Ultrafast X-ray pulses will enable scientists to take stop-motion pictures of individual atoms and molecules, revealing the frenetic action of the atomic world.

SLAC National Accelerator Laboratory (SLAC) in Menlo Park, California, which possesses the world’s longest linear accelerator, has been producing high-energy electrons for four decades. Starting in 2009, SLAC will use its linear accelerator to drive a new kind of laser, the Linac Coherent Light Source (LCLS), a new scientific tool for studying the world of the ultrasmall and the ultrafast.

The LCLS will produce very short flashes of X-ray light more than one billion times brighter than any other X-ray source on earth. “It can be used to measure the separation and motion of atoms in molecules, solid materials and disordered [noncrystalline] materials in a very new way,” explained John Galayda, LCLS construction director. The LCLS began construction in October, 2006.

The LCLS will enable scientists to observe molecules in action and how chemical bonds form and break. It also will help explain how materials work on the quantum level.

The ultrashort LCLS X-ray pulses will enable such short measurements that atoms will not have a chance to move and blur the picture. “Rather like the way a strobe flash on a camera stops motion,” Galayda explained. In addition, the pulses are so intense single images can achieve atomic resolution.

The ultrafast LCLS X-ray’s “shutter speed” will be about 100 femtoseconds—one-tenth of a trillionth of a second, or the time it takes light to traverse the width of a human hair. Stop-action imaging has a long history at Stanford University: Eadweard Muybridge developed early stop-motion photography in the 1870s on the future site of Stanford. He photographed a horse galloping to resolve the long-debated topic of whether all four of a horse’s feet came off the ground at once (they do).

The LCLS represents a major advance in the use of large electron accelerators to produce X-rays. Many synchrotron radiation facilities are in operation around the world today, using accelerators to produce X-rays millions of times brighter than the X-rays used for medical purposes, enabling scientists to understand the arrangement of atoms in a range of materials including metals, semiconductors, ceramics, polymers, catalysts, plastics and biological molecules. In contrast with the intense, ultrashort pulses of X-rays from the LCLS, the X-rays from synchrotron sources come as a more continuous stream. They are more useful for static structure measurements than for studies of ultrafast atomic motions.

Synchrotron sources have been highly productive. In 2006, Stanford University researcher Roger Kornberg won the Nobel Prize for Chemistry for his research using X-ray crystallography at SLAC’s Stanford Synchrotron Radiation Lightsource to study proteins important in the transcription of DNA. By exposing crystallized samples of protein molecules, usually the size of a grain of salt or smaller, to highly focused X-ray beams and creating characteristic patterns on a detector, scientists could calculate the locations of the atoms within the protein molecules. The technique enabled Kornberg and colleagues to investigate the structure of RNA polymerase, key to understanding how information held in DNA translates into the proteins of life.

The LCLS’s ultrafast, ultrabright X-ray pulses will enable scientists to move beyond the study of static atomic structures and take stop-motion pictures of atoms and molecules, revealing the frenetic action of the atomic world. This will lead to better understanding of not only what atomic structures look like, but how they move and interact with each other.