X-ray FEL Science

Because x-ray FEL sources are not yet in use, their most fruitful applications have not yet been determined. However, the FEL characteristics have been carefully considered by the x-ray community, and five scientific areas that will clearly benefit have been determined and accepted by DOE review panels.

Atomic, molecular, and optical (AMO) physics. Very-intense, ultrashort x-ray pulses will interact with matter in ways that a beam from a conventional x-ray source will not. Such strong-field effects are important to understand and characterize, as they may alter the properties of the materials being studied or even the methods being used for studying them. In addition, the strong-field effects will open new paths for studying the basic physics of atoms and simple molecules.

Studies of laser-excited transient states. Most chemical reactions involve rearrangements of atoms that take place on a sub-picosecond time scale. So do many structural phase transitions. Many of these atomic motions can be triggered by an optical laser pulse, and they can thus be precisely synchronized with the LCLS x-ray pulse. The ultrafast x-ray pulses can be used to take snap-shot measurements of the atoms in motion. This capability will open up for scrutiny broad areas of atomic and molecular dynamics on a femtosecond time scale.

Imaging of nano-particles (including single biomolecules). X-ray scattering has long been used to determine atomic structures. However, to avoid radiation damage limitations, crystallographers require that their samples form crystals, in which billions of molecules are all precisely aligned. LCLS offers an alternative approach. A very intense and very short LCLS x-ray pulse could be focused onto a single molecule, which would be destroyed – but not before the scattered x-rays are already on their way to the detector carrying the information needed to deduce the image. This technique offers the possibility of determining structures for samples which do not form crystals, including important classes of biological macromolecules.

Nano-scale dynamics of condensed matter. Complex dynamics at the nanometer to micrometer scale lie at the frontier of research in condensed matter. Viscoelastic flow of liquids, polymer diffusion, domain switching, and countless other collective processes show both fast and slow equilibrium dynamics, which are often sensitive functions of temperature, applied fields, and fabrication parameters. The unprecedented brilliance and narrow pulse duration of the LCLS will enable the study of dynamical changes of large groups of atoms in condensed matter systems over a wide range of time scales.

High energy-density (HED) physics. LCLS will enable the detailed study of states of matter created when normal condensed matter is suddenly heated to very high temperatures, well above melting. During the brief period (picoseconds) before this matter flies apart, it can form transient phases with properties very different both from the low-temperature condensed phase and from the high-temperature rarified plasma phase. These phases are of interest to scientists studying astrophysics, planetary physics, fusion energy and the transition region from condensed matter to hot dense plasmas.

The initial instruments to be installed at LCLS will all be directed towards experiments that lie within these five areas. To cover the broad range of science that lies within these five areas, six LCLS instruments will be required.