Environmental Engineering Reference
In-Depth Information
ment to molecules. This can prevent a gas from undergoing electric breakdown or
can change the conditions of a stable gas discharge.
2.3
Elementary Processes Involving Ions and Atoms
2.3.1
Slow Inelastic Collisions of Heavy Atomic Particles
Collisions of heavy atomic particles, ions, atoms, and molecules, proceed in a plas-
ma with small relative velocities compared with typical electron velocities. For ex-
ample, the average thermal velocity of the helium ion at room temperature is
v
T
10
5
cm/s
10
4
e
2
/
e
2
/
is the atomic velocity.
Hence, we deal with slow collisions of heavy atomic particles in a plasma, where the
velocity of the relative motion of colliding atomic particles is small compared with
a typical atomic velocity
v
e
. This leads to a specific distribution of bound electrons
in the course of collision when electrons respond to internal fields of the particles,
and the electron distribution differs slightly from the distribution when the parti-
cles are motionless. Then one can analyze particle collisions within a framework
of the electron terms of the quasimolecule consisting of colliding atomic particles
with fixed nuclei, and the electron terms are the electron energies of the quasi-
molecule, where the distance between colliding particles is a parameter. In particu-
lar, in considering the resonant charge exchange process we compose this process
as a result of interference between even and odd states of the quasimolecule.
Transition between two electron states is characterized by the ratio of the differ-
ence of their energies
D
1.2
D
5.7
„
,where
v
e
D
„
Δ
ε
to the quantum indefiniteness in energy
„
v
/
a
,where
v
is the collision velocity and
a
is a typical distance between nuclei associated with
a significant change of the corresponding states. The above criterion is called the
Massey parameter
[127]:
D
Δ
ε
a
.
(2.83)
„
v
If the Massey parameter is large, the probability of the corresponding transition
is adiabatically small, that is, it behaves as exp(
1isanumerical
coefficient. Therefore, transitions between electron states in slow atomic collisions
can be a result of intersections or pseudointersections of corresponding electron
states.
Consider as an example the charge exchange process
a
), where
a
A
C
C
B
!
A
C
B
.
(2.84)
In this case the Coulomb interaction takes place in the initial channel of the pro-
cess, with only a weak interaction of neutral particles in the final channel. The
