Chemistry Reference
In-Depth Information
radiation has made it possible to perform detailed researches related to molecular
inner-shell excitation and ionization processes.
Photoabsorption or X-ray absorption spectroscopy of molecules is one of the
most fundamental probes to investigate their electronic and geometric structures.
The photoabsorption spectra obtained are also a prerequisite for understanding
the subsequent processes, such as dissociation and de-excitation. The molecular
inner-shell photoabsorption spectra contain a variety of pronounced resonances
corresponding to electronic transitions of an inner-shell electron to bound and un-
bound states near the ionization threshold. Below the ionization threshold, struc-
tures called discrete resonances are due to transitions of a K-shell electron into
weakly antibonding and nonbonding orbitals (e.g.,
π
∗
valence and Rydberg or-
bitals). Above the threshold, in addition to the continuum states, features are due
to transitions of a K-shell electron to shape resonances corresponding to quasi-
bound states arising from the centrifugal barriers in the molecular fields, and/or to
doubly excited states corresponding to the shake-up like transitions in conjunction
with the K-shell excitation.
A high-resolution K-shell photoabsorption spectrum with vibrational resolu-
tion was initially recorded for nitrogen molecule (N
2
) by Chen et al. [17] in 1989.
These authors have found several new features corresponding to the Rydberg states
with vibrational fine structures and to the doubly excited states, and given tentative
assignments to these structures. Similar investigations in the K-shell excitation re-
gions of CO were performed by Domke et al. [18, 19] in 1990. The first observation
of the fine structures on the two broad features corresponding to the Rydberg states
in the K-shell photoabsorption spectrum of oxygen molecule (O
2
) and tentative as-
signments of the structures were reported by Ma et al. [20] in 1991. They proposed
the tentative assignment of the two broad features as the exchange interaction split
σ
∗
shape resonances, pulled down from the continuum to below the ionization
threshold by a strong attractive potential [14, 21]. Remember that the assignments
for the spectral features proposed in the previous experimental studies are mostly
based on the
Z
+
1 equivalent core model. Owing to the great efforts in the previ-
ous studies, it seemed that general features in the K-shell photoabsorption spectra
of simple molecules were fairly understood. However, the assignments for the
resonance structures in the spectra had not been well established. Definite assign-
ments for the discrete resonances, as well as continuum resonances, on the basis
of direct experimental evidence and sophisticated
ab initio
quantum mechanical
calculations, were highly desired.
In the early 1990s, a new experimental technique of angle-resolved photoion-
yield spectroscopy (ARPIS) was developed [22-26], which is based on the po-
larization dependence of the ejection direction of fragment ions in the decay
channels after the K-shell photoabsorption of low-Z molecules. This spectroscopy
can provide a rare opportunity to reveal the symmetries of the K-shell excited
states in diatomic molecules, directly by the decomposition of the conventional
