Environmental Engineering Reference
In-Depth Information
Figure 4.14
The mobility of nitrogen ions in nitrogen [71, 72]. Arrows indicate the mobilities of
ions at zero electric field strength.
where [
X
] is the number density of particles
X
,
g
a
,
g
C
,and
g
mol
are the statistical
weights with respect to the electronic state for atoms, atomic ions, and molecular
ions respectively,
and
D
are the vibrational energy and the dissociation energy
for the molecular ion, respectively, and
B
is the rotational constant for the molec-
ular ion; we assume the temperature
T
expressed in energy units to be large com-
pared with the rotational constant
B
, and we assume the two nuclei of the molecular
ions to be different isotopes. As a demonstration, we give in Table 4.14 [34] these
parameters for molecular helium and argon ions.
If we substitute these data into (4.107), in the argon case we obtain
„
ω
C
exp
,
Δ
H
T
K
(
T
)
D
with the enthalpy
Δ
H
D
1.33 eV for a pressure range
p
1 Torr, and
C
D
10
11
To r r
1
. This means that the equilibrium number densities for atom-
ic and molecular argon ions are equal at pressure
p
1.8
D
1Torr and temperature
688 K. We find
that molecular ions dominate at low temperatures under equilibrium conditions,
whereas atomic ions dominate at high temperatures. Usually ions are formed in a
gas as a result of electron-atom collisions. At low temperatures they are converted
into molecular ions as a result of three body collisions, and the basic ion type de-
T
D
624 K or at pressure
p
D
10 Torr and temperature
T
D
Ta b l e 4 . 14
The parameters of the ground electronic state of He
2
and Ar
2
ions [34].
Molecular ion
He
2
Ar
2
D
e
, eV
2.47
1.23
„
ω
e
,cm
1
1698
309
„
ω
e
x
e
,cm
1
35.3
1.66
B
e
,cm
1
7.21
0.143