How LCLS X-ray pulses can be used to determine the atomic structure of a complicated molecule.
A New Kind of Laser
SLAC's two-mile-long linear accelerator (or linac) has begun a new phase of its career, with the creation of the Linac Coherent Light Source (LCLS).
For nearly 50 years, SLAC's linac has produced high-energy electrons for cutting-edge physics experiments. Now, scientists continue this tradition of discovery by using the linac to drive a new kind of laser, creating X-ray pulses of unprecedented brilliance.
LCLS produces pulses of X-rays more than a billion times brighter than the most powerful existing sources, the so-called synchrotron sources which are also based on large electron accelerators.
The ultrafast X-ray pulses are used much like flashes from a high-speed strobe light, enabling scientists to take stop-motion pictures of atoms and molecules in motion, shedding light on the fundamental processes of chemistry, technology, and life itself.
Probing the Ultrasmall
The diameter of a human hair is about 1/1000 of an inch. The wavelength of visible light is about 50 times smaller than this, so ordinary microscopes can easily resolve a hair. But a molecule, about 10,000 times smaller than a hair, is too small to be resolved with visible light. X-rays, with wavelengths that are even smaller than a molecule, are ideal for imaging at the atomic scale.
Capturing the Ultrafast
The atomic and molecular world is abuzz with frenetic motion. Because they are so small and light, molecules and atoms react incredibly quickly to forces that act on them. Chemical reactions, in which molecules join or split, can take place in mere quadrillionths of a second.
The ultrafast LCLS X-ray flash captures images of these events with a “shutter speed” of less than 100 femtoseconds (100 femtoseconds = 1/10 of a trillionth of a second).
A Long History of Imaging Breakthroughs
The LCLS photographs atomic motion much as a “strobe” flash is used to photograph the motion of a bullet in flight. This latest advance in stop-action imaging at Stanford has roots going back more than 100 years. Around 1872, Eadweard Muybridge started making stop-motion photographs of people, animals, and trains in motion on Leland Stanford’s farm. He is famous for showing that all four of a horse’s feet leave the ground during a gallop. To be able to click a shutter fast enough to show each stride a horse makes when galloping required tremendous engineering ingenuity. The LCLS provides X-rays of such shortness and precision that stroboscopic experiments can be done with materials on the nanoscale, and even with individual molecules and atoms.
How Fast Is a Femtosecond?
2.4 seconds: The time it takes light to travel the distance to the moon and back—about 480,000 miles.
100 femtoseconds: the time it takes light to travel the width of a human hair.