X-ray absorption spectroscopy has become an essential tool while studying the morphological details of chemical substances. X-rays are short-wavelength electromagnetic radiations. They are used in various ways to analyze the structural arrangements. They are high energy, so, they can penetrate deep up to core electrons to excite them to a higher-levels. That absorbed energy is calculated and used for further calculations.
Principle of X-ray absorption Spectroscopy
X-ray spectroscopy works on the principle of the excitation of core electrons that are orbiting in the lower shell(s). As the electron absorbs x-rays, it becomes excited and jumps to a higher level. The X-ray region used ranges from 1 to 100 nm. When x-rays interact with electrons it excites electrons to the higher levels. Energy absorbed by the electrons has a characteristic value for each element one can distinguish with the X-ray absorption spectrum.
Instrumentation of XAS
The main components of the x-ray absorption spectrophotometer are given below:
X-ray Generator or X-ray source
The electron beam when strikes the metal plate knocks out the core electrons. In order to fill the unoccupied spaces, higher energy electron releases their energy and goes down. The energy released is in the range of 0.1 -100nm wavelength. These short wavelengths and high-energy waves are called X-rays.
Monochromator
The x-ray produced by the source has a series of wavelengths that are separated or filtered by a monochromator. This makes sure that only a single, identical wavelength approaches the target as required.
Collimator
It has the property to align and narrow down the beam up to 0.1mm in diameter. The main reason to use a collimator is to focus the crystal more precisely.
Sample holder
In x-ray techniques, the sample is always a single crystal. A pallet maker press is used for powdered samples. In the case of liquids, pans are available with cling film to give a uniform shape to a sample.
X-ray Detectors
X-rays, after striking the sample analyte, are absorbed and the remaining part is reflected towards detectors. The X-rays can be detected by:
i) Photomultiplier tube
When the light signal falls upon these detectors, it is converted into electric signals and enhanced thousands of times to be easily readable at the absorption spectrum.
ii) Semiconductor diode
These are semiconductor diodes that contain electron-hole pairs. When light falls on them, they change their ratio. This change may be converted into a pulse.
Absorption spectra of XAS
The spectrum of XAS is simple to understand and can be used for quantitative illustrations. The graph is obtained between intensity and the energy of emitted radiations in ev. The graph has unique curves for all possible excitations in an atom of the analyte.
Other Types of X-Ray spectroscopy
- X-ray absorption spectroscopy (XAS)
- X-ray emission spectroscopy
- Energy dispersive spectroscopy
- Wavelength dispersive spectroscopy
- X-ray photoelectron spectroscopy
- Resonant inelastic x-ray scattering
- X-ray Raman spectroscopy
- Auger electron decay
Advantages and applications of XAS
It can detect elements specifically based upon their core levels and the graph is well separated, and have high resolution. Therefore, it can be used for both qualitative and quantitative analysis. It can also be used to determine the structure and orientations of the solids in metal alloys. Moreover, it can tell us about the transition that happened during the electron jumping and coming back.
X-ray absorption spectroscopy is more sensitive when studying the structure and it a widely used in explorations. For example, the study of amorphous solids in metallurgy.
Concepts Berg
What is the principle of X-ray absorption?
X-rays are high-energy beams that go deep into the atoms and interact with core electrons and make them excited to jump to higher energy levels.
What does X-ray spectroscopy do?
X-ray spectroscopy is a very important and versatile technique to study the solid and liquid morphology without damaging the properties of the analyte.
What is the difference between XAS and XPS?
X-ray absorption spectroscopy is the technique in which X-rays are directed on a sample and the core electrons absorb these radiations. X-ray photoelectron spectroscopy is different in the sense that X-rays are directed on the sample interact with the inner core electrons and when they came back to the original position emits characteristic wavelengths.
Can X-ray be absorbed?
Yes, X-rays can be absorbed. The energy required for core electrons to excite to a higher level is in the range of X-rays.
What are some applications of X-ray absorption spectroscopy?
X-rays are nowadays used in almost every scientific study to analyze the structure. In medical treatment, X-ray imaging is used to find the bones fractures, and cracks.
What is the main difference between X-ray fluoroscopy XRF and X-ray absorption spectroscopy XAS?
The main difference between X-ray fluorescence and X-ray absorption spectroscopy is the type of detectors used in these techniques.
Reference Books
- X-Ray Absorption Spectroscopy of Semiconductors edited by Claudia S. Schnohr, Mark C. Ridgway
- X-Ray Absorption and X-Ray Emission Spectroscopy: (Theory and applications) By Jeroen A. van Bokhoven, Carlo Lamberti
Reference Links
- X-ray Spectroscopy (iaea.org)
