Environmental Engineering Reference
In-Depth Information
Excited atoms are present in a gas discharge plasma and may be of importance
for its properties. One of the ionization channels in a gas discharge plasma is con-
nected with the Penning process, which proceeds in a pairwise collision process
according to the scheme [137, 138]
B
!
A
C
C
A
C
e
C
B
,
(2.97)
where the ionization potential of atom
A
is lower than the excitation energy of
atom
B
. Therefore, the state
A
B
of the system of colliding atoms is above
the boundary of the continuous spectrum, that is, it is an autoionizing state, and
this state decays at moderate distances between colliding atoms when the width of
this autoionizing state becomes enough for its decay during collision. Table 2.15
contains the cross sections of the Penning process involving metastable atoms of
inert gases.
The Penning process (2.97) consists in decay of an autoionizing state of two in-
teracting atoms, and the width of the state decreases with increasing separation be-
tween the atoms. Hence, there is an optimal distance between atoms that is com-
parable with the atomic size. The cross sections of the Penning process given in
Figure 2.25 [139] and related to collisions of metastable helium atoms with atoms
of heavy inert gases testify that the width of the autoionizing state for He(2
1
S
)is
larger than that for He(2
3
S
). Next, the Penning process (2.97) has resonant charac-
ter, which follows from the electron spectrum for this process, and the width of the
electron spectrum is determined by the region of separations for the Penning pro-
cess. Figure 2.26 [140] gives the electron spectrum for the Penning process involv-
ing xenon and mercury atoms. Each resonance corresponds to a certain channel of
this process.
Note that metastable states include lower excited atomic states such that the ra-
diative transition from these states to the ground atomic state is forbidden, and
therefore these states are characterized by a large radiative lifetime and may accu-
mulate in a plasma with a remarkable concentration. Hence, metastable atoms may
be of importance for ionization processes in a plasma. In particular, a small con-
centration of heavy inert gases (approximately 10
3
) in heliumdecreases the break-
C
Ta b l e 2 . 15
The rate constant for the Penning process (2.97) at room temperature [27, 52, 92].
The rate constants are given in cubic centimeters per second.
Colliding atoms He(2
3
S
) e(2
1
S
) e(
3
P
2
) r(
3
P
2
)
Kr(
3
P
2
)
Xe(
3
P
2
)
10
12
10
11
Ne
4
4
-
-
-
-
10
11
10
10
10
10
Ar
8
3
1
-
-
-
10
10
10
10
10
12
10
12
Kr
1
5
7
5
-
-
10
10
10
10
10
10
10
10
10
10
Xe
1.5
6
1
2
2
-
10
11
10
11
10
11
10
11
10
11
10
11
H
2
3
4
5
9
3
2
10
11
10
10
10
11
10
10
10
12
10
11
N
2
9
2
8
2
4
2
10
10
10
10
10
10
10
11
10
11
10
10
O
2
3
5
2
4
6
2
