An atom, molecule or Bacterium placed at the focal point of the world's most powerful x-ray laser doesn't stand a chance. Up to a trillion high-energy photons, moving in unison, sweep through the matter, heating it to more than one million degrees Celsius—hot as the solar corona—in less than a trillionth of a second. When a pulse of such extreme radiation hits neon atoms, all 10 electrons boil off each atom, and the denuded nuclei explode away from their similarly ionized neighbors. The obliteration leaves a trail of destruction that can illuminate some of the mysteries of nature.
In the case of exploding neon, for instance, the x-ray laser astonishingly strips away the atoms' electrons from the inside out. The electrons orbit each nucleus in onionlike shells, but the outer shells are nearly transparent to x-rays. So the beam continues on until it hits the two electrons in the innermost shell. They take the brunt of the radiation, much as coffee in a microwave oven warms long before the cup that holds it. Sprung from the center, the two electrons leave behind a hollowed-out atom. But within femtoseconds (quadrillionths of a second), other electrons move inward to fill the gap. The cycle of hollowing and filling repeats—all within a single, exceedingly brief pulse of x-rays—until no electrons are left.
The result is an exotic, ionized plasma in a state called warm dense matter—it is usually found only in extreme settings such as nuclear fusion reactions and the cores of giant planets. For just a few femtoseconds, the destructive environment at the focus of an x-ray laser beam has no parallels on Earth.
The x-ray laser itself is as remarkable as the exotic phenomena it reveals. Known as the Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory, it evokes memories of the 1980s-era “Star Wars” missile-defense system. Advocates of that scheme proposed wielding x-ray lasers to shoot down ballistic missiles and satellites.
This real-world x-ray laser actually owes much more to the nation's premier atom smashers built during that era—in particular, the SLAC linear accelerator, operated by Stanford University for the U.S. Department of Energy. That accelerator produced many of the discoveries and Nobel Prizes that kept the U.S. at the forefront of elementary particle physics for decades. And then it was rebuilt into the LCLS x-ray laser, which came online in October 2009.
Since then, the LCLS has become to atomic and plasma physics, chemistry, condensed matter physics and biology what the Large Hadron Collider at CERN near Geneva has been to elementary particle physics. The device gives physicists a formidable hammer to smash the building blocks of nature, creating from the debris new forms of matter, such as hollow atoms. We also use it like a powerful, high-speed microscope to zoom in on the quantum realm. The LCLS's x-ray pulses are not only exceedingly bright but also incredibly short—just a few femtoseconds long. We use them to make atoms freeze in their tracks, to observe chemical reactions in progress, and to image living microbes and viruses in exquisite detail.
Shadows of atoms
The x-ray laser fuses two of the main tools used by today's experimental physicists: synchrotron light sources and ultrafast lasers. Synchrotrons are racetrack particle accelerators. Electrons circling through them throw off x-rays, which enter instruments arrayed around the circumference of the machines like pinwheel spokes.
One of us (Berrah) has spent a career using synchrotron x-rays to study the deep interior of atoms, molecules and nanosystems. X-ray light is ideal for this purpose. Its wavelengths are atomic size, so atoms cast a shadow in an x-ray beam. In addition, x-rays can be tuned to pick out specific kinds of atoms—say, only those of iron—and show where they sit in a solid or in a large molecule such as hemoglobin.
X-rays from synchrotrons are limited in one crucial way, however: they cannot trace out atomic motion inside most molecules or solids. The pulses are not short enough or bright enough, so they produce only dim, blurry images unless the target is a crystal, where local forces hold millions of molecules in precise ranks like identical soldiers at attention.
Lasers, for their part, are far brighter because the light they emit is coherent. The electromagnetic field in a laser is not choppy, like the surface of a rough sea; it is a regular series of smooth oscillations.
Lasers can exploit that regularity to focus enormous amounts of energy onto a tiny spot of both space and time; they can switch on and off in as little as a femtosecond. One of us (Bucksbaum) uses ultrafast optical laser pulses as a strobe light to study the motion of atoms and the steps in chemical reactions.
But conventional lasers operate at or near visible wavelengths—wavelengths at least 1,000 times longer than those needed to resolve atoms. Just as weather radar can see a rainstorm but not resolve the raindrops, optical lasers can see how collections of atoms are moving, but they cannot distinguish individual atoms. To cast a sharp shadow, the wavelength of the light must be no bigger than the object under observation. So we need x-ray lasers to image atoms. Actually building such a device is no easy task, however.
Death rays
At one time, the idea of building an x-ray laser seemed outlandish. Standard lasers are hard enough to construct. They work because atoms are like miniature batteries: they can absorb, store and release small amounts of energy in the form of photons. Typically atoms emit photons spontaneously, but early in the 20th century Albert Einstein discovered a way to trigger the release, a process known as stimulated emission. If you cause an atom to absorb a certain amount of energy and then hit it with a photon having the same amount of energy, the atom can release the originally absorbed energy—thus producing a clone of the photon. The two photons (the original one and its clone) go forth to trigger the release of energy from a pair of other atoms, and so on, building up a clone army in an exponential chain reaction. Laser beams are the result.
Even when conditions are right, atoms do not always clone photons as you might expect. The probability that an energetic atom will emit a photon when hit by another is small compared with the likelihood that the atom will simply release its energy spontaneously. Conventional lasers overcome this limitation in two ways. They pump in energy to prime the atoms. And they use mirrors to send the cloned light surging back and forth, picking up new recruits along the way.
In a typical helium-neon laser used in supermarket price scanners, for example, a continuous stream of electrons collides with atoms in the gas, energizing it. And light bounces back and forth between mirrors 200 times before it exits the laser.
For an x-ray laser, every step of this process becomes much more difficult. An x-ray photon may contain 1,000 times more energy than an optical photon does, so each atom must absorb 1,000 times more energy. The atoms do not hold on to that energy for long. Moreover, x-ray mirrors are hard to come by. Although these impediments are not insurmountable, it takes an enormous input of energy to create the lasing conditions.
In fact, the first x-ray laser got its energy from an underground nuclear bomb test in the 1980s. It was built for a secret project, code-named Excalibur, carried out by Lawrence Livermore National Laboratory, east of San Francisco. The project is still classified, although some information about it has been made public. The device was a component of former president Ronald Reagan's Strategic Defense Initiative (aka “Stars Wars”) and was meant to act as a death ray to shoot down missiles and satellites.
During the same decade, Lawrence Livermore also built the first nonnuclear, laboratory-scale version of an x-ray laser. It was powered by giant optical lasers originally designed to test properties of nuclear weapons. But these early devices were not practical research instruments. For decades the possibility that x-ray lasers would ever be used routinely for science applications seemed remote.
Subatomic demolition derby
The breakthrough that finally enabled investigators to develop x-ray lasers for civilian use came from another Bay Area institution, using a device intended for a different purpose entirely. In the 1960s Stanford built the world's longest electron accelerator. At three kilometers long, the building can be seen from space—it looks like a needle pointing from the mountains to the heart of the university's campus. The SLAC linac (short for linear accelerator) boosts dense bunches of electrons to velocities extremely close to the speed of light—so close that in a one-second race, the photons would travel nearly 300 million kilometers, and the electrons would trail them by just one centimeter.
The SLAC machine led to three Nobel Prizes for experimental discoveries in particle physics, but eventually it reached the end of its useful life. Particle physicists now make their discoveries at the Large Hadron Collider. A decade ago Stanford and SLAC's parent agency—the Department of Energy's Office of Science—decided to turn part of the aging machine into an x-ray laser by outfitting it with the same device used to produce x-rays at modern synchrotrons: an undulator.
Undulators consist of a series of magnets that generate alternating magnetic fields. Electrons moving through undulators wiggle and emit x-rays. In synchrotrons, which are closed loops, electrons bend into arced trajectories once they leave the undulator. That way the particles get out of the way, and the x-rays can move unimpeded to experimental stations. The electrons keep circling the racetrack, emitting a burst of x-rays each time they pass through the undulator.
The SLAC accelerator, however, is a straight line, and the undulator is unusually long (130 meters). The electrons move along the same path as the photons do—and at nearly the same speed. The result is a subatomic demolition derby. The electrons cannot get out of the way of the x-ray photons they have emitted, so the photons sideswipe them again and again. In so doing, the photons induce the electrons to emit clone x-ray photons through the process of stimulated emission.
The process is analogous to what happens in an optical laser, but with a difference. Mirrors are not needed to bounce the light back and forth through the electrons, because they travel together. All it takes to produce the laser is an intense beam of fast electrons and a space big enough to house a long undulator. And SLAC possesses both.
If everything is lined up nearly perfectly, voilà, an extraordinarily bright x-ray beam. At the end of the line, the electrons are diverted, and the photons enter the experimental stations. The system is known technically as a free-electron laser.
Though not a gun for “Star Wars,” the LCLS is still a formidable device. Its peak focused intensity, 10
Peering into Jupiter's core
Demand for the laser is so great that it can accommodate fewer than one in four research proposals to use it. On-site staff scientists work with large visiting teams of students, postdocs and senior scientists in intense marathons, 12 hours a day for five days. Every microsecond counts.
The research made possible by x-ray lasers is broad—and not limited by the conventional boundaries of physics. Just this year one collaboration that included biologists reported using the LCLS to make the first high-resolution x-ray images of living bacteria, and another pieced a series of x-ray snapshots together to create a stunning 3-D model of a virus.
In our own research in atomic, molecular and optical physics, we have used the LCLS recently to investigate two scientific problems that particularly interest us. The first is how matter behaves under extreme conditions. The second is what can be learned from the ultrafast imaging of molecules.
Recall those strange, hollowed-out neon atoms we described earlier. It takes mere femtoseconds for the electrons from the outer shells of an atom to fall in to replace those that have been lost from the inner shells (a phenomenon called Auger relaxation). If we shine a shorter, one-femtosecond x-ray pulse on the atom, no outer electrons will have time to drop into the vacant inner shell. While the atoms are hollow, they should thus be transparent to any additional x-ray radiation, no matter how intense. And we have in fact detected this hollow transparency at the LCLS—not only in atoms but also in molecules and larger bits of matter.
Theorists suggest that inside giant planets such as Jupiter, temperatures reach 20,000 degrees Celsius—four times hotter than the surface of the sun. Hydrogen and helium, the planet's main constituents, presumably take on exotic solid phases having extreme densities and structures. Yet little is known about the specifics. Even the strength of the material, or its compression in response to pressure, is not easy to measure and not well understood from basic principles. So far research in this domain has relied heavily on theoretical models. Experiments that can validate the models have been scarce.
Some of the first experiments done at the LCLS attempted to re-create these hostile conditions. The laser's ultrahigh intensity can heat matter with dizzying speed, producing unusual effects. For instance, we observed for the first time how multiple x-rays can gang-tackle molecules made of many atoms to liberate electrons that are strongly bound to atomic nuclei, a process called multiphoton absorption.
Bright x-rays can, in addition, rapidly break all the bonds in molecules that are expected to reside inside giant planets, including water, methane and ammonia. Measurements of matter in extreme conditions induced by x-rays have helped determine the equation of state—the formula that governs the density, temperature and pressure—in cores of giant planets and during meteor impacts.
Exploding proteins
The second line of research—exploiting the laser as an x-ray high-speed camera to image molecules and record movies of physical, chemical and biological dynamics—is filling in a serious gap in our knowledge. Researchers know distressingly little about the structure of many biological molecules—in particular, membrane proteins and large macromolecular complexes. The standard technique, crystallography, starts by growing a crystal that is large enough and perfect enough to diffract a beam of synchrotron x-rays. The resulting pattern reveals the structure of the molecule.
Unfortunately, x-rays rapidly damage the molecules they are probing. To compensate, researchers must prepare large crystals, yet many molecules of interest, including membrane proteins, are very difficult to crystallize. The synchrotron technique is also slow and thus unable to observe transient phenomena that occur on the femtosecond chemical timescale.
At first glance, the LCLS seems exactly the wrong tool for the job. Because it is billions of times more intense than synchrotron light sources, fragile materials such as proteins or noncrystalline systems cannot survive even one pulse of its x-rays before they explode and turn into a very hot soup of plasma.
Ironically, that destructive intensity is just what we need. Because the pulse is so short and bright, it can capture an image faster than the molecule is able to blow up. Consequently, although the laser obliterates the sample, it captures a clear image of the molecule just before its demise.
This concept, called diffraction before destruction, is already beginning to pay off. Scientists have used femtosecond crystallography to record diffraction patterns of nanocrystals, proteins and viruses. Recent work has mapped out the structure of proteins involved in sleeping sickness, a fatal disease caused by protozoan parasites.
Now that the LCLS has pioneered the technology, other free-electron x-ray lasers are in the works. In Japan, the SACLA laser facility opened in 2011. In Europe, a large x-ray laser is being built near Hamburg, Germany, and is scheduled for completion in 2017. In the U.S., SLAC is building the LCLS II. This upgrade will provide soft x-rays at a high repetition rate, which will let us conduct new kinds of experiments.
This new generation of machines has been designed to create a more stable, better controlled and more intense laser beam. One particularly important goal is to make the x-ray pulses even shorter. By using pulses as short as 0.1 femtosecond (100 attoseconds, or quintillionths of a second), we might begin to observe the motion not just of atoms but also of electrons within atoms and molecules. New devices could even allow us to control this motion. The dream of making movies that show how chemical bonds break and how new ones form is within our reach.
