First Femtosecond X-Rays Generated
By Lynn Yarris

Scientists at Berkeley Lab have produced the first directed beams of femtosecond x-rays. Lasting only a few hundred millionths of a billionth of a second, the strobe-like pulses of x-ray light will be used to study the motion of atoms during ultra-fast physical and chemical processes.

The researchers, who published their findings in the journal Science, reported that sending a short, powerful pulse of infrared laser light across the path of a narrowly focused electron beam generated pulses of x-rays that were a mere 300 femtoseconds in duration. A femtosecond is to one second in duration what one second is to 30 million years.

The 10-member team that conducted this research was led by two scientists, Robert Schoenlein, with Berkeley Lab's Materials Sciences Division, and Wim Leemans, with the Lab's Accelerator and Fusion Research Division.

X-rays are ideal for investigating the atomic structure of matter because they interact directly with nuclei and core electrons. The production of directed beams of x-rays on a femtosecond time-scale has been highly prized by the scientific community, because at room temperature atomic motion takes place, in most cases, on a time-scale of approximately 100 femtoseconds. With femtosecond x-ray pulses, it would be possible, for example, to track the movement of atoms in a sample of material during a phase transition (solid to liquid to gas), a chemical reaction, or any other process of physical change.

Wim Leemans (left) and Robert Schoenlein in the Beam Test Facility at the Advanced Light Source, where the first beams of femtosecond x-rays were produced.
The directed beams of femtosecond x-rays were produced at a branch line off the 50 MeV (million electron volt) linear accelerator that feeds electrons into the synchrotron booster of Berkeley Lab's Advanced Light Source. Atop this branch line, which is called the Beam Testing Facility (BTF), is a femtosecond terawatt near-infrared laser. The BTF provides a tightly focused electron beam that is about 90 microns in width; the laser produces photons in 100 femtosecond pulses.

"By crossing the photon and electron beams at a right angle, we obtain scattered x-ray pulses lasting about 300 femtoseconds that travel along the direction of the electron beam," says Schoenlein. "The duration of the x-ray burst is determined by the transit time of the laser pulse across the waist of the focused electron beam." Once femtosecond pulses of x-rays are generated, a magnet is used to remove the electron beam. What is left are pure femtosecond pulses of x-rays. The Berkeley Lab research team detected these pulses using an x-ray sensitive phosphor screen. Visible photons from the phosphor were then imaged onto a charged-coupling device camera.

The images showed that the x-ray photons arrived as an elliptically shaped beam, similar to the shape of the electron beam from which they were generated. Additional measurements indicated that the beam was delivered at an energy of 30,000 electron volts and flux of about 105 photons per pulse. These results were in accordance with theoretical predictions.

"Tighter focusing of the electron beam holds the potential for generating even shorter x-ray pulses, on the order of 50 femtoseconds or less," says Schoenlein.

Leemans and his colleagues at Berkeley Lab's Center for Beam Physics will experiment with tightening the focus of the BTF electron beam to reach these shorter pulse-lengths. Schoenlein and his colleagues in the Materials Sciences Division will use the 300 femtosecond x-rays now achievable in a study on the ultrafast melting of silicon.

In addition to Schoenlein and Leemans, other researchers participating in this experiment were Alan Chin, Paul Volfbeyn, Ernest Glover, Peter Balling, Max Zolotorev, Kwang Je-Kim, Swapan Chattopadhyay, and Berkeley Lab Director Charles Shank.