Chemistry Reference
In-Depth Information
the overlap of the vibrational states. In order to apply this formula, the ionization
potential and the electron affinity have to be known. For atomic negative ions this
is not a problem, whereas for molecular negative ions this can be subtle because the
ion may be in an (unknown) excited state. Fortunately, molecular negative ions have
a rather short lifetime, as discussed earlier. They are thus not of major concern in the
commonly used gas discharges.
For molecular positive ions,
dissociative ion-ion annihilation
mayalsobepos-
sible. It contains two relative motions in the exit channel and is thus much harder
to analyze theoretically. Experimentally, it is not always possible to discriminate
between this process and the charge-transfer-type reaction discussed in the previous
paragraphs. The energy dependence of the two cross sections is however essentially
the same, except that they reach the constant value at different energies. Thus, when
R
x
is determined from the high energy asymptotics of empirical data, the annihilation
cross section is in fact an effective cross section, comprising both charge-transfer-type
and dissociative ion-ion annihilation.
In contrast to positive ions, negative ions can also be destroyed by slow collisions
withneutralparticles(detachment).Tobespecific,
associativedetachment
ofnegative
ions due to molecules is discussed. Microscopically, it is a resonant process similar to
dissociative attachment and dissociative recombination. An auto-detaching, resonant
state thus plays an important role. In Figure 9.15, the relevant potential energy
surfaces are shown. The asymptotically stable configuration
changes with
decreasing
R
into a quasi-bound, resonant collision compound,
A
2
B
−
, from which
the electron auto-detaches, leaving behind a free electron and a neutral particle in an
excited vibrational state [62-64].
(
A
−
,
AB
)
(
A
,
AB
)
E
a
A
−
,
(
A
AB
)
-
A
2
B
E
d
A
2
B
A
2
B
R
΄
R
x
Internuclear distance
R
FIGURE 9.15
Illustration of associative detachment. The potential energy surface of a bond-
ing state of the collision compound
A
2
B
−
crosses the potential energy surface of the
A
2
B
molecule.Auto-detachmentofthecompoundstatethenproducesanelectronandavibrationally
excited
A
2
B
molecule. Also shown is an antibonding
A
2
B
−
state (dashed line). Detachment
may also occur from such a state, but then the initial kinetic energy of the
(
A
−
,
AB
)
system has
to be larger than
V
A
2
B
−
(
R
x
)
−
V
A
2
B
−
(
∞
)
.
