Chemistry Reference
In-Depth Information
measurable by coincidence detection between the momenta of the Auger electron
and atomic ion that is produced by the prompt dissociation following the molecular
Auger decay. The first experimental realization of this kind was reported as the an-
gular distribution measurements of photoelectrons from fixed-in-space molecules,
which was achieved by introducing the coincidence detection between the photo-
electron and fragment ion [130]. After this pioneering work, many investigations
for the molecular frame photoelectron angular distribution (MFPAD), which are
essentially based on the same concept as [130], have been carried out intensively
[131-157]. Since the applications of the angular anisotropy in the dissociation
process following the inner-shell photoexcitation, some recent investigations other
than MFPAD are summarized in the following Section VII.
A. Auger Electron Emission from Spatially Fixed Molecules
It is quite natural to extend the experimental technique for measuring MFPAD to
the Auger electron emission. The most detailed information on the MOs involved
in the process is obtainable by measuring the angular distributions of Auger elec-
trons from fixed-in-space molecules. As demonstrated in this chapter, thanks to
the rapid progress of the experimental techniques associated with synchrotron
radiation, vibrational spectroscopy in the inner-shell excitation region of low-Z
molecules has become feasible. The findings through the studies on the resonant
Auger processes and their theoretical analyses impel us to give up the traditional
idea that the molecular dissociation begins with the Auger final state when the
Auger transition is terminated [158]. This means that the Auger decay cannot be
treated separately from the primary photoexcitation process. However, it is widely
believed that the decay process following inner-shell ionization, namely, the nor-
mal Auger decay, can be interpreted as a two-step process in which the Auger
decay is treated independently of the initial ionization process. The experiment to
show the limitation of this picture has been performed on CO after C1s ionization
[159]. The de-excitation of the
C
(1s)
−
1
core-hole state predominantly populates
the three lowest singlet states X
1
(5
σ
−
2
)
,
A
1
(5
σ
−
1
1
π
−
1
)
,
B
1
(5
σ
−
1
4
σ
−
1
)
of the doubly ionized CO molecule.
The angular distributions after both
σ
and
π
ionization for the unresolved X
and A states are shown in Fig. 19, in comparison to those for the B state: the
angular distribution patterns for the X and A states are almost isotropic, but those
for the B state are very rich in structure and remarkably different from each other.
The latter observation completely differs from both regular Auger electron angu-
lar distributions from randomly oriented molecules, and the theoretical prediction
for the fixed-in-space angular distributions of Auger electrons by Kuznetsov and
Cherepkov [160], where the angular distribution patterns after both
σ
and
π
ion-
ization should be the same. Sundin et al. [161] have demonstrated with vibrational
resolution that the PCI effect distorts the line shapes of both the photoelectron and
