Environmental Engineering Reference
In-Depth Information
the ionization potential of Ar(
3
P
1
) and EA
2.44 eV is the electron affinity for
Cl
2
. At large distances, a Coulomb interaction exists for the second state, and at the
distance
R
c
D
16
a
0
(
a
0
is the Bohr radius) these states intersect.
In reality, a pseudointersection of the states occurs, so the states are separated by
a gap at distance
R
c
(see Figure 2.22). At smaller distances between the particles
the total system is found in the lowest state, and the Coulomb interaction occurs
between the colliding particles, which decreases the distance of approach. If we
assume that the rearrangement of chemical bonds occurs at a distance
R
0
D
e
2
/(
J
EA)
D
R
c
be-
tween the colliding systems, then process (2.94) takes place. It follows from (2.85)
that the maximal cross section of this process is
<
R
c
/2. Since
R
c
is a large val-
ue, the rate constant for formation of excimer molecules under optimal conditions
is greater than that corresponding to the gas-kinetic value. The rate constant is
7
π
10
10
cm
3
/s for process (2.94) at room temperature, and the rate constant for
formation of other excimer molecules lies in the interval 10
10
10
9
cm
3
/s at ther-
mal energies.
Excimer molecules emit radiation in the UV range of the spectrum and are char-
acterized by short lifetimes. The parameters of these transitions are given in Fig-
ure 2.22. Excimer molecules can be formed with atoms such as mercury, magne-
sium, and calcium instead of inert gas atoms, and oxygen atoms can replace the
halogens. These properties of excimer molecules are used in excimer lasers which
emit UV radiation [130-132].
2.3.4
Associative Ionization and the Penning Process
Associative ionization proceeds according to the scheme
A
C
AB
C
C
B
!
e
.
(2.95)
This process is the inverse, in principle, to dissociative recombination (see Fig-
ure 2.16). But in reality these processes proceed through different vibrational states
of a molecular ion
AB
C
and different electron states of the molecule
AB
.Itisfa-
vorable for the energetics of the process that the electron energy of the final state
should be lower than that of the initial state. For this reason the dissociative recom-
bination process proceeds through repulsive terms of molecule
AB
,whereasin
the case of associative ionization it goes through attractive states of molecule
AB
.
Hence, the cross section for associative ionization is of the order of gas-kinetic
cross sections for those excited states for which this process is effective.
Thus, the atomic states which can partake in the associative ionization process
are determined by the behavior of the electron states of the quasimolecule consist-
ing of colliding atoms. As a demonstration of this, Figure 2.24 shows the electron
states for two helium atoms, one of these is found in the ground state and the
other one is in an excited state [133]. According to this figure, associative ioniza-
tion may proceed with participation of the excited helium atom He(3
1
D
), and in
the first stage of the process of associative ionization the transition takes place
in the repulsive term He
He(2
1
P
) and then the molecular ion He
2
C
is formed.
