X-Ray Spectroscopy

BY- K. Sai Manogna (MSIWM014)

X-ray spectroscopy is a tool that detects and analyses photons with wavelengths in the X-ray section of the electromagnetic spectrum or particles of light. It is used to help scientists understand an object’s chemical and elemental properties. Many distinct X-ray spectroscopy techniques are used in science and technology, including archaeology, astronomy, and engineering. These approaches are used separately to construct a complete image of the substance or entity being studied. 

History:

1. In 1901, a German physicist, Wilhelm Conrad Roentgen, was awarded the first Nobel Prize in physics for the discovery of X-rays in 1895. 

2. According to the SLAC National Accelerator Laboratory, his new invention was rapidly put to use by other scientists and doctors. 

3. Between 1906 and 1908, Charles Barkla, a British physicist, conducted research that contributed to his discovery that X-rays could be typical of individual substances. He also received a Nobel Prize in physics for his work, but not until 1917. 

4. In fact, the use of X-ray spectroscopy started a bit earlier, in 1912, beginning with William Henry Bragg and William Lawrence Bragg, a father-and-son team of British physicists. 

5. To research how X-ray radiation interacted with atoms inside crystals, they used spectroscopy. 

6. By the following year, their method, called X-ray crystallography, had become the standard in the field, earning the Nobel Prize in physics in 1915. 

X-ray Absorption Spectroscopy (XAS) 

The absorbed photon’s energy lifts an electron from a deeply bound state into unoccupied bound states in x-ray absorption spectroscopy (XAS), or it gains enough energy to exit the atom. Thus, the absorption spectrum provides extensive information on the density of empty states and makes it possible to conclude coordination, the state of oxidation, and much more about the local structure. If the photon’s energy is sufficient to surpass the electron’s binding potential, the likelihood of absorption is affected by the mechanism of electron dispersion from the local atmosphere of the surrounding atoms. This system, called EXAFS, can be used to determine the local structure around the absorbed atoms. 

Instrumentation: 

Main parts of this instrument include:

  1. An X-ray source, 
  2. The sample holder, 
  3. An X-ray monochromator, and 
  4. A detector 

An X-ray is produced from the source by bombarding a heavy metal target with high energy electrons. The spectrum of the energy of the released X-rays influences the option of heavy metal. For instance, a tungsten (W) target generates X-rays of more incredible energy than a silver (Ag) target. Most of the energy that drives this bombardment process is lost as heat, so it is essential to cool the target electrode. More modern sources and other X-ray sources, such as the Stanford synchrotron beamline, are more effective. 

a. Source for X-ray: 

1. the X-ray source aims to supply the sample with X-ray radiation so that either X-ray Fluorescence or absorption experiments can be carried out. 

2. Atoms absorb X-rays from the source, and the wavelength of the absorbed X-ray in X-ray absorbance and the strength of that absorbance provides the identity of that atom, and concentration is consumed. 

3. This X-ray absorption causes the electron that absorbs the X-ray to be ionized. 

4. The atomic orbital electrons that absorb this light, in their orbitals, are very similar to the nucleus. 

5. Absorbed X-rays in X-ray fluorescence cause an atomic electron to be expelled, that is, atomic ionization, and the void is subsequently filled by an electron from an orbital further from the nucleus. 

6. The outer electron must emit energy in order to drop down to fill the hole. This emitted light is fluorescence from X-rays. 

b. Samples are exposed to X-rays: 

1. The monochromator in an X-ray instrument is very different from a wavelength grating or prism monochromator that is visible (400 to 700 nm) or UV (200 to 400 nm). 

3. Since the X-ray wavelengths are so short (say, 0.1 to 1 nm), it is not practical to scatter light using a prism or (grating) closely spaced grooves. Instead, contact with crystals of high purity is required; they function like a grating. 

4. The crystals are mounted on a movable stage at which the angle at which the incoming X-rays hit the crystal can be continuously and smoothly varied. 

5. The angle at which X-rays are detected to disperse from the crystal is also varied at twice the angle of entry. 

6. The crystal stage has rotated by 80 degrees in the picture below, so the detector stage has rotated by 160 degrees at this moment. 

7. By the collision of X-rays with high purity argon (Ar) gas, the detector mentioned here produces an electronic signal, an X-ray photon transducer. 

8. The collision causes Ar ionization and the development of free electrons that flow to a positive electrode. The detector’s current flux between the electrodes is proportional to the incoming X-rays: a signal, Voilà. 

c. Detector for X-ray: 

1. this double-stage rotation generates the X-ray spectrum with a wavelength on the x-axis and absorbance on the y axis as the detector signal is captured. 

2. Energy plotted for a fluorescence spectrum is on the x, and fluorescence emission is on the y axis. 

3. An absorbance spectrum is given below. K-edges are considered the shortest wavelength (highest energy) absorbance of elements studied by X-ray. 

4. Longer absorbances for wavelengths are L-edge, M-edge. The absorbance edge shape is very typical of the atoms involved in the absorbance when the function is closely examined. 

5. To assess the oxidation state of heavy metal atoms and whether the heavy metal atom is bound to carbon or hydrogen, modern K-edge X-ray spectra can be used. 

6. In other words, X-ray spectroscopy can be used to determine the chemical environment of heavy metal atoms in complex samples by spectral fitting to the available specifications. 

7. The atmosphere here means the environment for atomic bonding. 

:

One of the pioneers who helped in the production of X-ray emission spectroscopy was Karl Manne Georg Siegbahn from Uppsala, Sweden (1924 Nobel Prize). He painstakingly developed numerous diamond-ruled glass diffraction gratings for his spectrometers (also called X-ray fluorescence spectroscopy). He measured high precision X-ray wavelengths of several elements, using high-energy electrons as a source of excitation. 

With synchrotrons, intense and wavelength-tunable X-rays are now usually produced. In a material, relative to the incoming beam, the X-rays can suffer a loss of energy. This energy loss of the re-emerging beam reflects the atomic system’s internal excitation, an analogous X-ray to the well-known Raman spectroscopy typically used in the optical field. 

Highly accelerated electrons are bombarded with a piece of metal wire called an anticathode. The metal piece becomes a source of radiation from X-ray. With a crystal spectrometer, this radiation can be analyzed. 

The spectrum of emissions is composed of two parts: 

(a) Continuous spectrum 

(b) Line spectrum 

It consists of a line spectrum with a continuum of history. X-ray fluorescence generates X-radiation that only has a line spectrum without a continuous spectrum background. 

(a). Continuous spectrum: 

1. The continuous spectrum depends little on the metal used for the anticathode; with the increase of the metal’s Z, the curve’s height increases, but the curve’s form is independent of z. νmax is entirely independent of the anticathode metal used. 

I (v) = constant Z (vmax– v)

2. The curve depends heavily on the voltage V used for electron acceleration. 

3. With the voltage V, the maximum frequency increases proportionally. 

eV = hvmax 

4. Since the continuous spectrum is highly dependent on the velocity of the incident electron, it can be concluded that these electrons emit the corresponding X radiation. 

Classical Explanation: 

1. They are subjected to intense electrostatic forces arising primarily from the nuclei of the constituent atoms as electrons traveling at high velocities enter the anticathode. 

2. The electron is enormously accelerated, and the electrostatic charges emit electromagnetic waves, according to classical radiation theory, and the higher the acceleration, the higher the frequency. 

3. It is the sudden slowing down of the electrons responsible for the continuous spectrum when they penetrate the anticathode; this can be defined as deacceleration radiation, but the German term Bremsstrahlung is also used. 

The characteristics: 

1. The spectrum of the line primarily depends on the material from which the X-rays come, either the X-ray tube anticathode or the absorbing material used in a fluorescence experiment. 

2. The spectral lines’ frequencies are independent of the electron-accelerating voltage and the incident radiation frequency. It only depends on the chemical components of which the substance is composed. 

3. The frequencies are properties of the chemical elements’ atoms.

Several Applications :

In science and technology fields, including archaeology, astronomy, engineering, and health, X-ray spectroscopy is used today. 

– By studying them with X-ray spectroscopy, anthropologists and archaeologists can reveal secret knowledge about the ancient artifacts and remains they discover. For example, to determine the sources of obsidian arrowheads produced by prehistoric people in the North American Southwest, Lee Sharpe, associate professor of chemistry at Grinnell College in Iowa, and his colleagues used a tool called X-ray fluorescence (XRF) spectroscopy. 

– X-ray spectroscopy also allows astrophysicists to learn more about how space phenomena function. 

– Researchers at Washington University, for instance, are preparing to observe X-rays that come from interstellar phenomena, such as black holes, for the future prospectus. 

– The team, led by an experimental and theoretical astrophysicist, Henric Krawczynski, is preparing to launch a form of X-ray spectrometer called an X-ray polarimeter. 

– The instrument will be suspended in the Earth’s atmosphere by a long-term, helium-filled balloon beginning in December 2018. 

– Yury Gogotsi, a chemist and materials engineer at Drexel University in Pennsylvania, uses materials analyzed by X-ray spectroscopy to create spray-on antennas and water desalination membranes. 

– The invisible spray-on antennas are only a few hundred nanometers thick but can relay radio waves and steer them. 

A technique called X-ray absorption spectroscopy (XAS) ensures that the fragile material composition is right and helps assess the conductivity. To study the surface chemistry of complex membranes that desalinate water by filtering out particular ions, such as sodium, Gogotsi, and his colleagues also use X-ray spectroscopy. 

– X-ray spectroscopy can also be used in various medical research and practice areas, such as modern CT scanning machines. 

– According to Phuong-Anh T. Duong, Director of CT at Emory University Department of Radiology and Imaging Science, Phuong-Anh T. Duong, Director of CT at Emory University Department of Radiology and Imaging Science, Collecting X-ray absorption spectra during CT scans (via photon counting or spectral CT scanner) may provide more accurate information and contrast on what is going on inside the body, with lower radiation exposures from the X-rays and fewer or no need to use contrast materials (dyes)

X-rays advantages

a. Cheapest

b. most convenient and commonly used tool.  

c. X-rays are not absorbed by air, so the specimen does not have to be in an evacuated chamber. 

Disadvantages of X-rays: 

With lighter elements, they do not interact very strongly.

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