as a source of infor-
mation for the spectroscopic diagnostics of non-equilibrium plasmas [202,203]. This
new method has been used in studies of capillary arc [202] and pulsed microwave dis-
charges [199]. In both cases a remarkable difference between
T
vib
and
T
was observed.
This observation is in accordance with previous CARS measurements in other dis-
charges [200,204]. In pure H
2
plasmas the values of
T
vib
obtained by two independent
OES methods are in rather good agreement for
T
vib
=
g
→
b
3
3 000
−
5 000 K [202].
6.3.3.4 Degree of Dissociation of Hydrogen
Dissociation processes in low-temperature hydrogen-containing plasmas can be
investigated by several different approaches, namely, actinometry (see Section
6.3.3.2.1), absolute atomic line intensities [201,205,206], relative atomic and molecu-
lar line intensities [207,208], laser induced fluorescence [209], or vacuum ultraviolet
absorption methods [210]. When using atomic emission the electron impact dissocia-
tive excitation process should be taken into account in molecular plasmas which have
a low degree of dissociation [206]. Since this work, various methods based on relative
intensity measurements have been assessed and compared [208]. It was shown that
the fine structure makes an important contribution in the balance equations for the
calculation of emission rate coefficients of the direct and dissociative excitation of
H
α
and H
β
lines [210].
A new spectroscopic method considers the non-resolved fine structure of
Balmer lines in the kinetics of excitation and deactivation of hydrogen atoms in
non-equilibrium plasmas [212-215].
Based on measurements of the relative line intensities of atomic and molec-
ular hydrogen and the gas temperature and using a simple excitation-deactivation
model the density of molecular hydrogen was studied in microwave discharges
in H
2
-Ar-B
2
H
6
gas mixtures (
f
8 mbar).
The experimental arrangement of the planar microwave plasma reactor and the
optical system is shown in Figure 6.9. Details of the reactor and the arrangement for
emission spectroscopy can be found elsewhere [171,216]. The experimental values
of
=
2.45 GHz,
P
=
1.2
−
3.5 kW,
p
=
1
−
obtained for two different distances from the microwave window are
shown in Figure 6.10. From Figure 6.10 one may see that the density ratio increases
noticeably with MW power at a distance of 2 cm from the optical axis from the MW
window, while for the greater distance of 4 cm the variation of the density ratio does
not exceed the experimental error.
[
H
]
/
[
H
2
]
6.3.4 A
BSORPTION
S
PECTROSCOPY
6.3.4.1 General Considerations
The methods of absorption spectroscopy are of great importance in plasma diagnos-
tics because they provide a means of determining the population densities of species
