Environmental Engineering Reference
In-Depth Information
Figure 2.18
The cross section of electron attachment to the oxygen molecule as a function of
the electron energy
at room temperature [114].
for the cross section of electron attachment to the oxygen molecule [114, 121]. Note
that the autodetaching state O
2
(
2
u
) follows from the cross section dependence
on the electron energy [122] for this process. One can see the resonant character
of the electron attachment process and the parameters of this resonance are de-
termined by the relative positions of the electronic states for the molecular and
autodetaching states and by the coupling between these states. For vibrationally ex-
cited molecular states this resonance is broadened and is shifted, as is seen from
Figure 2.19, where the left part of the resonance for electron attachment to the oxy-
gen molecule is given at different temperatures [123]. As is seen, in this case the
resonance increases and shifts to lower electron energies with increasing tempera-
ture. Note that the vibrational excitation energy for the oxygen molecule is 0.196 eV,
so at a temperature of 1930 K approximately 69% of oxygen molecules are found
in the ground vibrational state, and 21% of molecules are in the first excited vibra-
tional state. Hence, from Figure 2.19 it follows that the resonance energy decreases
with increasing vibrational state of the oxygen molecule, and the maximum cross
section increases also.
Letusgiveonemoreexampleforthedependenceofthemaximumcrosssec-
tion in accordance with measurements [124] for electron attachment to HCl and
DCl molecules in the temperature range 300-1200 K. If we denote the ratio of elec-
tron attachment cross sections for different vibrational states as
Π
D
σ
(
v
D
i
)/
η
i
(
v
D
D
σ
0), for the HCl molecule we obtain from these measurements that
η
38
1
D
D
D
and
580.
In reality, a resonance dependence of the cross section of electron attachment
on the electron energy is typical for diatomic molecules with a small number of
autodetaching states of a forming negative ion. For complex molecules with a large
number of autodetaching states the dependence of the cross section on the electron
energy becomes smooth, and the electron attachment process becomes possible at
η
880, whereas for the DCl molecule we have
η
32 and
η
2
1
2
