Chemistry Reference
In-Depth Information
photoabsorption spectra into their symmetry components. Parallel to the devel-
opments in theoretical calculations, as well as experimental advances offered by
introducing the combined utilization of a high-performance monochromator and
undulator radiation, further extensions of the ARPIS technique not only to linear
polyatomic molecules [27-32], but also nonlinear ones [33], have been demon-
strated successfully. A review paper on the application of the ARPIS technique to
some simple linear molecules offering a summary of the previous ARPIS studies
appeared recently, where Rydberg-valence mixings, Renner-Teller couplings and
vibrationally induced transitions, and vibronic couplings and Jahn-Teller distor-
tions in the inner-shell excited states of some low-Z molecules have been examined
precisely [34].
Through the previous studies mentioned above, it has been widely recog-
nized that a combination of high-resolution ARPIS experiments and sophisticated
ab initio
quantum chemical calculations is an extremely powerful tool to inves-
tigate the electronic structures of molecular inner-shell excited states. The main
purpose of this chapter is to summarize the latest experimental data and theoreti-
cal interpretation of some typical linear and nonlinear low-Z molecules [nitrogen
(N
2
), oxygen (O
2
), acetylene (C
2
H
2
), and sulfur dioxide (SO
2
)], measured by pho-
toabsorption spectroscopy, as well as angle-resolved photoion-yield spectroscopy
with highest resolving power.
II. ANGLE-RESOLVED PHOTOION-YIELD SPECTROSCOPY
A. Photofragment Angular Distribution
A complete treatment and separation of the photofragmentation dynamics into an-
gular and radial parts are allowed by the Born-Oppenheimer and Franck-Condon
(FC) principles. According to the FC principle, molecular electronic transitions
are most favored in which the position of the nuclei change little during the ab-
sorption of a photon. Generally, if the excited state is dissociative, the dissociation
occurs in a short time compared to the rotational period of the molecule. Thus
the distribution of the trajectories of the fragments reflects the initial orientation
of the molecule. The photodissociating molecules are not isotropically distributed
relative to the exciting radiation since the absorption probability is greatest when
the transition dipole
of the incident radiation.
Therefore, the angular distribution of the fragments should show a corresponding
anisotropy. The polarization character of synchrotron radiation is quite useful for
producing the anisotropic orientational distribution of molecules.
Let us introduce the general expression with a semiclassical treatment for
the photofragment angular distribution of the molecule whose transition dipole
moment
μ
is aligned with the electric vector
ε
is at an angle
χ
to the ejection direction of the fragment [35, 36]. Here,
the cases where the molecules are excited by linearly polarized light and the
μ
