DOE News
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    The DOE Science News Source is a Newswise initiative to promote research news from the Office of Science of the DOE to the public and news media.
    • 2018-04-04 18:05:05
    • Article ID: 692278

    Tick, Tock on the 'Attoclock:' Tracking X-Ray Laser Pulses at Record Speeds

    When it comes to making molecular movies, producing the world's fastest X-ray pulses is only half the battle. A new technique reveals details about the timing and energy of pulses that are less than a millionth of a billionth of a second long, which can be used to probe nature's processes at this amazingly fast attosecond timescale.

    • Credit: Terry Anderson / SLAC National Accelerator Laboratory

      In this illustration, ultrashort X-ray pulses (pink) at the Linac Coherent Light Source ionize neon gas at the center of a ring of detectors. An infrared laser (orange) sweeps the outgoing electrons (blue) across the detectors with circularly polarized light. Scientists read data from the detectors to learn about the time and energy structure of the pulses, information they will need for future experiments.

    • Credit: Frank Scholz & Jens Buck / DESY

      An illustration of the ring-shaped array of 16 individual detectors arranged in a circle like numbers on the face of a clock. An X-ray laser pulse hits a target at the center and sets free electrons that are swept around the detectors. The location, where the electrons reach the “clock,” reveals details such as the variation of the X-ray energy and intensity as a function of time within the ultrashort pulse itself.

    To catch chemistry in action, scientists at the Department of Energy’s SLAC National Accelerator Laboratory use the shortest possible flashes of X-ray light to create “molecular movies” that capture the motions of atoms in chemical reactions and reveal new details about the most fundamental processes in nature.

    Future experiments at the Linac Coherent Light Source (LCLS), SLAC’s X-ray free-electron laser, will use pulses that last just attoseconds (billionths of a billionth of a second). Such experiments will be even more powerful because they'll be able to detect the motions of electrons within molecules during chemical reactions. However, to design such ultrafast experiments, researchers need meticulous measurements of the X-ray pulses so they can use that information to interpret the data they collect on the samples they study.

    Now an international team, including SLAC scientists, has created an X-ray “attoclock” that lets them analyze X-ray pulses on the attosecond timescale of electron motions.

    “Using this method, we can resolve details of the pulses in the attosecond domain for the first time,” says Ryan Coffee, a senior scientist at LCLS and the Stanford PULSE Institute and a principal investigator on the team. “This paves the way for X-ray free-electron laser science at a timescale that is key to understanding physical chemistry.”

    The team’s research was published in Nature Photonics on March 5.

    Timekeeping in Attoseconds

    The term “attoclock” was coined by Swiss physicist Ursula Keller, who first demonstrated a technique to study attosecond processes with circularly polarized light 10 years ago. However, the LCLS version is the first one designed to measure individual X-ray pulses, one by one.

    It consists of a ring of detectors arranged like numbers on the face of a clock. When an X-ray pulse hits a target at the center of the clock, it knocks electrons out of the target's atoms. Those electrons are hit by circularly polarized laser light that whirls the electrons around the ring before they land on one of the detectors. The position of that detector – the number on the clock face – tells scientists how much energy the X-ray pulse contained and when exactly it hit the target.

    “It’s like reading a watch,” Coffee says. “An electron may strike a detector positioned at one o’clock or three o’clock or anywhere around the clock face. We can tell from where it hits exactly when it was generated by the X-ray pulse.”

    In an experiment designed to test the technique, the researchers hit neon gas with an attosecond X-ray pulse and then read which of the 16 detectors arrayed around the attoclock the freed electrons hit.

    “In coming up with this technique, we combined ideas from different fields,” says principal investigator Wolfram Helml, then a Marie Curie research fellow at SLAC and the Technical University of Munich and now at the Ludwig-Maximilian University of Munich. “For our purposes, it just made sense to combine the circularly polarized light used in the original attoclock with a ring-shaped detector that has been used in other kinds of experiments.”

    Finding the True Colors

    The technique will be especially important for pump-probe experiments, in which a molecule is first excited with a “pump” pulse and then analyzed by a second “probe” pulse to see how it reacted.

    As short as they are, these pulses can contain many different colors or wavelengths. “The color can also vary widely from pulse to pulse, and our technique can sift through the pulses, finding those that are interesting for the experiment,” Coffee says, noting the importance that such sifting will have for the data deluge expected when an upgrade to the X-ray laser, LCLS-II, comes online a couple years from now. With pulses that arrive up to a million times per second, LCLS-II will produce as much data in a few minutes as LCLS currently collects in a month.

    “For instance, only a certain color may excite a molecule when it is ‘pumped,’” Helml said. “With the attoclock we can see what part of the pulse is actually exciting the molecule because we know exactly when particular colors of light arrive. This lets us pinpoint more precisely when changes occur in the molecule as a result of the interaction with light.”

    What’s more, scientists can potentially excite individual elements in separate parts of the molecule at the same time using different colors of X-rays.

    “With this technique we could look within a single molecule at the interplay between atoms. For example, what’s going on with an oxygen atom and how might that affect the chemical environment surrounding a nearby nitrogen atom?” Helml says. “With that level of detail, we can discern completely new chemical behavior.”

    Progress in Motion

    The attoclock team is now working on a proposal to build more refined detectors.

    “With the next detector, we are aiming to precisely identify a broader spectrum of energies,” Coffee says. “This will be an important feature for our upgraded X-ray laser, LCLS-II, which will produce pulses with an even wider energy range and more multi-color flexibility than our current machine.”

    This is one of several ideas being tested at SLAC to give scientists detailed information about attosecond pulses. Two other teams are building similar systems with different types of detectors, including one at LCLS and PULSE that recently published a study in Optics Express

    The international team on the Nature Photonics study also included scientists from Deutsches Elektronen-Synchrotron (DESY) and the European X-ray Free-electron Laser (Eu-XFEL), both in Germany, who also provided the unique ring-shaped detector; University of Kassel in Germany; University of Gothenburg in Sweden; University of Bern in Switzerland; University of Colorado at Boulder; University of the Basque Country in Spain; and Lomonosov Moscow State University in Russia.

    LCLS is a DOE Office of Science user facility.

    The Stanford PULSE Institute is a joint institute of SLAC National Accelerator Laboratory and Stanford University. PULSE seeks to advance the frontiers of ultrafast science, with particular emphasis on research using SLAC's Linac Coherent Light Source (LCLS). For more information, please visit

    SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, Calif., SLAC is operated by Stanford University for the U.S. Department of Energy's Office of Science. For more information, please visit

    SLAC National Accelerator Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit

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    Science Up-Close: Developing a Cookbook for Efficient Fusion Energy

    To develop a future fusion reactor, scientists need to understand how and why plasma in fusion experiments moves into a "high-confinement mode" where particles and heat can't escape. Scientists at the Department of Energy's Princeton Plasma Physics Laboratory simulated the transition into that mode starting from the most basic physics principles.

    Peering into the Mist: How Water Vapor Changes Metal at the Atomic Level

    New insights into molecular-level processes could help prevent corrosion and improve catalytic conversion.

    Neutron science publications reach new highs at ORNL's flagship facilities

    The High Flux Isotope Reactor and the Spallation Neutron Source at the Department of Energy's Oak Ridge National Laboratory have reached new levels of increased science productivity. In 2018, a record high of more than 500 scientific instrument publications were produced between HFIR and SNS--based on neutron beamline experiments conducted by more than 1,200 US and international researchers who used the world-leading facilities.

    Fiery sighting: A new physics of eruptions that damage fusion experiments

    Feature describes first direct sighting of a trigger for bursts of heat that can disrupt fusion reactions.

    Microbial Types May Prove Key to Gas Releases from Thawing Permafrost

    Scientists discover key types of microbes that degrade organic matter and release carbon dioxide and methane into the atmosphere.

    An effect that Einstein helped discover 100 years ago offers new insight into a puzzling magnetic phenomenon

    Experiments at the Department of Energy's SLAC National Accelerator Laboratory have seen for the first time what happens when magnetic materials are demagnetized at ultrafast speeds of millionths of a billionth of a second: The atoms on the surface of the material move, much like the iron bar did. The work, done at SLAC's Linac Coherent Light Source (LCLS) X-ray laser, was published in Nature earlier this month.

    Found: A precise method for determining how waves and particles affect fusion reactions

    Like surfers catching ocean waves, particles within plasma can ride waves oscillating through the plasma during fusion energy experiments. Now a team of physicists led by PPPL has devised a faster method to determine how much this interaction contributes to efficiency loss in tokamaks.

    Discovery adapts natural membrane to make hydrogen fuel from water

    In a recent study from the U.S. Department of Energy's (DOE) Argonne National Laboratory, scientists have combined two membrane-bound protein complexes to perform a complete conversion of water molecules to hydrogen and oxygen.

    How Plants Regulate Sugar Deposition in Cell Walls

    Identified genes involved in plant cell wall polysaccharide production and restructuring could aid in engineering bioenergy crops.

    Scientists Identify Gene Cluster in Budding Yeasts with Major Implications for Renewable Energy

    How yeast partition carbon into a metabolite may offer insights into boosting production for biofuels.

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    Top 10 Discoveries of 2018

    Every year, the U.S. Department of Energy's (DOE) Brookhaven National Laboratory compiles a list of the biggest advances made by the Lab's staff scientists, engineers, and visiting researchers. From uncovering mysteries of the universe to building better batteries, here, in no particular order, are our picks for the top 10 discoveries of 2018.

    U.S. Department of Energy Announces $33 Million for Small Business Research and Development Grants

    The U.S. Department of Energy announced it will award 189 grants totaling $33 million to 149 small businesses in 32 states.

    DOE to Provide $16 Million for New Research into Atmospheric and Terrestrial Processes

    The U.S. Department of Energy (DOE) announced a plan to provide $16 million for new observational research aimed at improving the accuracy of today's climate and earth system models.

    Machine learning award powers Argonne leadership in engine design

    When attempting to design engines to be more fuel-efficient and emissions-free, automotive manufacturers have to take into account all the complexity inherent in the combustion process.

    ORNL partners with industry to address multiple nuclear technology challenges

    The Department of Energy's Oak Ridge National Laboratory is collaborating with industry on six new projects focused on advancing commercial nuclear energy technologies that offer potential improvements to current nuclear reactors and move new reactor designs closer to deployment.

    Lithium earns honors for three physicists working to bring the energy that powers the sun to Earth

    Feature describes research of three PPPL physicists who have won the laboratory's 2018 outstanding research awards

    DOE approves technical plan and cost estimate to upgrade Argonne facility; Project will create X-rays that illuminate the atomic scale, in 3D

    The U.S. Department of Energy has approved the technical scope, cost estimate and plan of work for an upgrade of the Advanced Photon Source, a major storage-ring X-ray source at Argonne.

    Costas Soukoulis elected to National Academy of Inventors

    Costas Soukoulis, Ames Laboratory senior scientist and Iowa State University Frances M. Craig Endowed Chair and Distinguished Professor, has been named as a 2018 National Academy of Inventors (NAI) Fellow.

    Biophysicist F. William Studier Elected Fellow of the National Academy of Inventors

    F. William Studier, a Senior Biophysicist Emeritus at the U.S. Department of Energy's Brookhaven National Laboratory and Adjunct Professor of Biochemistry at Stony Brook University, has been elected as a Fellow of the National Academy of Inventors (NAI). He is among 148 renowned academic inventors being recognized by NAI for 2018.

    Blast to the future

    A grant from DOE's Technology Commercialization Fund will help researchers at Argonne and industry partners seek improvements to U.S. manufacturing by making discovery and design of new materials more efficient.

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    Rapid Lake Draining on Ice Sheets Changes How Water Moves in Unexpected Ways

    Widespread fracturing during lake drainage triggers vertical shafts to form that affect the Greenland Ice Sheet.

    New Historical Emissions Trends Estimated with the Community Emissions Data System

    The data system will allow for more detailed, consistent, and up-to-date global emissions trends that will aid in understanding aerosol effects.

    Peering into the Mist: How Water Vapor Changes Metal at the Atomic Level

    New insights into molecular-level processes could help prevent corrosion and improve catalytic conversion.

    Microbial Types May Prove Key to Gas Releases from Thawing Permafrost

    Scientists discover key types of microbes that degrade organic matter and release carbon dioxide and methane into the atmosphere.

    New Method Knocks Out Yeast Genes with Single-Point Precision

    Researchers can precisely study how different genes affect key properties in a yeast used industrially to produce fuel and chemicals.

    How Plants Regulate Sugar Deposition in Cell Walls

    Identified genes involved in plant cell wall polysaccharide production and restructuring could aid in engineering bioenergy crops.

    Scientists Identify Gene Cluster in Budding Yeasts with Major Implications for Renewable Energy

    How yeast partition carbon into a metabolite may offer insights into boosting production for biofuels.

    More Designer Peptides, More Possibilities

    A combined experimental and modeling approach contributes to understanding small proteins with potential use in industrial, therapeutic applications.

    Deep Learning for Electron Microscopy

    Artificial intelligence on Summit to discover atomic-scale structures.

    Clarifying Rates of Methylmercury Production

    New model provides more accurate estimates of how fast microbes produce a mercury-based neurotoxin.


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