X-ray absorption spectroscopy

This technique has the following projects (and possibly other techniques) related to it:

• Light induced processes in artificial reaction centers
• Steady-state synchrotron measurements

X-ray Absorption Spectroscopy

Despite its name, X-ray Absorption Spectroscopy (XAS) is essentially a form of electron spectroscopy. X-rays absorbed by matter excite and promote an absorbing atom's core electron to higher unoccupied states or into a free unbound state (the continuum). The latter is the photoelectric effect. At low X-ray energies of interest in this work (1-50 keV), the photoelectric mechanism is the dominant interaction between X-rays and atoms. The XAS technique has the advantage of being able to probe disordered and amorphous materials. It is capable of obtaining structural information in the immediate neighbourhood of the absorbing atom.

In practice one observes that the X-ray absorption rises very suddenly as soon as the X-ray energy is sufficient to reach the binding energy of particular electrons in a particular atom. The corresponding feature in an X-ray absorption spectrum is called an absorption edge. These edges are at different energies due to the different nuclear charges of each element and different orbitals involved. The whole spectrum typically covers a ~1 keV region, mostly on the high energy side of the absorption edge. The XAS spectrum can be aproximately divided into three regions, illustrated below. In practice there is not an obvious division of these regions and the XANES and NEXAFS regions are often modeled together.

XANES: X-ray Absorption Near Edge Structure
The energy of incoming X-rays is sufficient to transfer core electrons to higher unoccupied valence states. Analysis of this part of the spectrum (around the edge) provides information concerning the oxidation state of the absorbing atom and its site symmetry.

NEXAFS: Near Edge X-ray Absorption Fine Structure
The core electron is excited into the continuum, but the photoelectron has low kinetic energy and is strongly backscattered by all of the neighbouring atoms. This part of the spectrum is sensitive to the number, kind and symmetry of atoms adjacent to the absorber atom.

EXAFS: Extended X-ray Absorption Fine Structure
The energy of the photoelectron is high enough that its de Broglie wavelength becomes comparable to the distance to neighbouring atoms. The photoelectron wave is weakly backscattered by and among the neighbouring atoms. The analysis of this part of the absorption spectrum gives information regarding the number, kind and distances of neighbouring atoms from each other and the absorber.

Here we depict an energy level diagram showing excitation of core electrons, along with a cartoon of an X-ray absorption spectrum, observed at each of the illustrated transitions.

A neighbouring atom scatters the wave, resulting in interference at the origin. This can be constructive or destructive, depending on the interatomic distance and the photoelectron wavelength. Since the photoelectron wavelength is connected to the X-ray energy, there is a modulation of the likelihood of X-ray absorption at different energies.

Further reading and Techniques applied

General overview and motivation for X-ray based measurements
X-ray absorption spectroscopy (This page)
X-ray Diffraction and the argumentation for broad bandwidth

Overview and motivation for ultrafast X-ray measurements
Developments done on ultrafast X-ray sources in Lund
Developments on X-ray Detectors

Measurements done on ultrafast synchrotron user facilities
Steady state measurements done on synchrotrons