Environmental Engineering Reference
In-Depth Information
In reality, both regimes can be fulfilled for formation of bound states through
formation of a long-lived complex. For example, the lifetime of the long-lived com-
plex consisting of electrons and large molecules is of the order of 10
6
10
4
s, so
10
17
cm
3
.Atsmall
number densities of particles
C
, formation of the bound state proceeds in the three
body process. Note that the Thomson formula for the three body process follows
from (2.105) if the transition time for colliding particles is used as the lifetime of a
long-lived complex.
criterion (2.106) is satisfied for number densities [
C
]
10
15
2.3.7
Types of Elementary Processes
A summary of the elementary processes occurring in weakly ionized gases is given
in Tables 2.17-2.19 [146]. The comments in the table footnotes contain details and
special features of these processes.
2.4
Radiative Processes in Excited and Ionized Gases
2.4.1
Interaction of Radiation with Atomic Systems
Interaction of a plasma with radiation affects plasma properties, and in systems
such as gas lasers or the plasma generated by laser radiation, these processes de-
termine the plasma parameters. Electromagnetic fields cause transitions in atomic
systems, and Table 2.20 gives a list of single-photon processes that are of interest
for plasmas. The weakness of interaction is characterized by the fine structure con-
stant
e
2
/(
α
D
„
c
)
1/137, which is the ratio of the average electron velocity in the
Ta b l e 2 . 20
Elementary interactions between atoms and radiation [146].
Process
Scheme of the process
A
Excitation as a result of photon absorption
„
ω
C
A
!
A
!„
ω
C
Spontaneous radiation of an excited atom
A
A
!
Stimulated photon emission
„
ω
C
2
„
ω
C
A
A
C
C
Atomic photoionization
„
ω
C
A
!
e
A
!
Photodetachment of a negative ion
„
ω
C
A
C
e
Photodissociation of a molecule
„
ω
C
AB
!
A
C
B
A
C
!
Photorecombination of an electron and an ion
e
C
A
C„
ω
A
C„
ω
Radiative attachment of an electron to an atom
e
C
A
!
Atomic photorecombination
A
C
B
!
AB
C„
ω
Bremsstrahlung in electron-atom or electron-ion collisions
e
C
A
!
e
C
A
C„
ω
