The experimental techniques in use at the ALS fall into three broad categories: spectroscopy, microscopy/imaging, and scattering/diffraction. Click on a heading to learn more about these techniques at the ALS.
These techniques are used to study the energies of particles that are emitted or absorbed by samples that are exposed to the light-source beam and are commonly used to determine the characteristics of chemical bonding and electron motion.
In spectroscopy experiments, a sample is illuminated with light and the various product particles (electrons, ions, or fluorescent photons) are detected and analyzed. The unifying feature is that some “property” of a material is measured as the x-ray (photon) energy is swept though a range of values. At the most basic level, one measures the absorption, transmission, or reflectivity of a sample as a function of photon energy.
Probes that use the vacuum ultraviolet (VUV) region of the spectrum (10–100 eV) are very well matched to the elucidation of bonding in solids, surfaces, and molecules; to the investigation of electron–electron correlations in solids, atoms, and ions; and to the study of reaction pathways in chemical dynamics. At the lowest end of this energy range (below 1 eV) we have infrared, far-infrared, and terahertz spectroscopies, which are well matched to vibrational modes and other modes of excitation.
Soft x-ray spectroscopies employ the excitation of electrons in relatively shallow core levels (100–2000 eV) to probe the electronic structure of various kinds of matter. Elemental specificity is the watchword for this kind of spectroscopy. Each element has its own set of core levels that occur at characteristic energies. The photon-energy tunability of synchrotron radiation is essential.
Hard x-ray spectroscopy is applied in a wide variety of scientific disciplines (physics, chemistry, life sciences, and geology) to investigate geometric and electronic structure. The method is element-, oxidation-state-, and symmetry-specific. It is a primary tool in the characterization of new and promising materials. It is also used in the elucidation of dilute chemical species of environmental concern.
To see which ALS beamlines use spectroscopy techniques, go to the Beamline Directory and select “Spectroscopy” in the “ALL TECHNIQUES” drop-down menu.
To see examples of spectroscopy research at the ALS, click the blue button below.
These techniques use the light-source beam to obtain pictures with fine spatial resolution of the samples under study and are used in diverse research areas such as cell biology, lithography, infrared microscopy, radiology, and x-ray tomography.
The wavelengths of soft x-ray photons (1–15 nm) are very well matched to the creation of “nanoscopes” capable of probing the interior structure of biological cells and inorganic mesoscopic systems. Topics addressed by soft x-ray imaging techniques include cell biology, nanomagnetism, environmental science, and polymers. The tunability of synchrotron radiation is absolutely essential for the creation of contrast mechanisms. Cell biology CAT scans are performed in the “water window” (300–500 eV). Nanomagnetism studies require the energy range characteristic of iron, cobalt, and nickel (600–900 eV).
Mid- and far-infrared (energies below 1 eV) microprobes using synchrotron radiation are being used to address problems such as chemistry in biological tissues, chemical identification and molecular conformation, environmental biodegradation, mineral phases in geological and astronomical specimens, and electronic properties of novel materials. Infrared synchrotron radiation is focused through, or reflected from, a small spot on the specimen and then analyzed using a spectrometer. Tuning to characteristic vibrational frequencies serves as a sensitive fingerprint for molecular species. Images of the various species are built up by raster scanning the specimen through the small illuminated spot.
Lithography, a technique used in the art world for many centuries, has been adopted and adapted with phenomenal success by the high-tech industry. In microchip manufacturing, a silicon wafer is coated with a thin layer of photosensitive material called a resist. An image of a mask containing the desired pattern is projected onto the resist. The exposed (or unexposed) parts of the resist are etched away and, with further processing, the desired circuit is built up. The same basic process can be used in the manufacture of small mechanical components. Work at synchrotron light sources focuses primarily on the exposures of the resists.
To see which ALS beamlines use microscopy and imaging techniques, go to the Beamline Directory and select “Microscopy/Imaging” in the “ALL TECHNIQUES” drop-down menu.
To see examples of microscopy and imaging research at the ALS, click the blue button below.
These techniques make use of the patterns of light produced when x-rays are deflected by the closely spaced lattice of atoms in solids and are commonly used to determine the structures of crystals and large molecules such as proteins.
When a crystalline sample is illuminated with x-rays, the x-rays are scattered (diffracted) into very specific directions with various intensities. Detectors are used to measure this “diffraction pattern,” which is then processed by computers to deduce the arrangement of atoms within the crystal.
Hard x-rays have wavelengths comparable to the distance between atoms. Essentially everything we know about the atomic structure of materials is based on results from x-ray and neutron diffraction. From advanced ceramics to catalysts, from semiconductor technology to the frontiers of medicine, and from new magnetic materials and devices to framework compounds used to sequester radioactive waste, crystallography using hard x-ray diffraction techniques at synchrotron radiation facilities plays a crucial role in our ability to understand and control the world in which we live.
The scattering of x-rays from protein crystals is the most powerful method of determining the three-dimensional structure of large biological molecules (macromolecules). Because macromolecules are large and flexible, their crystals tend to be small, imperfect, and weakly diffracting. In many cases, the intensity, small beam size, and collimation of a synchrotron beam is vital for successful results.
Soft x-ray scattering techniques employ the excitation of electrons in relatively shallow core energy levels (100–2000 eV) to probe the electronic structure and other properties of various kinds of matter. The sample is illuminated with monochromatic soft x-rays, and the scattered photons are detected over a small angular range. In the elastic scattering mode, one measures the speckle diffraction pattern. In the inelastic mode, the scattered photons are passed through a spectrometer and analyzed.
To see which ALS beamlines use scattering and diffraction techniques, go to the Beamline Directory and select “Scattering/Diffraction” in the “ALL TECHNIQUES” drop-down menu.
To see examples of scattering and diffraction research at the ALS, click the blue button below.