Day 7 - Spectroscopy

It's day 7 of the Diamond Industrial Liaison advent calendar. Today is all about spectroscopy, another of the X-ray techniques we offer researchers here at Diamond.

X-ray spectroscopy is a powerful tool for the determination of local atomic structure in materials that are not necessarily characterised by crystalline order. Spectroscopy can therefore be applied to materials whose constituents, such as atoms, molecules or ions, are not arranged in a highly ordered microscopic structure and do not form a crystal lattice that extends in all directions, creating a very powerful direct probe of chemical environments.

Atoms and molecules have unique spectra. These spectra can be used to detect, identify and quantify information about the atoms and molecules in a sample; in particular elemental composition, chemical state and physical properties of both inorganic material and biological systems. By sweeping through a range of photon energies and measuring the response, the absorption, reflectivity or fluorescence of the sample is measured.

Spectroscopy allows researchers to gain valuable information about the internal consituents of a sample and how they may change over varying conditions such as time, temperature or pressure.

When a beam of white light strikes a triangular prism it is separated into its various components (ROYGBIV). This is known as a spectrum.

The electromagnetic spectrum extends from below the low frequencies used for modern radio communication to gamma radiation at the short-wavelength (high-frequency) end, thereby covering wavelengths from thousands of kilometers down to a fraction of the size of an atom. Visible light lies toward the shorter end, with wavelengths from 400 to 700 nanometres.  

Nearly all types of electromagnetic radiation can be used for spectroscopy to study and characterise matter. When using X-rays the photoelectric effect occurs.

The photoelectric effect refers to the emission, or ejection, of electrons from the surface of a sample in response to electromagnetic radiation, or light. Energy contained within the electromagnetic radiation is absorbed by electrons within the sample, giving the electrons sufficient energy to be 'knocked' out of, that is, emitted from, the sample. 

In the X-ray region atoms absorb X-rays sharply at certain wavelengths (called absorption edges) that are characteristic of that particular atomic species. So if you shine a particular frequency, or energy, of X-rays onto a sample, you can control the type of electron that is emitted.

The process occurs as follows:

1. The core level electron absorbs the energy from the X-ray;

2. If sufficient energy is provided, the electron is ejected from the atom forming a photoelectron which affects neighbouring atoms;

3. The atom is left in excited state with an empty electronic level i.e. core hole.  
 

An absorption graph is produced which provides researchers with element-specific information from the sample. This can include which elements are present, where they are located, the density of the element, what are the neighbouring atoms and the distance between those atoms.

At Diamond, we employ three main techniques in the field of spectroscopy:

• X-ray Absorption Spectroscopy
• X-ray Emission Spectroscopy
• X-ray Fluorescence

A range of X-ray Absorption Spectroscopy techniques are available at Diamond, including X-ray Absorption Near-Edge Structure (XANES), Extended X-ray Absorption Fine Structure (EXAFS), Resonant Inelastic X-ray Scattering (RIXS) and X-ray Emission Spectroscopy (XES).

At characteristic wavelengths the X-ray absorption of an element changes dramatically, these are called absorption edges. XAS is a technique that can be divided into XANES - near the absorption edge, the spectra may contain fine structure that reveals the electronic and geometrical environment of the absorbing atom - and further from the edge EXAFS reveals the local atomic environment to the element. Both can be used to follow reactions on timescales down to the millisecond. 

Benefits of Synchrotron Techniques

• The high intensity of synchrotron X-rays make it possible to detect elements present in very low concentrations.
• The tuneability of synchrotron radiation allows XAS spectra of virtually all the elements to be collected in short time periods.
• Combining the high brightness with the small spot size available makes it possible to examine samples with very high spatial resolution.

Useful for

X-ray absorption spectroscopy is important in gaining structural understanding of a range of materials, including catalysis, biomaterials, novel materials with special electronic properties such as superconductivity, dilute species in fluids, and complex inhomogeneous materials.

It can provide information on bio-remediation processes, study minute minerals returned from space missions and be used to understand chemical reactions such as heterogeneous catalysis and hydrothermal synthesis of industrial materials.

Beamlines

B18 - I08 - I18 - I20

X-ray emission spectroscopy (XES) is one of the so-called "photon-in - photon-out" spectroscopies in which a core electron is excited by an incident X-ray photon and then this excited state decays by emitting an X-ray photon to fill the core hole. The energy of the emitted photon is the energy difference between the involved electronic levels. The analysis of the energy dependence of the emitted photons is the aim of the X-ray emission spectroscopy.

Benefits of synchrotron techniques

The high resolution available at a synchrotron opens up the possibility of novel studies of phase transitions in large macromolecules as a function of temperature, leading to improved methods for the preparation of large single crystals. In addition, the high intensity X-rays can probe more deeply into the sample than laboratory techniques, and the use of resonant diffraction allows complex structures with low "normal" electron contrasts to be studied.

XES is a technique complementary to X-ray absorption spectroscopy (XAS) that provides valuable information with respect to the electronic structure (local charge- and spin-density) as well as the nature of the bound ligands.

Useful for

Catalysts - direct studies of the structure and interactions of catalysts with chemical reagents under rapidly changing environmental conditions – three-way catalysts, fuel cells, photochemical processes and solution chemistry.
Environmental -  metal speciation of toxic materials to handle the remediation of environmental contamination, processes used for the disposal of toxic materials, studying rocks, soils, sediments, plant materials, pollutants and radioactive waste issues in climate change.
Material Science - samples under realistic conditions of high pressures and temperatures, kinetic processes in operating electrochemical cells, design and characterisation of novel, advanced materials.
Biology - determination of the structure of metalloproteins, studying biochemical processes such as the life mechanisms of photosynthesis or respiration.

Beamlines

I20   

X-ray Fluorescence (XRF) occurs when the inner shell electrons of atoms in the sample get excited by the X-rays and subsequently release X-ray photons when the system relaxes, that is when the electrons transition from the higher energy levels of the atom to the vacant inner shell.

The beauty of this process is that each secondary X-ray photon (sometimes called characteristic radiation) emitted from the sample has a specific energy which is a fingerprint of the atom from which it has originated. By measuring the energy of the secondary photons it is possible to establish the elemental composition of the sample at the point where the X-ray beam hits the sample. Typically a special type of detector called energy-dispersive detector is used to precisely measure the energy of each photon. The plot of the number of photon counts versus their energy, the X-ray spectrum, typicallly shows a number of peaks which are directly associated with specific elements, so by just glancing at the spectrum it is possible to quickly deduce which elements are present in the sample.

Shifts in peak position are used to measure internal strains. By scanning the sample through the beam, a 2D or 3D map of strains in the sample can be produced.

Benefits of synchrotron techniques

The high energy of X-rays from a synchrotron allows the technique to be used on thick samples, such as real engineering components. The high intensity reduces data collection times, so larger samples can be scanned to map internal strains. The great strength of this technique is that it allows for non-destructive testing of materials.

Useful for

X-ray fluorescence spectroscopy can be used to study the chemical composition of virtually anything and is becoming an advanced and essential analytical technique in life and environmental sciences, medical applications, archaeological and cultural heritage applications, forensic chemistry, industrial applications, and earth and planetary sciences. Some of the research taking place at synchrotrons has helped scientists to study historical artefacts, develop highly nutritious food, and investigate Alzheimer’s disease.

Beamlines

I18 - I08