This result was so weird that the leader of the experiment, SLAC Professor Joachim Stöhr, devoted the next three years to developing a theory that explains why it happened. Now his team has published a paper in Physical Review Letters describing the 2012 experiment for the first time. What they saw was a so-called nonlinear effect where more than one photon, or particle of X-ray light, enters a sample at the same time, and they team up to cause unexpected things to happen. "In this case, the X-rays wiggled electrons in the sample and made them emit a new beam of X-rays that was identical to the one that went in," said Stöhr, who is an investigator with the Stanford Institute for Materials and Energy Sciences at SLAC. "It continued along the same path and hit a detector. So from the outside, it looked like a single beam went straight through and the sample was completely transparent." This effect, called "stimulated scattering," had never been seen in X-rays before. In fact, it took an extremely intense beam from SLAC's Linac Coherent Light Source (LCLS), which is a billion times brighter than any X-ray source before it, to make this happen. A Milestone in Understanding How Light Interacts with Matter The observation is a milestone in the quest to understand how light interacts with matter, Stöhr said. "What will we do with it? I think we're just starting to learn. This is a new phenomenon and I don't want to speculate," he said. "But it opens the door to controlling the electrons that are closest to the core of atoms – boosting them into higher orbitals, and driving them back down in a very controlled manner, and doing this over and over again." Nonlinear optical effects are nothing new. They were discovered in the1960s with the invention of the laser – the first source of light so bright that it could send more than one photon into a sample at a time, triggering responses that seemed all out of proportion to the amount of light energy going in. Scientists use these effects to shift laser light to much higher energies and focus optical microscopes on much smaller objects than anyone had thought possible. The 2009 opening of LCLS as a DOE Office of Science User Facility introduced another fundamentally new tool, the X-ray free-electron laser, and scientists have spent a lot of time since then figuring out exactly what it can do. For instance, a SLAC-led team recently published the first report of nonlinear effects produced by its brilliant pulses. "The X-ray laser is really a quantum leap, the equivalent of going from a light bulb to an optical laser," Stöhr said. "So it's not just that you have more X-rays. The interaction of the X-rays with the sample is very different, and there are effects you could never see at other types of X-ray light sources." Stöhr stumbled on this latest discovery by accident. Then director of LCLS, he was working with Andreas Scherz, a SLAC staff scientist, who is now with the soon-to-open European XFEL in Hamburg, Germany, and Stanford graduate student Benny Wu to look at the fine structure of a common magnetic material used in data storage. To enhance the contrast of their image, they tuned the LCLS beam to a wavelength that would resonate with cobalt atoms in the sample and amplify the signal in their detector. The initial results looked great. So they turned up the intensity of the laser beam in the hope of making the images even sharper. That's when the speckled pattern they'd been seeing in their detector went blank, as if the sample had disappeared. "We thought maybe we had missed the sample, so we checked the alignment and tried again," Stöhr said. "But it kept happening. We knew this was strange – that there was something here that needed to be understood." Stöhr is an experimentalist, not a theorist, but he was determined to find answers. He and Scherz dove deeply into the scientific literature. Meanwhile Wu finished his PhD thesis, which described the experiment and its unexpected result, and went on to a job in industry. But the team held off on publishing their experimental results in a scientific journal until they could explain what happened. Stöhr and Scherz published their explanation last fall in Physical Review Letters. "We are developing a whole new field of nonlinear X-ray science, and our study is just one building block in this field," Stöhr said. "We are basically opening Pandora's box, learning about all the different nonlinear effects, and eventually some of those will turn out to be more important than others." Explore further: First Test of New X-ray Laser Strips Neon Bare More information: B. Wu et al. Elimination of X-Ray Diffraction through Stimulated X-Ray Transmission, Physical Review Letters (2016). DOI: 10.1103/PhysRevLett.117.027401 J. Stöhr et al. Creation of X-Ray Transparency of Matter by Stimulated Elastic Forward Scattering, Physical Review Letters (2015). DOI: 10.1103/PhysRevLett.115.107402
The experiments took place at the Linac Coherent Light Source (LCLS) X-ray laser, a DOE Office of Science User Facility. Its pulses are so bright that they can be used to turn solids into highly ionized gases, or plasmas, that blow up within a fraction of a second. Fortunately, for many samples researchers can take the data they need before the damage sets in – an approach that has been used to reveal never-before-seen details of a variety of samples relevant to chemistry, materials science, biology and energy research. The ultimate limits of this approach are, however, not well understood. One of the key visions for X-ray laser science is to image individual, one-of-a-kind particles with single X-ray pulses. To do so in a quantitative manner, researchers need to understand precisely how each molecule responds to the intense X-ray light. The new study, published today in Science Advances, provides an unexpected insight into this aspect. "So far, all models have assumed that a very small system would immediately explode when we pump a lot of energy into it with the X-ray laser," says former LCLS researcher Christoph Bostedt, who is now at Argonne National Laboratory and Northwestern University. "But our experiments showed otherwise." At LCLS, Bostedt and his fellow researchers exposed minuscule clusters of xenon atoms to two consecutive X-ray pulses. The clusters, which were merely three millionths of an inch across, were heated by the first pulse for 10 quadrillionths of a second, or 10 femtoseconds. The second pulse then probed the clusters' atomic structures over the next 80 femtoseconds. "The unique nature of the LCLS X-ray pulse allowed us to create a freeze-frame movie of the response, with a resolution of about a tenth of the width of a single xenon atom," says LCLS and Stanford University graduate student Ken Ferguson, who led the data analysis. The researchers believe that the effect is a result of how electrons, which were initially localized around individual xenon atoms, redistribute over the entire cluster after the first X-ray pulse. "This phenomenon had never been observed before, nor had it been predicted by any of the existing theories," he says. "We expect it to have implications for many ultrafast X-ray laser experiments, especially those geared toward single-particle imaging with very intense X-ray pulses." The research could benefit studies in other areas as well, such as investigations of warm dense matter – a state of matter between a solid and a plasma that exists in the cores of certain planets and is also important in the pursuit of nuclear fusion with high-power lasers. More information: K. R. Ferguson et al. Transient lattice contraction in the solid-to-plasma transition, Science Advances (2016). DOI: 10.1126/sciadv.1500837
News Article | September 12, 2016
To understand how molecules undergo light-driven chemical transformations, scientists need to be able to follow the atoms and electrons within the energized molecule as it gains and loses energy. In a recent study, scientists used the ultrafast high-intensity pulsed X-rays produced by the Linac Coherent Light Source to take molecular snapshots of these molecules.
Fraud punished A Parkinson’s disease researcher in Australia pleaded guilty to research fraud and was handed a two-year suspended prison sentence by a court in Brisbane on 31 March. Bruce Murdoch, formerly of the University of Queensland in Brisbane, was found to have falsified results published in the European Journal of Neurology in 2011; three of his papers have been retracted. In a statement to the blog Retraction Watch, University of Queensland vice-chancellor Peter Høj said that the university had reimbursed around Aus$175,000 (US$132,000) to funding bodies associated with Murdoch’s work. Ice wall to stem Fukushima leak The Tokyo Electric Power Company (TEPCO) on 31 March began freezing the soil surrounding reactors 1 to 4 of the disaster-stricken Fukushima Daiichi nuclear power plant. A refrigeration system (pictured) is creating a 30-metre deep, 1.5-kilometre-long wall of frozen ground that aims to stop groundwater from flowing under the plant and carrying radioactive isotopes into the sea. More than 300 tonnes of water per day are pumped into the damaged reactors to stop their cores from overheating and contributing to the radioactivity leak. TEPCO expects that it will take months for the ¥35-billion (US$316-million) project to seal the zone. Emergency over The Ebola outbreak that has killed more than 11,000 people in Guinea, Liberia and Sierra Leone is no longer a public-health emergency, said the World Health Organization (WHO) in Geneva, Switzerland, on 29 March. The WHO noted that the virus is now unlikely to spread internationally, and that the affected countries have the capacity to deal with new cases. A campaign in Guinea last month administered an experimental Ebola vaccine to nearly 800 people who may have come into contact with 8 individuals with the virus. Liberia has recorded two new cases since the announcement: a woman who died on 31 March and her five-year-old son. ET search starts The SETI Institute in Mountain View, California, has kicked off a search for signals from extraterrestrial civilizations that might be living on planets orbiting any of the 20,000 nearest red dwarf stars. Red dwarfs are dimmer and cooler than the Sun, but they make up the bulk of stars in the Galaxy, increasing the odds of finding life there. The two-year search will be conducted at the Allen Telescope Array in northern California, the institute announced on 30 March. China’s Go A team of Chinese scientists plans to challenge Google DeepMind’s Go-playing artificial-intelligence algorithm with its own program by the end of 2016, Chinese state news has reported. The DeepMind program, known as AlphaGo, beat a leading human player, South Korea’s Lee Sedol, by four games to one in March. Reporting from an event organized by the Chinese Go Association and the Chinese Association for Artificial Intelligence, Shanghai Securities News said on 31 March that a team from China will issue the challenge by the end of the year. Science in space Two scientific payloads travelled to the edge of space on 2 April in the latest test of Blue Origin’s commercial space vehicle, New Shepard. Amazon founder Jeff Bezos’s reusable spacecraft took off from a Texas launch site, flew to an altitude of 103 kilometres and successfully landed 11 minutes later. On board was an experiment from the University of Central Florida in Orlando investigating how dust and rubble settle in microgravity, and the ‘box of rocks experiment’ from the Southwest Research Institute in San Antonio, Texas, to work out how regolith forms on asteroids. Nuclear security More than 50 countries, most represented by their heads of state, made a variety of commitments intended to prevent nuclear terrorism at the conclusion of a nuclear summit in Washington DC on 1 April. The meeting was the fourth biennial summit in a process initiated in 2009 by US President Barack Obama (pictured with Canadian Prime Minister Justin Trudeau). Much of the focus has been on reducing civilian stocks of highly enriched uranium at research reactors. At least 28 reactors have been shut down or converted to low-enriched uranium since 2009, but challenges remain in converting 11 high-performance research reactors. See Editorial for more. Laser beam added The Linac Coherent Light Source (LCLS), the world’s brightest X-ray free-electron laser, began a US$1-billion construction project on 4 April to add a second beam. LCLS-II, based at the SLAC National Accelerator Laboratory in Menlo Park, California, will accelerate electrons through superconducting niobium cavities to produce X-ray pulses 10,000 times more concentrated and firing 8,000 times faster than X-rays produced by the $414-million LCLS, which started operations in 2009. This will enable it to image processes that occur on smaller scales and faster timescales. Construction will last until the early 2020s. Crater drilling A two-month expedition to drill into the 200-kilometre-wide Chicxulub crater, which formed 66 million years ago when an enormous asteroid smashed into the planet, began on 1 April. The aftermath of the impact obliterated most life on Earth, including the dinosaurs. From a drill-ship off the coast of Yucatán, Mexico, researchers will start to penetrate one of Chicxulub’s most striking features — its ‘peak ring’, a circle of mountains within the crater floor. Scientists have yet to fully explain how peak rings form, even though they are common in big impact craters across the Solar System. See go.nature.com/pgxb18 for more. Gorilla decline Numbers of the largest primate on the planet, Grauer’s gorilla (Gorilla beringei graueri), have plummeted since 1995, according to a report from the Wildlife Conservation Society (WCS). The report, published on 4 April, says that the numbers have dropped from an estimated 17,000 in 1995 to 3,800 today, a 77% decrease. Grauer’s gorillas live in the wild only in the eastern Democratic Republic of the Congo, and the WCS report blames the decline on illegal hunting around mining sites, the civil war in the country from 1996 to 2003, and habitat loss. Patent pledge Drugmaker GlaxoSmithKline (GSK) has announced plans to improve access to its medicines in the world’s poorest countries. The company said on 31 March that it would stop filing drug patents in many developing countries. That means that generic manufacturers in those nations would be able to supply copycat versions of GSK’s drugs without worrying about lawsuits. GSK also signalled that it intended to improve access to low-cost drugs that can help to address the growing burden of cancer in the developing world. Public-health advocates have embraced the news and are urging other drug companies to follow suit. See go.nature.com/nqhggj for more. Career boost Four philanthropic organizations have created an international research programme focused on early-career scientists. Announced on 29 March, the International Research Scholars Program will select up to 50 members who are not originally from one of the G7 countries, but have trained in the United States or the United Kingdom for at least one year. Awardees will each receive a total of US$650,000 over five years. The sponsors are: the Howard Hughes Medical Institute in Chevy Chase, Maryland; the Bill & Melinda Gates Foundation in Seattle, Washington; the London-based Wellcome Trust; and Lisbon’s Calouste Gulbenkian Foundation. More of the global population is now obese than is underweight, according to a study of 186 countries from 1975 to 2014 (see Lancet 387, 1377–1396; 2016). The proportion of obese men more than tripled and that of obese women more than doubled during that period. Many people are still underweight in the world’s poorest regions, particularly in Asia and Africa. But the global average weight of a person grew by 1.5 kilograms each decade. See go.nature.com/yslifh for more. 11–13 April The 43rd session of the Intergovernmental Panel on Climate Change convenes in Nairobi. go.nature.com/bdodfh 12–15 April Water across the Universe and its origins will be discussed in Noordwijk, the Netherlands. go.nature.com/lncjsb
News Article | September 1, 2016
An ultrafast “electron camera” at the Department of Energy’s SLAC National Accelerator Laboratory has made the first direct snapshots of atomic nuclei in molecules that are vibrating within millionths of a billionth of a second after being hit by a laser pulse. The method, called ultrafast electron diffraction (UED), could help scientists better understand the role of nuclear motions in light-driven processes that naturally occur on extremely fast timescales. Researchers used the UED instrument’s electron beam to look at iodine molecules at different points in time after the laser pulse. By stitching the images together, they obtained a “molecular movie” that shows the molecule vibrating and the bond between the two iodine nuclei stretching almost 50 percent — from 0.27 to 0.39 millionths of a millimeter — before returning to its initial state. One vibrational cycle took about 400 femtoseconds; one femtosecond, or millionth of a billionth of a second, is the time it takes light to travel a small fraction of the width of a human hair. “We’ve pushed the speed limit of the technique so that we can now see nuclear motions in gases in real time,” says co-principal investigator Xijie Wang, SLAC’s lead scientist for UED. “This breakthrough creates new opportunities for precise studies of dynamic processes in biology, chemistry and materials science.” The UED method has been under development by a number of groups throughout the world since the 1980s. However, the quality of electron beams has only recently become good enough to enable femtosecond studies. SLAC’s instrument benefits from a high-energy, ultrabright electron source originally developed for the lab’s femtosecond X-ray laser, the Linac Coherent Light Source (LCLS), a DOE Office of Science User Facility. The results will be published in Physical Review Letters. Physicists have long known that chemical bonds between atoms are flexible — like springs connecting spheres. This flexibility allows molecules to change shape in ways that are crucial for biological and chemical functions, such as vision and photosynthesis. However, methods to study these motions on a femtosecond timescale have so far been indirect. Spectroscopy, for example, infers these changes from the way laser light interacts with electron clouds around atomic nuclei, and requires theoretical calculations to turn these data into a picture of the nuclear geometry. This can be done very precisely for small molecules — an accomplishment that earned the late Ahmed Zewail, a pioneer in the field of femtochemistry, the 1999 Nobel Prize in Chemistry — but quickly becomes very challenging for larger molecules. Researchers also use X-rays to study ultrafast molecular motions. Although X-rays deeply penetrate the electron clouds, interacting with the electrons closest to the nuclei, they don't yet do so with high enough resolution to precisely determine the nuclear positions in current femtosecond X-ray studies. In contrast, UED uses a beam of very energetic electrons that interacts with both electrons and atomic nuclei in molecules. Therefore, it can directly probe the nuclear geometry with high resolution. “We previously used the method to look at the rotation of molecules — a motion that doesn’t change the nuclear structure,” says lead author Jie Yang from SLAC, who was at the University of Nebraska, Lincoln at the time of the study. “Now we have demonstrated that we can also see bond changes due to vibrations.” The concept behind the iodine UED experiment is similar to the classical double-slit experiment often demonstrated in physics classrooms. In that experiment, a laser beam passes through a pair of vertical slits, producing an interference pattern of bright and dark areas on a screen. The pattern depends on the distance between the two slits. In the case of UED, an electron beam shines through a gas of iodine molecules, with the distance between the two iodine nuclei in each molecule defining the double slit, and hits a detector instead of a screen. The resulting intensity pattern on the detector is called a diffraction pattern. “The characteristic pattern tells us immediately the distance between the nuclei,” says co-principal investigator Markus Guehr from Potsdam University in Germany and the Stanford PULSE Institute. “But we can learn even more. As the iodine molecules vibrate, the diffraction pattern changes, and we can follow the changes in nuclear separation in real time.” Co-principal investigator Martin Centurion from the University of Nebraska, Lincoln, says, “What’s also great about our method is that it works for every molecule. Unlike other techniques that depend on the ability to calculate the nuclear structure from the original data, which works best for small molecules, we only need to know the properties of our electron beam and experimental setup.” Following their first steps using the relatively simple iodine molecule, the team is now planning to expand their studies to molecules with more than two atoms. "The development of UED into a technique that can probe changes in internuclear distances of a dilute gas sample in real time truly is a great achievement," says Jianming Cao, a UED expert from Florida State University and a former member of the Zewail lab at the California Institute of Technology, who was not involved in the study. "This opens the door to studies of atomic-level motions in many systems — structural dynamics that are at the heart of the correlation between structure and function in matter." The research was funded in part by the DOE Office of Science.