Chemistry Reference
In-Depth Information
0
25
50
75
100
125
150
175
200
e + O
2
4
4
777.4 nm
3
3
2
2
844.6 nm
1
1
0
0
0
25
50
75
100
125
150
175
200
Electron energy in eV
FIGURE 3.21
Opticalemissionexcitationcrosssectionforatomicoxygenmultipletradiation
at 777.4 and 844.6 nm from dissociative excitation of oxygen molecule. (Data from Schulman,
M.B. et al.,
Phys.Rev.A
, 32, 2100, 1985.)
lamp
A
s
, the solid angle of observation
, the spectral transmittance of the light
path γ
, the measured photomultiplier signal
R
without background and dark
current with the index s for the reference measurements with the standard lamp. The
function
F
(
λ
)
(
)
·
λ is the photon emission rate of the standard lamp at the operating
temperature
T
in the interval between λ and λ
λ,
T
+
λ is the spectral
passband by the monochromator. The experimentally determined optical emission
excitation cross sections of the two prominent multiplet transition of atomic oxygen
at 777 and 844 nm from the dissociative electron impact excitation of molecular
oxygen are shown in Figure 3.21. The shoulder observed at about 35 eV has its origin
from additional contribution of the dissociative excitation of atomic oxygen from the
molecular oxygen ion [20].
The
electronimpactionization
is themost prominent elementarycollisionprocess
for charged species generation in the volume of nonthermal plasmas. The ioniza-
tion of a neutral atom or molecule due to the collision with an electron has to be
described exactly by quantum physics and needs extensive calculations. Already the
single ionization of a neutral atom represents a three-body problem. For the calcu-
lation of the ionization cross section, mostly methods of the perturbation theory are
applied using the Born approximation. Here, the wave function of the free electron
is expressed by partial wave development. This can be used if the scattering poten-
tial of the neutral is small compared with the energy of the impinging electron. In
result the ionization cross section at higher energy is proportional to ln
E
λ, whereas
E
. Tak-
ing into account specific interaction potentials, the different quantum mechanical
approximations, such as
Coulomb-Born-approximation
and the
distorted plane wave
approximation
, provide reliable results in special cases. Therefore, semiclassical
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