X-ray spectroscopy

X-ray spectroscopy is a powerful scientific method used to characterize materials. It uses X-ray radiation to study the atomic structure of a specimen. This technique allows scientists to identify which elements are present in a sample. By analyzing the radiation emitted, researchers can learn about the material's properties. It is a vital tool for understanding the arrangement of atoms in different substances. This method works for almost every element in the periodic table. The only exceptions are hydrogen, helium, and lithium.

The process begins by exciting the atoms within a specimen. This excitation can happen using a high-energy beam of charged particles. These beams might consist of electrons, such as in an electron microscope. They can also consist of protons or a beam of X-rays. When a photon hits an electron in an inner shell, that electron becomes excited. It moves from its original position to a higher energy level. Eventually, the electron returns to its low energy level. As it drops back down, it emits a photon. This photon has a specific wavelength that is uniquely characteristic of that element.

There are two primary techniques used to analyze these characteristic X-rays. The first is energy-dispersive X-ray spectroscopy, commonly known as EDS. In an EDS spectrometer, a semiconductor detector measures the energy of incoming photons. This method is very useful for scanning an entire spectrum to identify unknown elements. It is widely used in electron microscopes and portable XRF units. However, EDS can face challenges with resolution. Trace elements can sometimes create peaks that obscure the results. To keep the detector working well, it often requires cooling with liquid nitrogen or Peltier cooling.

The second main technique is wavelength-dispersive X-ray spectroscopy, or WDS. Unlike EDS, WDS is a method of sequential spectrum acquisition. This means it analyzes one peak at a time rather than the whole spectrum at once. A single crystal is used to diffract the photons according to Bragg's law. By moving the crystal and the detector, a wide region of the spectrum can be observed. WDS is slower than EDS and is sensitive to sample positioning. However, it offers much higher spectral resolution and sensitivity. This makes WDS superior for the accurate quantification of trace elements.

The history of this field is deeply connected to the Bragg family. William Lawrence Bragg and William Henry Bragg were pioneering scientists. This father-and-son team won the Nobel Prize in 1915. They used X-ray emission spectroscopy to investigate the structure of crystals. They used a cathode-ray tube or an X-ray tube to pass electrons through crystals of various elements. They even painstakingly produced diamond-ruled glass diffraction gratings for their instruments. Their work led to the development of Bragg's law. This law describes how X-rays are diffracted by a crystal.

Modern scientists use various sophisticated designs to detect X-ray spectra. One method involves grating spectrometers. In these devices, X-rays pass through a slit and hit optical elements like mirrors or gratings. These elements disperse the X-rays by diffraction based on their wavelength. Another design uses spherical grating mounts, which were devised by Henry Augustus Rowland. A spherical grating can both diffract and focus light using a single optical element. This is done using the Rowland circle geometry. In this setup, the entrance slit and the detector both lie on an imaginary circle. This allows for high spectral energy resolution without extra focusing optics.

Advanced research also utilizes resonant inelastic X-ray scattering, or RIXS. This occurs during a photon-in-photon-out process that acts like a scattering event. When the X-ray energy matches the binding energy of a core-level electron, the process is enhanced. Because orbital energies are widely separated, scientists can select a specific atom of interest. RIXS provides valuable information about the local electronic structure of complex systems. Today, intense and tunable X-rays are often generated using massive machines called synchrotrons. These tools continue to push the boundaries of how we understand the atomic world.