What would you do with a light source that could flash beams of x-ray light in strobe-like pulses lasting only a few millionths of a billionth of a second? In other words, what could you do with a femtosecond x-ray source? This has become a serious question now that Swapan Chatto-padhyay and Kwang-Je Kim, physicists at LBL's Center for Beam Physics (CBP), are having such a device made.
The story starts at the Advanced Light Source, which produces its x-ray and ultraviolet light from the state-of-the-art electron beam orbiting around its storage ring. This high quality beam originates in a linac that accelerates electrons to 50 million electron volts (MeV) of energy prior to sending them into a booster synchrotron for acceleration to 1.5 billion electron volts. Once the ALS storage ring has been filled with electrons, the 50 MeV linac sits idle for several hours.
To make good use of this "down" time, CBP researchers have designed the Beam Test Facility (BTF), a branch line that will utilize the electron beam of the 50 MeV linac without disturbing the operations of the ALS. The BTF is now under construction and is expected to be operational in the Fall of 1993. One of the first major experiments at this new facility will be the production of x-rays in femtosecond pulse lengths.
With current technology, it is difficult to generate a sufficiently intense electron beam much shorter than a few picoseconds. To overcome this limitation, Chattopadhyay and Kim have proposed a technique involving orthogonal Thomson scattering -- the scattering of photons after collisions with electrons at 90-degree angles. The technique begins with the focusing of an electron beam down to about 100 microns in width -- the equivalent of 330 femtoseconds.
By scattering the laser beam across the waist of such an electron beam we can generate femtosecond pulses of x-rays in the same direction as the electrons, Chattopadhyay says. A magnet can then be used to separate the electron beam from the x-ray beam.
At the BTF, Chattopadhyay, Kim, and other CBP researchers will initially demonstrate the feasibility of this technology using a femtosecond laser system that is now being built by LBL Director Charles Shank and Robert Schoenlein. The laser will be positioned on top of the BTF's roof shielding block and its light will be directed through a hole to cross paths with the linac beam.
Once they have produced x-ray pulses in the 300-femtosecond range, as proof-of-principle, Chattopadhyay and Kim will try squeezing the electron beam into a width of about 10 microns in order to generate x-ray beams with pulse lengths of only 30 femtoseconds. This takes us back to our opening question.
X-rays are the ideal photons for investigating the atomic structure of matter because they interact directly with nuclei and core electrons -- a distinct advantage over visible light photons which interact only with certain electronic states. Furthermore, atomic motion takes place on a time-scale of approximately 100 femtoseconds, which means that femtosecond x-ray pulses could be used to track the shifting positions of a given material's atoms during a chemical reaction, phase-transition, desorption, or any other process that involves atomic motion.
Says Schoenlein: "Well-developed static measurements such as x-ray diffraction, x-ray microscopy, x-ray absorption spectroscopy (XAS), and extended x-ray absorption fine structure spectroscopy (EXAFS) can be readily adapted to transient structural measurements that will provide time-resolved information about ultrafast events."
One of the first applications of a femtosecond x-ray laser might be a study of solids when they melt. No one really understands melting, the materials scientists will tell you.
It is known that if you apply heat to a solid, its atoms will begin to vibrate until there is a change in the lattice and the ordered structure of the solid phase becomes the disorderly arrangement of the liquid phase. However, no one has a clear picture about how this phase-transition process works.
With a femtosecond x-ray source and time-resolved x-ray diffraction techniques, snapshots could be taken that reveal the changing positions of the atoms in the lattice as energy is transferred from electrons to phonons (atomic vibrational frequencies). This should answer the important question of precisely when melting occurs and provide a much better understanding of why it happens.
Similar time-resolved x-ray diffraction experiments could help explain some of the many mysteries behind such exotic materials as fullerenes (also known as "buckyballs"), high-temperature superconductors, semiconductor nanocrystals, bulk semiconductors, and organic crystals.
Femtosecond x-ray pulses will have tremendous applications in broad areas of science, including physics, chemistry, biology, and engineering, Chattopadhyay says. And the ability to time-resolve atomic motion could open entirely new fields of scientific research.