X-ray emission spectroscopy

Introduction

In X-ray emission spectroscopy (XES), X-ray line spectra are measured with a spectral resolution sufficient to analyze the impact of the chemical environment on the X-ray line energy and on branching ratios. This is done by exciting electrons out of their shell and then watching the emitted photons of the recombinating electrons.

It is possible to differentiate between:

• non-resonant XES (XES)

1. $K_{\beta }$-measurements
2. Valence-to-Core (VtC/V2C)-measurements
3. ($K_{\alpha }$)-measurements

• resonant XES (RXES oder RIXS)

1. XAS+XES 2D-measurements
2. high-resolution XAS
3. 2p3d RIXS
4. Mössbauer-XES-combinated measurements

Fig.1: K-Beta Mainline and V2C

The first XES experiments were published by Lindh and Lundquist in 1924 In these early studies, the authors utilized the electron beam of an X-ray tube to excite core electrons and obtain the $K_{\beta }$-line spectra of sulfur and other elements. Three years later, Coster and Druyvesteyn performed the first experiments using photonexcitation. Their work demonstrated that the electron beams produce artifacts, thus motivating the use of X-ray photons for creating the core hole. Subsequent experiments were carried out with commercial X-ray spectrometers, as well as with high-resolution spectrometers.

While these early studies provided fundamental insights into the electronic configuration of small molecules, XES only came into broader use with the availability of high intensity X-ray beams at synchrotron radiation facilities, which enabled the measurement of (chemically)dilute samples. In addition to the experimental advances, it is also the progress in quantum chemical computations, which makes XES an intriguing tool for the study of the electronic structure of chemical compounds.

Energy Level Diagramm K-Lines

History

Henry Moseley, a british physicist was the first to discover a relation between the $K_{\alpha }$-lines and the atomic numbers of the probed elements. This was the birth hour of modern x-ray spectroscopy. Later these lines could be used in elemental analysis to determine the contents of a sample.

William Lawrence Bragg later found a relation between the energy of a photon and its diffraction within a crystal. The formula he established, $n\lambda =2d\,\sin(\theta )$ says that an X-ray photon with a certain energy bends at a precisely defined angle within a crystal.

Analyzers

A special kind of monochromator is needed to diffract the radiation produced in X-Ray-Sources. This is because X-rays have a refractive index n ≈ 1. Bragg came up with the equation that describes x-ray/neutron diffraction when those particles pass a crystal lattice.(X-ray diffraction)

For this purpose "perfect crystals" have been produced in many shapes, depending on the geometry and energy range of the instrument.

In the Von Hamos geometry, a cylindrically bent crystal disperses the radiation along its flat surface’s plane and focuses it along its axis of curvature onto a line like feature. The spatially distributed signal is recorded with a position sensitive detector at the crystal’s focusing axis providing the overall spectrum. Alternative wavelength dispersive concepts have been proposed and implemented based on Johansson geometry having the source positioned inside the Rowland circle, whereas an instrument based on Johann geometry has its source placen on the Rowland circle.