X-ray lasers can spot elusive electron motion

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Greg Stewart/SLAC National Accelerator Laboratory

Scientists can track the movements of an atom’s nucleus relatively easily, but electrons have proven elusive — they move so fast that they tend to be reduced to blurs. Now, however, those movements could be crystal clear. Researchers at the SLAC National Accelerator Lab have developed a technique, X-ray laser-enhanced attosecond pulse generation (XLEAP), that can observe even the fastest motions of electrons. The laser pulses at just 280 attoseconds, or billionths of a billionth of a second, and can create snapshots of electrons to track their progress. The trick was to modify the laser in a way that squeezed electrons into tighter groups, making for shorter X-ray bursts.

X-ray lasers like SLAC’s Linac Coherent Lightsource have an undulator, or a magnet that converts some of the energy from electron beams into X-ray bursts. The team added two magnets in front of the undulator to shape the electron groups into narrow, very intense spikes (some nearly 500 megawatts) with a wide variety of energies. From there, they could get attosecond-level X-ray flashes.

It was another matter to measure the X-rays. That required creating a device that sent the X-rays through a gas and stripped them of some of their electrons to create an electron cloud. An infrared laser gives a “kick” to those electrons, leading to different movement speeds that help scientists calculate the length of an X-ray pulse.

This method could lead to breakthroughs in… well, virtually any scientific field that studies atoms. Biologists, chemists and material scientists could more accurately study processes that start at the electron level, such as photosynthesis. And the technology should get better — SLAC expects both refinements and the next-gen LCLS-II laser (which shoots X-ray pulses 8,000 times faster) to allow for more intense and potentially shorter pulses. It might soon be possible to study the activities of molecules at the shortest possible intervals.

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