Turning on a femtosecond X-ray strobe.

Conventional X-ray imaging can capture the pocket of decay extending into a tooth or the fractured bone in an injured leg. X rays that penetrate crystals can also reveal details of internal structure.

Now, researchers have access to a new type of X-ray source that flashes in strobe-like pulses lasting only 300 femtoseconds. Such ultrashort pulses open up the possibility of tracking rapid changes in the atomic and molecular structures of materials and the progress of chemical reactions.

Robert W. Schoenlein of the Lawrence Berkeley (Calif.) National Laboratory and his colleagues describe their X-ray source in the Oct. 11 Science. "We've actually been able to observe and characterize the femtosecond X rays," Schoenlein says. "Right now, we're working on the first application."

The researchers generate X rays with a wavelength of 0.4 angstrom by sending a stream of pulsed infrared light from a powerful laser across a beam of tightly focused bunches of high-energy electrons. When an electron collides with an infrared photon at a 90#161# angle, it gives up some of its energy to the photon, which is converted to an X ray. These interactions produce a cone of X rays that fans out from the electron beam.

Getting the experiment to work was no simple matter. "The trick is to collide a bullet of electrons with a bullet of light," Schoenlein says. "You've got two things that are moving at the speed of light or very close to it."

Moreover, the bunches of electrons and photons are less than 100 micrometers wide and must meet within a time interval of only about 10 picoseconds.

At present, the LBNL team is using the source to study the scattering of X rays by atoms in a silicon crystal. The researchers plan to observe how a crystal lattice breaks apart as it melts.