Environmental Engineering Reference
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states in such processes. For instance, the cross section for vibrational excitation of
molecules by electron impact increases by two to three orders of magnitude if this
process proceeds through excitation of autodetaching states instead of by direct
excitation. There are some processes that can proceed only through excitation of
autoionizing or autodetaching states, such as dissociative attachment of electrons
to molecules and dissociative recombination of electrons with positive molecular
ions. Then the cross section of the process as a function of the electron energy
exhibits resonances corresponding to autoionizing or autodetaching states. As a
demonstration of this, Figure 2.12 gives a schematic display of states which shows
the process of dissociative attachment of an electron to a molecule. The electronic
states related to dissociative recombination of electrons and molecular ions have
the same form as those for dissociative attachment of electrons to molecules and
are represented in Figure 2.13. In both cases the first stage in the process, the
electron capture by a molecular ion or molecule, has the resonant character. As
Figure 2.12 Electronic states of a molecule
and its negative molecular ion which are
responsible for electron attachment to a
molecule. Variation of these states proceeds
along a coordinate that determines this pro-
cess (the distance between nuclei for a di-
atomic molecule). The probability of electron
capture depending on the electron energy is
indicated at the right.
Figure 2.13 Electronic states of a molecule
and its positive ion which determine the pro-
cess of dissociative recombination; e is the
energy of the captured electron when the dis-
tance between nuclei is equal to the equilibri-
um distance R e . The autoionizing state exists
until the distance between nuclei does not
exceed R c .
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