Organic molecular semiconductors
Dilute magnetic semiconductors
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Synchrotron radiation and the x-ray spectroscopies that it enables is a
fascinating areas of modern physics research which can touch on the most basic
and fundamental physical questions as well as addressing issues related to
modern technology and specifically the materials which enable this technology.
Only the briefest definitions of my research 'tools' are given here, for more
complete definitions why not start with wikipedia !
One of my students wished this book existed, but there are in fact some great books out there not necessarily covering all of these spectroscopies.
I would recommend this one
for all x-ray absorption spectroscopies, this one
for an introduction to XPS before moving on to this one
and then finally this one
for almost everything else.
- X-ray sources : - Synchrotrons
Laboratory based x-ray sources are not practical for the study of
processes such as x-ray absorption or x-ray emission or any resonant
spectroscopy. This is due to two factors: the fixed wavelength of such
sources and the low intensity that they typically provide. Laboratory
sources are however
suitable for x-ray photoemission spectroscopy. Tunable, high intensity
x-ray sources are available and are known as synchrotron radiation sources. High
intensity undulator beamlines are most suited to x-ray emission
- X-ray Spectroscopy:
See X-ray spectroscopy.
- XAS = X-ray absorption spectroscopy:
(An inclusive term which may refer to NEXAFS, to XANES and also EXAFS though each has a ).
This spectroscopy involved tuning the incident x-ray photon energy
through an x-ray absorption edge and recording the variation of intensity with photon energy, the XAS spectrum, via either the electron
yield or the fluorescence yield. The XAS absorption process in the soft
x-ray regime, close to the x-ray absorption edge, is a dipole excitation where the electron is promoted from
the core level to the conduction
band and the intensity reflects the electronic structure and is senstive to bond orientation (i.e. NEXAFS). The recorded absorption spectrum (often) reflects the conduction band
density of states, but not always due to atomic multiplet effects for e.g. transition metal L-edges. In the hard x-ray region, analysing the XAS signal away from the edge gives structural information particularly when applied to the metal K edges e.g. in EXAFS.
- XES = X-ray emission spectroscopy:
See soft X-ray emission spectroscopy.
When a core hole is created through absorption of an x-ray photon by a
core electron, that core hole rapidly becomes filled again. This core
hole can be filled in two ways. The first is where an electron in a
higher level fills the hole and transfers the released energy to other
electrons as kinetic energy, these electrons then leave the atom and
are known as Auger electrons. This non-radiative process thus gives
rise to Auger electron spectroscopy. Alternately, an electron may fill
the core hole and the released energy is emitted as electromagnetic
radiation. The most probable radiative process is a dipole transition.
When the core hole that has been created may be linked by a dipole
transition to the states in the valence band of the material, then the
emitted radiation forms a spectrum rather than a single emission
feature. This emission spectrum then reflects the partial density of
states of the valence band. For example: on creation of an oxygen 1s core hole in a metal oxide
material, the emitted spectrum then reflects the oxygen 2p partial density of states. Thus
soft x-ray emission can be used as a complementary probe to soft x-ray
absorption to obtain the partial density of states of a material.
- XPS = X-ray photoemission
spectroscopy : When a core hole is created through absorption of
an x-ray photon, the photoemitted electron can escape the material with
a kinetic energy equal to the difference between the photon energy and
the binding energy of the core electron. The variation of the
spectrum of recorded kinetic energies of the photoelectrons can reveal
differences in binding energy for the same core level but differing
depending on the local chemical environment within the material. This
information is vital for a complete interpretation of the results
obtained from the other spectroscopies listed here.
- RXES = Resonant soft x-ray
emission spectroscopy : When the incident x-ray photon energy is tuned to the
threshold of an absorption edge then the soft x-ray emission resulting
from the decay of the core hole may differ considerably from the
emission spectrum recorded with incident energies high above the
threshold. These differences can be due to site selectivity where the
core hole is created only on one of a number of differing atoms of the
same element within the material. This may be due to differing binding
energies for the core electrons in these different chemical
environments and is revealed by x-ray photoemission spectroscopy. When
either the binding energies are the same or there is the same chemical
environment then the differences in x-ray emission can be due to state
selectivity. This is where a given excitation energy corresponds to a
particular state in the conduction band (particular feature in the soft
x-ray absorption spectrum) and the resultant core hole is of a given
symmetry such that it couples through dipole transitions with only some
of the states in the valence band.
- RIXS = Resonant inelastic x-ray
scattering : Apart from the resonant soft x-ray emission
described above one can also have resonant elastic and inelastic x-ray
scattering. Elastic x-ray scattering occurs where the core electron is
excited from an initial to an intermediate state and immediately
returns to the initial state with the emission of an x-ray of the same
energy as the incident x-ray photon. Inelastic scattering occurs where
the final state is not the same as the initial state and the emitted
x-ray differs in energy from the incident x-ray according to the
difference in energies between the initial and final states. This is
described fully by the Kramers-Heisenberg equation. The low energy
electronic excitations probed in this manner are frequently
inaccessible through optical spectroscopies due to parity
considerations. These parity considerations are bypassed due to two
succesive dipole transitons. The origins of these low energy electronic
excitations can be of particular importance in transition metal oxide
materials where these low energy electronic transitions have great
consequences in terms of the electronic and/or magnetic properties of
- XMCD :
XMCD is a difference spectrum of two x-ray absorption spectra (XAS) taken in a magnetic field, one taken with left circularly polarized light, and one with right circularly polarized light. By closely analyzing the difference in the XMCD spectrum, information can be obtained on the magnetic properties of the atom, such as its spin and orbital magnetic moment.
In the case of transition metals such as iron, cobalt, and nickel, the absorption spectra for XMCD are usually measured at the L-edge. This corresponds to the process in the iron case: with iron, a 2p electron is excited to a 3d state by an x-ray of about 700 eV. Because the 3d electron states are the origin of the magnetic properties of the elements, the spectra contain information on the magnetic properties.