What began as an idea to build weapons for the "Star Wars" project to deal with the missile threats of the 80s era, today functions as an unprecedentedly powerful microscope, capable of creating exotic forms of matter that cannot be found anywhere else in the universe.
If you put an atom, a molecule or a grain of dust in the focus of the most powerful X-ray laser in the universe, they wouldn't stand a chance. The radiation will heat the material in less than a billionth of a second to temperatures higher than a million degrees Kelvin, the temperature prevailing in the Sun's corona. Neon atoms, for example, that were exposed to such extreme radiation, lost all ten of their electrons in a flash, and as soon as they lost the electron mantle that protected them, they scattered away from the nearby atoms. In the eyes of physicists, the path of destruction holds a strange charm.
This process is astonishing, because the laser causes electrons to leave the atom from the inside out. The electrons, which surround the atomic nucleus in onion-like shells, or orbitals, do not all react uniformly to the X-ray beam. The outer shells are almost transparent to X-rays, so it is the inner shell that absorbs most of the radiation, much like how coffee in a microwave oven heats up long before the cup that contains it. The two electrons in the innermost shell are shot out, leaving behind an empty space; The atom becomes hollow. Within a few femtoseconds (a millionth of a billionth of a second), other electrons are drawn in and replace the lost electrons, and this cycle of forming an empty orbital in the nucleus of the atom and filling it with outer electrons continues until there are no more electrons left. This process occurs both in single molecules and in solid matter.
The resulting exotic matter state does not last more than a few femtoseconds. In solids, it decays into an ionized state, plasma, known as hot compressed matter, which can usually only be found in extreme conditions such as nuclear fusion reactors and the cores of giant planets. The short-lived but extreme environment that prevails at the focus of an X-ray laser beam has no equal on Earth.
The X-ray laser itself is as noteworthy as the exotic phenomenon it reveals. The device, called "Linac Coherent Light Source, LCLS" operates at the American National Accelerator Laboratory at Stanford University in the USA (SLAC). The facility evokes memories of the anti-missile defense system from the 80s, "Star Wars," whose followers proposed using laser beams in the X-ray range to intercept ballistic missiles and satellites. But the existence of this realistic x-ray laser is much more to the credit of the large particle accelerators that were developed around the same time to smash atoms. The X-ray laser device is actually a conversion to a new purpose of one of the most important US particle accelerators, the SLAC linear accelerator, operated by Stanford University for the US Department of Energy. This machine produced many of the discoveries and Nobel prizes that left the USA at the forefront of elementary particle physics for decades. Since its conversion in October 2009, to the X-ray laser LCLS, it has fulfilled for atomic physics, plasma physics, chemistry, condensed state physics and biology the role that the Large Hadron Collider (LHC), at the CERN laboratory near Geneva, fulfills for elementary particle physics: it provides A way in which the building blocks of nature can be slammed into each other with enormous energies, and by the way create new forms of matter, such as hollow atoms, or simply observe the quantum realm very closely as if through a fast and powerful microscope. LCLS X-ray pulses can be so short (single femtoseconds) that they freeze the motion of atoms, allowing physicists to observe the course of chemical reactions in real time. The pulses are also very bright, allowing us to image proteins and other biological molecules that have so far been very difficult to study using other X-ray sources.
opaque shadows
The X-ray laser unites two of the main tools used by experimental physicists today: synchrotron light sources and super-frequency pulsed lasers. Synchrotrons are particle accelerators built like racetracks. Electrons moving inside them in a circle shoot out x-rays, which enter devices located around the circle and organized like the wings of a paper vane. Nora Berra, one of the authors of this paper, has dedicated her career to using synchrotron X-ray radiation to probe the deep bowels of atoms, molecules and nanosystems. X-ray radiation is ideal for this purpose. Its wavelengths are of the order of magnitude of atoms, therefore atoms cast a shadow when illuminated by a beam of X-rays. Also, X-rays can be tuned so that they can detect certain types of atoms - for example, only iron atoms - and see where they are located within a solid or within a large molecule such as hemoglobin. (The iron is responsible for the red color of the blood.)
However, there is one thing that synchrotron X-ray radiation cannot do: track the movement of atoms within a molecule or solid body. All we see is a dim cloud; The pulses are too long or not bright enough. A synchrotron source can simulate molecules only if they are arranged in a crystalline array, where local forces hold millions of them in precise rows, like identical soldiers in formation.
Lasers, on the other hand, are much brighter because they produce coherent light: the magnetic field inside a laser does not change wildly like the surface of a stormy sea but oscillates in smooth oscillations at a regular and controlled rate. Thanks to the coherence, the lasers can focus enormous energy on a tiny point and can be turned on and off in no more than a femtosecond. Boxbaum, the second author of the article, uses optical laser pulses that are generated at ultra-fast rates as an equivalent to strobe lights, which are lights that flash at a constant frequency, therefore allowing the viewing of oscillating objects as if they were fixed in place. With their help, he studies the movement of atoms and phases in chemical reactions.
But normal lasers operate in the visible light range or in wavelengths close to it, which are more than 1,000 times greater than the wavelength needed for the resolution of single atoms. Just as a rain radar is able to see a rainstorm but is unable to separate the drops, so ordinary lasers are able to see how groups of atoms move but are unable to distinguish between atoms. For light to cast a sharp shadow, it must have a wavelength at least as small as the observed object. That's why we need an X-ray laser.
In general, the X-ray laser overcomes the disadvantages that the existing tools pose for material imaging at the smallest scales. But building such a device is not an easy task at all.
death rays
There were times when the idea of building an X-ray laser sounded far-fetched, as even building a normal laser was a challenge. Standard lasers work because atoms are like tiny batteries: they can absorb, store and release small amounts of energy in the form of photons or particles of light. Atoms normally release the energy they have received spontaneously, but in the early 20th century, Albert Einstein discovered a way to trigger the release, a process known as forced emission. If you make an atom absorb a certain amount of energy and then hit it with a photon containing the same amount of energy, the atom will be able to release the energy it absorbed in the first place and create a clone of the photon. The two photons (the original and the clone) continue and trigger the release of energy from another pair of atoms, and so on, until an army of clones is established in an exponential chain reaction. The result is laser beams.
However, even when the conditions are right, atoms do not always trap photons. The probability that a given atom will emit a photon when hit by another photon is quite low, and it is more likely that the atom will spontaneously release its energy before this happens. Normal lasers overcome this limitation through a process called "energy extraction" that prepares the atoms for action and through the use of mirrors that send the cloned light rushing forward and backward, picking up new soldiers along the way. In a typical helium-neon laser used in supermarket barcode scanners, a continuous stream of electrons collides with atoms in the gas, and each photon of light is recycled 200 times by bouncing back and forth between mirrors.
With an X-ray laser each of these steps becomes much more complicated. An X-ray photon may contain 1,000 times more energy than an optical photon, so each atom would have to absorb 1,000 times more energy. The atoms do not hold onto their energy for long. More than that, X-ray mirrors are hard to come by. And even though these failures are not fundamental, it is still necessary to invest a huge amount of energy to create the conditions for producing a laser (for the arena).
In fact, the first X-ray laser got its energy from an underground nuclear bomb test. It was built for a secret project that received the code name "Excalibur" and was carried out at the American Lawrence National Laboratory in the city of Livermore, east of San Francisco. The project is still classified, although much of the information about it has already been revealed to the public. The device was a component of former President Ronald Reagan's 80s Strategic Defense Initiative, dubbed "Star Wars," and was supposed to function as a death ray that intercepted missiles and satellites.
During the same decade, the Lawrence Laboratory at Livermore also built the first non-nuclear lab-scale version of an X-ray laser, powered by powerful optical lasers designed to test the properties of nuclear weapons. However, these devices were impractical as research tools, and the possibility that X-ray lasers would ever be routinely used for scientific applications seemed unlikely.
Movement in line
The breakthroughs that allowed researchers to finally develop X-ray lasers for civilian use came from another institution in San Francisco Bay, which used a device intended for an entirely different purpose. In the 60s, Stanford University built the longest electron accelerator in the world, a three-kilometer-long structure that, viewed from space, looks like a needle emerging from the mountains and pointing toward the heart of the university campus. The linear accelerator SLAC (Stanford Linear Accelerator Center) accelerates compressed clusters of electrons to speeds very close to the speed of light (as close as one centimeter per second). This machine yielded three Nobel Prizes for experimental discoveries in particle physics.
However, the accelerator has reached the end of its life as a useful machine, and particle physicists are making their discoveries today in the Large Hadron Collider in Switzerland. Ten years ago, the Science Department of the US Department of Energy, the parent agency of Stanford and SLAC, decided to turn part of the aging machine into an X-ray laser. SLAC equipped the accelerator with the same device that is used to produce X-rays in modern synchrotrons: a device called an undulator.
Undulators consist of a series of magnets that generate changing magnetic fields. Electrons moving through undulators oscillate and emit X-rays. In synchrotrons, which are closed loops, as soon as the electrons leave the oscillator their paths curve and become curved. Thus the particles are removed from the path of the X-rays, which are directed to experimental stations. The electrons continue to move down the racetrack, emitting a burst of X-rays each time they pass through the undulator.
But SLAC is a straight-line accelerator, and its inverter is unusually long (130 meters). The electrons move along the same path as the photons and at almost the same speed. The result is a subatomic collision car race. The electrons cannot move out of the way of the photons in the field of X-ray radiation they emitted, so the photons "hit" them from the side again and again. Thus, the photons cause the electrons to emit cloned photons of X-ray radiation through a process of forced emission.
There is no need for mirrors to cause the light to bounce back and forth through the electrons, since they move together. All that is needed to produce the laser is a strong beam of fast electrons and a space large enough to accommodate a long undulator. And at SLAC you can find both. If you just arrange everything perfectly, more or less, and there will appear an astonishingly bright beam of X-rays. At the end of the line, the electron trajectory is diverted, and the photons enter the experimental stations. The technical term for this system is a free electron laser.
Although the LCLS is not used as a cannon for "Star Wars", it is still a monstrous device. The highest focused power reached, 1018 watts per square meter, is several billion times greater than synchrotron light sources. The laser can cut steel. Its oscillating electromagnetic field can be 1,000 times stronger than the fields that bind atoms together in molecules.
as material in the hand of the creator
Demand for the laser is so great that its schedule allows at most one research proposal in four to use it. The lab's permanent staff scientists work with large visiting groups of students, postdocs and senior scientists in packed marathons, 12 hours a day for five days. Every microsecond counts.
X-ray lasers open up extensive research fields for us. To give you a taste of the possibilities, we will focus here on two scientific problems that are of particular interest to us: how matter behaves under extreme conditions and what can be learned from the ultra-fast imaging of molecules. There is a close connection between these two problems and between the fundamental processes studied in atomic, molecular and optical physics, our field of expertise.
When the LCLS creates hollow atoms in molecules and solids, it takes advantage of the tendency of electrons from the outer shells of the atom to fall in to replace the electrons lost from the inner shells. This phenomenon, known as Auger relaxation, lasts for several femtoseconds. Therefore, if we irradiate the system with an x-ray pulse of one femtosecond in length, no outer electron will have enough time to fall into the hollow places in the inner shell. Under these conditions, the hollow atoms will be transparent to any additional X-ray photon, even if the intensity is very high. We discovered this hollow transparency at LCLS, not only in atoms but also in molecules and larger samples of matter.
According to the theory, it is possible that inside giant planets like Jupiter, for example, temperatures reach 20,000 degrees Kelvin, four times the temperature of the surface of the Sun. Hydrogen and helium, the main components of the planet, probably reach exotic solid states of aggregation there, with extreme densities and structures. However, very little is known about the details. Even the strength of the material, i.e. its compression in response to pressure, is not easy to measure and is not well understood from basic principles. Until now, research in this area has relied heavily on theoretical models. Only a few experiments were conducted that could confirm the models.
Some of the first experiments conducted at LCLS attempted to recreate these hostile conditions. The great power of the laser can heat a material at a dizzying speed, and by the way create unusual effects. For example, we observed for the first time several X-rays striking together molecules made up of many atoms, releasing from them electrons that were tightly bound to the atom's nucleus, a process called multiple photon absorption. The high photon density can also remove some electrons from a single atom, molecules or solids, turning them into hollows as described here, in a process called serial absorption. Bright X-rays can, in addition, quickly break all bonds in molecules expected to be found inside giant planets, including water, methane and ammonia. Measurements of matter under extreme conditions have helped determine the equation of state—the formula that governs density, temperature, and pressure—in the cores of giant planets and during meteorite impacts.
Proteins explode
The second line of research – using a laser as a fast X-ray camera to simulate molecules and record movies of physical, chemical and biological dynamics – fills a serious gap in our knowledge. The knowledge in the hands of the researchers regarding the structure of many biological molecules is disturbingly poor, in particular, regarding molecules on the cell membrane and macromolecular couplings of giant molecules. In the usual method, crystallography, the first step is to grow a crystal that will be large and perfect enough to be able to cause the x-ray beam from a synchrotron to undergo diffraction. The resulting pattern reveals the structure of the molecule. The disadvantage of this method is that the X-rays easily damage the molecules they test. The researchers must therefore prepare large crystals to compensate for the defects, but it is very difficult to crystallize many of the interesting molecules, such as cell membrane proteins. Also, the synchrotron method does not allow for a fast pulse rate, so it is impossible to observe transient phenomena that occur on the chemical scales of femtoseconds.
At first glance, the LCLS seems to be the least suitable tool for this task. Its power is several billion times greater than the power of synchrotron light sources, so fragile materials such as proteins or non-crystalline systems will not be able to withstand even a single pulse of its X-rays without exploding and turning into a hot plasma soup. But ironically, this destructive power is exactly what we need. Because the pulse is so short and bright, it can take a picture in less time than it takes for a molecule to explode. Therefore, even though the laser erases the sample, it is enough to take a clear picture of the molecule just before its elimination.
This concept, called bypass before destruction, is already beginning to bear fruit. Scientists have used femtosecond crystallography to record diffraction patterns of nanocrystals, proteins and viruses [see box on pages 58 and 59]. In recent work, scientists have mapped the structure of proteins involved in sleeping sickness, a deadly disease caused by single-celled parasites.
Now that the LCLS has pioneered the technology, labs in Europe and Asia are also planning to build their own free-electron X-ray lasers. This new generation of machines will be more stable and provide better beam control. One of the particularly important goals is the creation of even shorter X-ray pulses. If we have pulses of 0.1 femtosecond (100 attoseconds, that is, 10-18ths of a second), we may be able to begin to observe not only the movement of atoms but also the movement of electrons within atoms and molecules. New devices may even allow us to control this movement. The dream of making movies showing how chemical bonds break and how new bonds form is now within our reach.
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About the authors
Nora Berrah, head of the physics department at the University of Connecticut, leads research tests and manages the construction of advanced instrumentation at the LCLS X-ray laser. Bera's field of expertise is the study of the interactions between photons and atoms, molecules and nano-systems. She is a research fellow in the American Physical Society and the recipient of the Davison-Germer Award for research in atomic physics or surface physics, one of the most prestigious decorations in the field.
Philip H. Bucksbaum holds the Marguerite Blake Wilbur Chair in Natural Sciences at Stanford University and at SLAC, where he directs the PULSE Institute dedicated to research using ultrafast lasers and LCLS. He is a research fellow of the American Physical Society and a member of the National Academy of Sciences and the American Academy of Arts and Sciences.
in brief
X-ray lasers have always been one of the staples of science fiction, but the first X-ray laser intended for scientific purposes began operating at Stanford University, as a facility of the Department of Energy's Department of Science, only four years ago. It is known as the "Linear Coherent Light Source" (LCLS), and the energy to operate it comes from the world's largest linear particle accelerator at the American National Accelerator Laboratory SLAC.
Exotic states of matter that do not appear anywhere else in the universe have been created by exposing atoms, molecules and solids to high-intensity X-ray pulses.
The laser, which is also used as a flash light, froze the movement of atoms, took very fast pictures of proteins and viruses and recorded physical and chemical transformations that last less than a billionth of a second.
More on the subject
Femtosecond Electronic Response of Atoms to Ultra-Intense X-rays. L. Young et al. in Nature, Vol. 466, pages 56-61; July 1, 2010.
Femtosecond X-ray Protein Nanocrystallography. Henry N. Chapman et al. in Nature, Vol. 470, pages 73–77; February 3, 2011.
Single Mimivirus Particles Intercepted and Imaged with an X-ray Laser. M. Marvin Seibert et al. in Nature, Vol. 470, pages 78-81; February 3, 2011.
Double Core-Hole Spectroscopy for Chemical Analysis with an Intense X-ray Femtosecond Laser. N. Berrah et al. in Proceedings of the National Academy of Sciences USA, Vol. 108, no. 41, pages 16,912-16,915; October 11, 2011.
Creation and Diagnosis of a Solid-Density Plasma with an X-ray Free-Electron Laser. SM Vinko et al. in Nature, Vol. 482, pages 59-63; February 2, 2012.
Natively Inhibited Trypanosoma brucei Cathepsin B Structure Determined by Using an X-ray Laser. Lars Redecke et al. in Science, Vol. 339, pages 227-230; January 11, 2013.
Watch the animation of powerful x-ray beams blasting helpless atoms and comment on the article On the website of Scientific American Israel.
The article was published with the permission of Scientific American Israel
