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Research

Glossary

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.

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!

Brief definitions:

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 spectroscopy.

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 the material.

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.