Chemistry Reference
In-Depth Information
of that discharge phenomena is much lower than the characteristic energy relaxation
times.
In some restricted cases the approximation of a local
partial thermodynamic
equilibrium state
(PTE) can be applied for nonequilibrium plasmas which implies
the definition of separate temperatures for
1. Translational energy of electrons
T
e
, ions
T
+
and neutrals
T
n
2. Internal energy distribution in excited electronic
T
exc
, vibrational
T
vib
and
rotational
T
rot
states
As an example, the
nonthermal plasma of the positive column in low-pressure DC
glow discharge
represents a weakly ionized nonthermal plasma characterized by
total pressure
p
<
10
3
Pa, degree of ionization χ
=
10
−
6
−
10
−
4
, and electron density
n
e
<
10
18
m
−
3
.
The electron temperature is much higher than the ion and neutral gas temperature,
typically
T
e
∼
300 K. In such plasmas, also indicated as
cold
plasmas
, the electrons represent the main energetic particles with mean kinetic energy
of few electron volts (1eV corresponds to 11,600 K).
10
4
K
T
+
∼
T
n
∼
3.3 ELEMENTARY COLLISION PROCESSES AND CROSS SECTIONS
3.3.1 I
NTRODUCTION AND
O
VERVIEW
Elastic and inelastic collisions between electrons, ions, and neutral atoms and
molecules represent key processes in nonthermal plasmas for the dissipation of
translational and internal energy. In particular, the inelastic collisions of the hot
electrons with heavy particles are of great importance as the main source in the gen-
eration of charged, excited, and/or highly reactive radicals. The elementary collisions
processes between plasma particles have to be described by conservation laws such
as the total momentum and energy of the participating particles under consideration
of the quantum states of the atomic or molecular collision partners and their selection
rules for electronic transitions as well as the vibrational and rotational transitions in
the case of molecules, respectively. Collisions in nonthermal plasmas at low pressure
are dominated by binary collisions. Generally, the result of the collision between two
particles
A
and
B
depends on their specific physical interaction determined by the
energy, mass, and charge of the particles, and their specific collision cross section
σ
AB
, as exemplarily demonstrated in Figure 3.7.
With increasing particle density, e.g., in nonthermal plasmas at atmospheric
or higher pressure, the collisions involving three particles must be taken into
account, too.
The collision of charged, metastable excited and reactive plasma particles as
well as the VUV/UV photons with surfaces at the plasma boundaries has to be
additionally considered, such as on discharge electrodes, immersed probes for diag-
nostics, material surfaces for plasma processing, and the wall of the plasma vessel.
The plasma-surface interactions may result in emission of secondary species, or the
plasma boundary acts as a third collision partner, see Figure 3.8. Therefore, many
