By Lynn Yarris, LCYarris@lbl.gov
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 became a serious question after Swapan Chattopadhyay and Kwang-Je Kim, at LBL's Center for Beam Physics (CBP), determined that they could possibly make such a device. Answers to the question were subsequently provided by a team consisting of Chattopadhyay and Kim, two experts in visible light femtosecond lasers--physicist Robert Schoenlein and LBL Director Charles V. Shank (the inventor of the femtosecond laser)--and CBP researcher Wim Leemans, an expert in laser-plasma interactions.
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, 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 understands melting," says Marvin Cohen, a leading theorist with LBL's Materials Sciences Division.
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, Cohen says, scientists do not have 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.
Says Schoenlein, "Femtosecond experiments on these materials in the optical regime have provided evidence of vibrational coherence (a possible key to their novel properties). Femtosecond x-rays could shed new light on these coherent phonons by providing direct information about atomic displacements and enabling us to map out the energy transfer between vibrational modes and vibrational damping."
Studies of chemical reactions could begin with the photodissociation of iodine--a simple molecule that undergoes a big chemical change in a hurry. The complicated atomic motion of iodine during dissociation is important because of iodine's value as a solid electrolyte (it is used in pacemakers, for example), but it is poorly understood.
Especially promising for such studies would be the use of femtosecond x-rays with EXAFS, a technique in which one of an atom's inner-shell electrons is excited by an x-ray photon, causing the atom to emit light that is then diffracted by a neighboring atom. EXAFS diffraction patterns reveal much about the spacing between atoms in a lattice. With femtosecond pulses, scientists could for the first time directly observe the shifts in interatomic spacings that take place during reactions.
"Femtosecond x-ray pulses will have tremendous applications in broad areas of science, including physics, chemistry, biology, and engineering," says Chattopadhyay. "And the ability to time- resolve atomic motion could open entirely new fields of scientific research."