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Seeing the Invisible

For thousands of years, our knowledge of the world around us was limited to objects we can see with our eyes and more recently with optical microscopes. However, the wish to see what nature denies the human eye has ever been the aspiration of research. The success of an investigation to make the invisible visible depends, among other things, on how light of the correct wavelength and a sufficient intensity can be focused on to a specimen.

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Diagram of electron ejection from the innermost shell of an atom via x-ray probe at the Advanced Photon Source (APS). (Credit: Argonne National Laboratory)

During the last four decades, the growth of light sources that are bright and wavelength-selectable (tunable) has markedly expanded the scope of investigation of the structure of matter in the most general sense. Structure can mean the positions of the atoms (atomic structure), the behavior of the electrons around the atomic nuclei (electronic structure), and the spectrum of atomic vibrations in molecules and solids. Magnetic structure is another category in the broad structure spectrum. Composition (what atoms and compounds are present) is yet another kind of generalized structure.

The atomic structures of solid materials span the extremes from completely ordered with atoms arrayed around the points of a repeating lattice to completely disordered, akin to the arrangements in liquids but frozen in place. To “see” atoms, which have dimensions of the order of a tenth of a nanometer (which is one billionth of a meter), we need to use a form of light that has a much shorter wavelength than visible light. As a general (but not invariable) rule, short-wavelength (hard) x rays are most useful for probing atomic structure. As a result, since their discovery by Röntgen in 1895, they have been our principal means of unraveling the positions of atoms in solids from the comparatively simple structures in metals and semiconductors to the highly complex arrangements in biological molecules, such as proteins and DNA.

As for the electronic structure of materials, the inner electrons are bound tightly in orbitals around the atomic nuclei, whereas the outer, more loosely bound electrons participate in chemical bonding between atoms as well as influence most of the common properties of matter, such as electrical conductivity, magnetic behavior, and optical activity. Again as a general (but not invariable) rule, long-wavelength (soft) x rays and ultraviolet are good choices for studying the many aspects electronic structure and their consequences, such as chemical reactions. Infrared is ideally suited to studying atomic vibrations in molecules and solids, and at its very long wavelength end (terahertz waves), it is also turning out the be useful for certain types of electronic structure experiments.

Elemental analysis (identification of the elements present in a sample and their concentrations) is the province of x rays. For chemical analysis both soft x rays and infrared can provide molecular fingerprints that identify chemical compounds. Some properties of matter, such as the strength of materials, are optimized only when the material is spatially inhomogeneous. To study such materials, imaging akin to that in an ordinary microscope can be carried out throughout the entire light source spectrum with a spatial resolution determined by the wavelength of the light. A particularly useful kind of imaging combines imaging with the other structure components to yield maps of the spatial distribution of all these kinds of structure.

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