Environmental Engineering Reference
In-Depth Information
A comparison of this balance equation and the definition of the rate constant
for three body processes shows that the third process (2.102) can be characterized
as a three body process only if
[
C
](
k
q
C
k
d
)
1 .
(2.104)
Then the three body rate constant is
K
D
k
q
k
d
τ
.
(2.105)
Ta b l e 2 . 17
Collision processes of electrons with atoms and molecules.
Process
Scheme
Comment number,
section of [146]
a
, Section 2.1.7
Elastic collision of electrons with atoms
and molecules
e
C
A
!
e
C
A
A
Inelastic collision of electrons with atoms
and molecules
e
C
A
!
e
C
Section 2.2.1
A
C
Ionization of atoms or molecules by
electron impact
e
C
A
!
2
e
C
Section 2.2.2
AB
(
J
0
)
b
Transitions between rotational levels of
molecules
e
C
AB
(
J
)
!
e
C
v
0
)
c
Transitions between vibrational levels of
molecules
e
C
AB
(
)
!
e
C
AB
(
v
B
Dissociative attachment of an electron to
amolecule
e
C
AB
!
A
C
Section 2.2.7
AB
C
!
B
Dissociative recombination
e
C
A
C
Section 2.2.5
d
Dissociation of a molecule by electron
impact
e
C
AB
!
e
C
A
C
B
B
Electron attachment to an atom in three
body collisions
e
C
A
C
B
!
A
C
Section 2.2.3
A
C
!
Electron-ion recombination
2
e
C
e
C
A
A
C
C
A
C
in three body collisions
e
C
B
!
B
Section 2.2.3
A
C
Z
[
A
C
(
Z
1)
]
Dielectronic recombination
e
C
!
Section 2.2.6
A
C
(
Z
1)
!
C„
ω
a
Typically, the cross section is of the order of the gas-kinetic cross section.
bJ
and
J
0
are angular momenta of the molecule. The cross section is, typically, less by one or two
orders of magnitude than the cross section for elastic collision of a slow electron and molecule. A
selection rule determines the difference
J
J
0
depending on the molecular species and on the
collision energy.
c
Processes proceed via formation of an autodetaching state
AB
. The probability of the process
depends on the relative position of the electron energy and the autodetaching state energy. The
cross section for the process has a resonant energy dependence, and the maximum cross section
is less than or of the order of the atomic cross section.
d
The process proceeds via excitation of a molecule into a repulsive electron term. The dependence
of the cross section on the electron energy is the same as for excitation of vibrational levels of the
molecule by electron impact.
