August 19, 2005  


Dancing Atoms

By Monica Bobra

An international community of over 250 engineers and users will gather for the 27th annual free-electron laser conference at from August 21 to 26 at the laser’s birthplace—Stanford University.

In 1976, a group of scientists wanted to create the world’s first free-electron laser, then a theoretically proposed instrument that required an intensely bright electron beam. They decided to alter the electron beam of the existing Stanford Superconducting Accelerator.

“We thought we’d try it just to see if it could be done,” said Todd Smith, one of the scientists present on that eventful day. Though scientists now hope the world’s first hard x-ray free-electron laser at the future LCLS at SLAC will revolutionize the fields of biology, chemistry, material science and atomic physics when it turns on in 2009, the impetus for the world’s first ever free-electron laser was simply curiosity.

In a free-electron laser, a beam of electrons surf through magnets on electromagnetic waves. Both the magnets and waves act like fences, keeping the electron jostling along a specific path at near-light speeds. As the electrons bump their way through this ocean of waves, they emit photons—small bundles of light at specific energies. The numerous photons are collected and focused, forming the light of a laser.

Operating a Free-Electron Laser is advantageous because its light is tunable, like a dial tunes a radio. Run different bunches of electrons through these magnets, and the electrons will emit photons of different energies. Together, these photons supply the laser’s light.

In conventional lasers, atoms sit inside a substance called a lasing medium, like a chocolate chips in cookie dough. Many different lasing media are present in modern-day lasers; for example, some use glass or silicon. Applied electrical discharges or bursts of light surge through the medium and excite the atoms, which then release photons. The energy of the photon depends on the type of atom. For example, argon lasers only produce certain wavelengths of visible light; carbon dioxide lasers produce specific wavelengths of infrared light. Free-electron lasers, however, can produce larger ranges of light—the trick, however, lies in manipulating electron bunches.

The group of six Stanford scientists successfully created the first free-electron laser, which operated at infrared wavelengths between approximately 1.5 and 7 microns. Soon after, chemists like Michael Fayer, now the director of the Fayer Research Group at Stanford University, “were pretty excited about it—there was a lot of scientific interest,” said Smith.

In one of Fayer’s first experiments with the newfound laser, he analyzed protein dynamics. Proteins are constantly moving, allowing them to execute biological tasks. Using the protein myoglobin bound to carbon monoxide, he analyzed how the protein’s motions caused the carbon monoxide to vibrate. The free-electron laser acts like a strobe light, measuring the carbon monoxide’s motions every picosecond. Like a dance, the motions change over time—sometimes moving quickly, at other times, moving slowly. Analyzing the dance enabled Fayer to understand basic aspects of protein motions.

The LCLS laser will probe at x-ray wavelengths as short as an angstrom, which is about the size of an atom. The x-ray bunches, called pulses, will be hundreds of times shorter than a picosecond. Such an instrument will enable scientists to study how individual atoms inside molecules, proteins, liquids and solids dance. In previous instruments, scientists had to infer the nature of atomic dances; for example, Fayer’s group deduced protein motions based on how the carbon monoxide vibrated over time. But LCLS’s laser will permit researchers to watch the motion of atoms directly.

As a result, researching with free-electron lasers is a booming enterprise. Over thirty extremely large free-electron lasers are scattered worldwide, in places as diverse as Italy, Japan, and India. And many more are in development, including the LCLS’s fighting gun — the first hard x-ray free-electron laser in the world, which will use the electron beam from the linear accelerator and operate at energies up to 14.3 GeV.

“The source of the excitement is that nobody has been able to do this before,” said the free-electron laser conference co-chair, SLAC scientist John Galayda. It’s the same type of remark Stanford scientists already heard, nearly three decades ago.





The Stanford Linear Accelerator Center is managed by Stanford University for the US Department of Energy

Last update Thursday August 18, 2005 by Chip Dalby