Verlag des Forschungszentrums Jülich
JUEL-3544
For broad band materials the intensity distribution in a SXE spectrum can
be related to the local
partial density of states (LPDOS) which is the density of states (DOS)
projected on the site of the
atom carrying the core hole (local) and on the well-defined angular
momentum of the core state
(partial). We demonstrate this for three different polytypes of SiC for
which we study the local Si
s+d and C p symmetric DOS. The occupied LPDOS is found to be very similar
although the
optical band gap varies by 50%. The advantages of SXE spectroscopy in
comparison to photoelectron
spectroscopy are demonstrated.
The insensitivity to sample charging and the bulk sensitivity of SXE are
exploited in the study of
the electronic structure of mass-selected CDs nanocrystals stabilized by an
organic ligand shell.
We find a strong variation of the electronic structure with the size of the
nanocrystals. Especially,
we show that the band gap opening is caused by nearly equal shifts of the
valence band maximum
and conduction band minimum.
The DOS interpretation of SXE spectra breaks down when the initial core
hole state is prepared
using photon absorption with energies close to an absorption threshold.
Absorption and emission
process have then to be treated as to be dependent on each other. For broad
band materials this
opens up a technique to study electronic band structures resolving the
local symmetry character
of the states. We demonstrate this for GaN and the three SiC polytypes. The
sensitivity of this
technique manifests itself in distinct differences between the spectra of
the three SiC polytypes.
For correlated materials, e.g., materials where the single electron picture
breaks down, the study
of the excitation energy dependence of the SXE spectra enables a detailed
analysis of the states
of different electronic configurations. This yields in information about
the different electron
interaction energies within the configuration of the initial and final
state of the emission process.
We demonstrate this for 4d -> 4f excitations in LaF_3. In addition, the
excitation energy
dependence of the emission spectra also offers an access to the dynamic of
the intermediate
state on the time scale of the core hole lifetime, typically in the
femtosecond time range. Within
a simple model we derive the shape of the centrifugal barrier localizing
the excited 4f electron in
the intermediate La3+ 4d^{-1}4f state.
Lüning, Jan
Soft X-ray spectroscopy of board band, size confined, and correlated materials
206 S., 1998
Soft x-ray emission (SXE) spectroscopy is a unique tool for the study of
the electronic structure
of matter as it is element specific, bulk sensitive, and not affected by
sample charging. The latter
properties result from the fact that the information about the electronic
structure is carried by photons
which in the soft x-ray energy range typically have a penetration length of
several tenths of a micron.
The element specificity stems from the atom specific binding energy of the
core level
involved: The initial state of an emission process is a shallow core hole
state (excited, e.g., in a
previous soft x-ray absorption (SXA) process). In the emission process a
less deeply bound
electron (e.g., a valence electron) fills the core hole, and the energy
difference between the initial
and final state is emitted as a photon. SXE spectroscopy is therefore the
tool to study insulating
materials, buried structures like interfaces and embedded clusters, or
compound systems.
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