How do they work?
Lightsources can be compared to a ‘super microscope’, by providing intensely bright forms of X-ray, infrared and ultraviolet light, which enables research on samples in the tiniest detail. Each range of light is suited to a particular job.
To ‘see’ atoms, we need to use a form of light that has a much shorter wavelength than visible light. As a general rule, short-wavelength (hard) X-rays are most useful for probing atomic structure. Again as a general rule, long-wavelength (soft) X-rays and ultraviolet light are good choices for studying 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 useful for certain types of electronic structure experiments. The identification of elements in samples is the province of X-rays.
This range of the electromagnetic spectrum is known as ‘synchrotron light’, as it is produced by a dedicated synchrotron machine. A synchrotron light source typically begins with an electron gun, containing a manmade material, to which an electrical and thermal current is applied. This results in electrons ‘lifting off’ and beginning their journey by being propelled down a linear accelerator (linac). They then enter a circular-shaped booster ring, where they are accelerated to relativistic speeds. Finally they enter another ring, often called a ‘storage ring’, where they circulate for hours. The electrons will travel in a straight line, so at points around the ring, special ‘bending’ magnets help them keep to their circular path. As the electrons circulate, powerful magnets keep them bunched together and focused.
Synchrotron light is produced when the electrons change direction around the ring. In synchrotrons, this happens when they are manipulated by bending magnets, or as they pass through insertion devices. At the points where the electrons change direction, they emit a fan of radiation (known as synchrotron light). This radiation branches off the storage ring, and enters laboratories, or ‘beamlines’. Here it is refined with devices such as monochromators and mirrors, before it is shone on the sample, enabling researchers to obtain detailed data about the sample’s structure and behaviour.
The moving graphic below (courtesy Diamond Light Source) gives a demonstration of the electrons (in red) starting in the linac and speeding around the booster synchrotron and storage ring, where the synchrotron light (in blue) is produced. The team at the Swedish synchrotron, MAX IV Laboratory, have created a friendly animation to help explain how synchrotron light is created: watch the video