Chemistry Reference
In-Depth Information
More in detail, the inelastic electron-molecule interaction results in a highly
excited intermediate molecule complex. The electronic transitions in molecules takes
place at constant internuclear distance and the transition probability is determined
by the Franck-Condon factors which imply the optimum overlapping of the wave
functions of the considered electronic states. The relaxation of the excited molecule
complex may result in the stabilization of the complex if a binding molecular state
exists and the excess energy can be distributed in internal degrees of freedom or
transferred in collisions with the background gas at higher gas pressure like that in
swarm experiments. In the case of a repulsive potential curve the relaxation of the
molecule complex results in dissociation and the products are excited or they have
excess in translational energy,
A
∗
+
, see Figure 3.14.
The
charge transfer collisions
between positive ions and neutrals are charac-
terized by the transfer of an electron from the neutral to the positive ion without
exchange of translational energy. Charge transfer collisions are classified in
resonant
or
nonresonant
collisions concerning the same or different ionization energy of the
collision partners, and in
symmetric
or
asymmetric
collisions concerning the same or
different nature of the collision partners. As an example, the symmetric and resonant
charge transfer collision is observed between a single charged positive argon ion and
the neutral argon atom in ground state
B
+
(
+
ε
T
)
Ar
fast
+
Ar
slow
.
Ar
slow
→
Ar
fast
+
(
AB
+
)*
+
(+ ε
T
)
A
* +
B
+
AB
A
+
B
AB
Internuclear distance
FIGURE 3.14
Principle of direct electron impact ionization of a diatomic molecule. At
higher electron energy the highly excited ionized molecule complex results in dissociative
ionization.
