Environmental Engineering Reference
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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
 
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