Scientists from the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University demonstrated that they could create ripples in molecules through “Impulsive Raman Scattering” using powerful attosecond x-ray laser pulses. This experiment is the first to take advantage of the new technology to produce attosecond x-ray laser pulses. The study is published in Physical Review Letters.
Generally, x-ray pulses cause the electrons in the innermost core of molecules to jump to higher energies. These core-excited states are highly unstable, and the electrons return to their stable state releasing energy to the neighboring electron. This decay process happens just in a millionth of a billionth of a second, forcing the excited electron out of the atom producing a charged ion.
If the x-ray pulses are sufficiently short and intense, it can excite the core electrons while driving an outer electron to fill the gap. It allows the molecule to enter into an excited state while keeping its atom in a stable neutral state. Since this Raman process relies on the electronic excitation of the core-level electrons, it is easier to trace its origin and evolution.
According to the co-author and SLAC scientist James Cryan, the Raman interaction is similar to taking a rock and tossing it into the water. Similar to how the rock creates a rippling wave on the water surface, the x-ray excitation creates charge waves that ripple across the molecule’s surface. They provide researchers with an entirely new way to measure the response of a molecule to light.
Excited-state molecules can also be created using pulsed visible light; however, these are more like small earthquakes that ripple the entire surface. The impulsive Raman X-ray excitation, on the other hand, is equivalent to dropping rocks in various places to produce and observe different ripple patterns.
Earlier LCLS experiments demonstrated the Raman process in atoms, but this is the first time scientists observed this in molecules. The success is due to the recent developments in producing x-ray free-electron laser (FEL) pulses 10 to 100 shorter than before. The X-ray Laser-Enhanced Attosecond Pulse project (XLEAP), led by SLAC scientist Agostino Marinelli, provided a method to generate intense pulses that are just 280 attoseconds, or billionths of a billionth of a second, long. These pulses were critical to the success of the experiment.
This new observation will allow scientists to study how electrons zipping around molecules kick off key processes in biology, chemistry, materials science, and more.
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