We are all theoreticians, engaged in several fields of solids state
physics. The electronic structure in that or other form is a common
denominator of our activities. As concerns
x-ray spectroscopy, we have been involved in three areas so far:
X-ray absorption spectroscopy (XAS):
X-ray absorption spectroscopy means that you irradiate the sample
with x-rays and measure the dependence of the absorption coeficient
on the energy of the incoming radiation. The dominant process contributing
the the photoabsorption on x-rays in solids is the
photoefect: A core electron absorbs the photon and is exited into
the unoccupied states energy region. There is a steep rise in the
absorption coeficient whenever the energy of incoming x-rays is high
enough to excite another core electron. Such steep jumps are called absorption edges. The energy of the ejected photoelectron
as measured from the bottom of the conductin band is just zero at the
edge. The energy dependence of the absorption
coeficient above the absorption edge exhibits fine oscillations -
spectroscopists call it fine structure. This fine structure
reflects angular-projected local density of unoccupied electron
states in the solid under consideration. Our interest lies predominantly in the near-edge region of the absorption
spectrum (XANES, a.k.a.
NEXAFS) - it covers the region of photoelectron
energies less than, say, 50 eV.
We participated in interpretation of XANES spectra of several compound.
The computational techniques we employ comprise the real-space multiple-scattering
(RS-MS) formalism for calculating electronic structure of finite clusters
and the pseudopotential method for calculating band-structure of solids.
Our RS-MS calculations rely on the RSMS code, which evolved from the
DLXANES/ICXANES
code via gradual and never-ending corrections, amendments
and extensions. You may read something more about our code here
one day (sooner or later).
X-ray emission spectroscopy (XES):
In x-ray emission, the electron goes just just the opposite way
as in the x-ray absorption: It jumps from an occupied valence level
into a core state (which had to be made vacant before). The more
valence electrons have certain energy, the higher intensity
of the emitted radiation of the coresponding energy will be
observed. Hence, we are mapping occupied states in this way.
As in the case of x-ray absorption, this mapping is selective with
respect to the chemical type and angular-momentum symmetry. Calculating and interpreting x-ray emission spectra of solids has
got a long tradition in our group. Historically, this was the first research
activity this group got engaged to. We still somehow cannot just say
good-bye to this old love of ours.
In (x-ray) bremsstrahlung isochromat spectroscopy, you shoot
electrons from an electron gun into a piece of material and measure
the bremsstrahlung radiation which is generated during the
deceleration of
those electrons inside the solid sample. By varying the energy of
incoming electrons and simultaneously measuring the intensity of
bremsstrahlung radiation of a fixed energy (hence "isochromat"
spectroscopy), you perform an energy scan of the low-lying unoccupied
electron states. If the energy difference between the incoming and
decelerated electrons (=isochromat energy) is high, the bulk of the
deceleration takes place in the vicinity of nuclei and, thus, the
process is governed by essentially the same physics as x-ray
absorption spectroscopy. We got involved into the BIS business by applying a local XAS-like
approach to the calculation of BIS spectra.
The x-ray bremsstrahlung isochromat spectroscopy has a limited
potential in comparison with XAS (most notably, it is not
element-specific), nevertheless, it can be helpful
in certain systems and/or situations. Apart of that, we find it
quite interesting that such a seemingly diffuse process like
deceleration of electrons is solids can be described within a
local framework. This is possibly only due to the high energy
loss of the incoming electrons (typical isochromat energies are
1487 eV or 5415 eV). That means that one can hardly expect
the local description to work for, e.g., inverse UV photoemission.
Other areas of scientific interest of ours
For some of us, x-ray spectroscopy is the most important research
field. For others, it is just a secondary issue. Other research
activities of ours (well, maybe it is more fair to say just
research interest) include, e.g.,
all-electron approach to electronic structure calculation
via pseudopotential formalism,
core level binding energies and core level shifts,