Chemistry Reference
In-Depth Information
1. Cross sections measured, for instance, in crossed beam or swarm exper-
iments, can be used without modifications as input data for plasma
modeling.
2. Theoretical calculations of cross sections can neglect the plasma environ-
ment, which is a tremendous simplification.
Whereas the physics of a collision is unaffected by the plasma, the reverse is of
course not true. Elementary processes determine to a large extent the properties of a
plasma (see previous chapters). First of all, a gas discharge can only be maintained
electrically because of electron impact ionization, and for an electro-negative gas,
dissociative electron attachment produces positively and negatively charged carriers.
Equally important are electron impact excitation and dissociation of molecules. They
not only transform external electric energy into internal energy, but most importantly
also produce the species that are eventually utilized in the technological application
of the discharge: electronically excited species if the discharge is used as a light
source or laser and reactive fragments (radicals) if the discharge is applied for surface
processing or catalysis.
Technologically interesting gas discharges contain complex molecular gases such
as
CF
4
,
CF
3
I
,
C
3
F
8
,
CCl
2
F
2
,or
SF
6
with a multitude of excited and fragmented
species. Even simple diatomic molecules,
O
2
or
N
2
for instance, give rise to a large
number of species with an accordingly large number of elementary processes. Leav-
ing aside elementary processes containing three particles in the entrance channel that
are only relevant at rather high densities, the most common collisions for a generic
electro-negative, diatomic gas are shown in Table 9.1, where they are also classi-
fied according to the collision compounds controlling the microphysics:
AB
−
,the
compound illustrated on the right-hand side of Figure 9.7,
A
2
−
,
A
2
B
,
A
2
B
−
, and
AB
.
The branching of the compounds and thus the probabilities for the various collisions
(cross sections) depend on the initial energy and properties of the compound states at
the distance where they have to lock-in into the asymptotic scattering states defining
the various collisions.
In view of the great technological and economical impact of reactive gas dis-
charges, it is somewhat surprising that the number of experimental groups measuring
cross sections for gases of plasma-chemical relevance is rapidly diminishing. Thus,
any listing of currently available cross-section data must be necessarily incomplete.
Since in addition the gases of interest change with time, cross-section data are not
included at all in this chapter. As far as they exist, they can be found, for instance, in
the review article by Brunger and Buckman [48], the monograph by Christophorou
and Olthoff [49], and in web-based cross-section compilations sponsored by national
research institutions and university groups whose activities depend on atomic and
molecular collision data. The largest ones, maintained, respectively, by the Inter-
national Atomic Energy Agency, the American National Institute of Standards and
Technology,theOakRidgeNationalLaboratory,USA,theJapaneseNationalInstitute
for Fusion Science, the Weizmann Institute of Science in Israel, and the Université
Paris-Sud in Orsay, France, are [50]:
