X-ray emission spectroscopy

History

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

β

{\displaystyle K_{\beta }}

-line spectra of sulfur and other elements. Three years later, Coster and Druyvesteyn performed the first experiments using photon excitation. 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 and 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, there has been progress in quantum chemical computations, which makes XES an intriguing tool for studying the electronic structure of chemical compounds.

Henry Moseley, a British physicist, was the first to discover a relation between the

K

α

{\displaystyle 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,

Equipment

Analyzers

A special 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. Although they are called perfect, there are miscuts within the crystal structure, which leads to offsets of the Rowland plane.

These offsets can be corrected by turning the crystal while looking at a specific energy (for example:

K

α

2

{\displaystyle K_{\alpha 2}}

-line of copper at 8027.83 eV).

When the signal intensity is maximized, the photons diffracted by the crystal hit the detector in the Rowland plane. The instrument's horizontal plane will now have a slight offset, which can be corrected by increasing or decreasing the detector angle.

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. In contrast, an instrument based on Johann geometry has its source placed on the Rowland circle.

X-ray sources

X-ray sources are produced for many different purposes, yet not every X-ray source can be used for spectroscopy. Commonly used sources for medical applications generally generate very "noisy" source spectra because the used cathode material must not be very pure for these measurements. These lines must be eliminated as much as possible to get a good resolution in all used energy ranges.

For this purpose, normal X-ray tubes with highly pure tungsten, molybdenum, palladium, etc., are made. Except for the copper they are embedded in, they produce a relatively "white" spectrum. Another way of producing X-rays are particle accelerators. They produce X-rays from vectorial changes in their direction through magnetic fields. Whenever a moving charge changes direction, it has to give off radiation with the corresponding energy. In X-ray tubes, this directional change is the electron hitting the metal target (anode). In synchrotrons, the outer magnetic field accelerates the electron into a circular path.

There are many X-ray tubes, and operators must choose accurately depending on what should be measured.

Modern spectroscopy and the importance of Kβ-lines in the 21st Century

Today, XES is less used for elemental analysis. Still, more and more measurements of

K

β

{\displaystyle K_{\beta }}

-line spectra find importance, as the relation between these lines and the electronic structure of the ionized atom becomes more detailed.

If a 1s-Core-Electron gets excited into the continuum (out of the atoms' energy levels in MO),

electrons of higher energy orbitals need to lose energy and "fall" to the 1s-Hole that was created to fulfil Hund's Rule (Fig. 2).

Those electron transfers happen with distinct probabilities (see Siegbahn notation).

Scientists noted that after an ionization of a somehow bonded 3d-transition metal-atom, the

K

β

{\displaystyle K_{\beta }}

-lines intensities and energies shift with oxidation state of the metal and with the species of ligand(s). This gave way to a new method of structural analysis:

High-resolution scans of these lines can determine the exact energy level and structural configuration of a chemical compound.

If we ignore every transfer that does not affect valence electrons, there are only two major electron transfer mechanisms.

Soft X-ray emission spectroscopy

Soft X-ray emission spectroscopy (SXES) is an experimental technique for determining the electronic structure of materials.

Uses

X-ray emission spectroscopy (XES) probes the partially occupied density of a material's electronic states. XES is element-specific and site-specific, making it a powerful tool for determining detailed electronic properties of materials.

Forms

Emission spectroscopy can take the form of either resonant inelastic X-ray emission spectroscopy (RIXS) or non-resonant X-ray emission spectroscopy (NXES). Both spectroscopies involve the photonic promotion of a core level electron and the measurement of the fluorescence that occurs as the electron relaxes into a lower-energy state. The differences between resonant and non-resonant excitation arise from the atom's state before fluorescence occurs.

In resonant excitation, the core electron is promoted to a bound state in the conduction band. Non-resonant excitation occurs when the incoming radiation promotes a core electron to the continuum. When a core hole is created this way, it can be refilled through several decay paths. Because the core hole is refilled from the sample's high-energy free states, the decay and emission processes must be treated separately dipole transitions. This contrasts with RIXS, where the events are coupled and must be treated as a single scattering process.

Properties

Soft X-rays have different optical properties than visible light, and therefore, experiments must take place in ultra high vacuum, where the photon beam is manipulated using special mirrors and diffraction gratings.

Gratings diffract each energy or wavelength present in the incoming radiation in a different direction. Grating monochromators allow the user to select the specific photon energy they wish to use to excite the sample. Diffraction gratings are also used in the spectrometer to analyze the photon energy of the radiation emitted by the sample.