image: Determining the structure of an X-ray laser pulse based on the angular streaking of photoelectrons. view more
Credit: Nikolay Kabachnik
Professor of Skobeltsyn Institute of Nuclear Physics (SINP), MSU and his foreign colleagues determined the physical parameters of ultrashort X-ray laser pulses with high temporary resolution. The results of the study were published in Nature Photonics journal.
The researchers carried out a series of unique experiments and determined energy and time characteristics of the pulses of the X-ray free-electron laser. The photon beam in such lasers is generated by a bunch of relativistic electrons, i.e. the electrons that move with the speed comparable to that of light. The electrons pass a number of specifically arranged magnets and lose a part of their energy that is turned into a laser radiation. Using this technology, the scientists managed to obtain high-power ultrashort X-ray pulses.
"X-ray free-electron lasers provide us with a unique opportunity to study the structure of atoms, molecules (including biological ones), and solids, as well as to research the dynamics of atomic and molecular processes and their development in time with unprecedented precision. To use these opportunities, one should know the parameters of the pulses including time properties, such as duration, temporary structure of the pulse, and the time of its arrival at the target," explained Nikolay Kabachnik, a co-author of the work and leading research associate at the department for physics of atomic nuclei of SINP, MSU.
To determine the parameters of laser emission the scientists used the angular streaking method. In an experimental unit, laser pulses hit the atoms of neon, and the latter lost a part of their electrons. The emitted electrons got into the rotating (circularly polarized) field of infrared radiation. As a result of their interaction with the field, electrons changed their energy and angular distribution (the dependence of electron intensity on the angle). After measuring these characteristics the scientists made a conclusion about the structure and properties of the initial laser pulse.
In modern X-ray free-electron lasers individual pulses differ by their parameters (energy, duration, intensity, etc.) and therefore are described statistically, i.e. by averaging out the characteristics of a big number of pulses. However, the method presented by the authors of the research provides for the measurement of properties of each particular pulse. The theoretical description of this process was suggested by Professor Nikolay Kabachnik and his colleague Professor Andrey Kazansky (DIPC, San Sebastian).
"This method was theoretically justified in our previous work. Its implementation by an international group of physicists allows to determine the parameters of individual X-ray free electron laser pulses with attosecond resolution (1 attosecond is a billionth part of one billionth of a second). This opens the way for experiments studying the development of ultra-fast molecular processes in time," added the scientist.
In the study, the scientists used as a target a very simple system of neon atoms. However, the work may serve as a basis for similar experiments on much more complicated objects, such as molecules that take part in photosynthesis. Knowing the details of photoexcitation (the initial stage of photosynthesis) will help the scientists better understand this important but understudied process. Detailed research of the photoionisation process and its development in time may increase the efficiency of solar panels and lead to the development of faster computer chips.
The work was conducted by the scientists from SLAC National Accelerator Laboratory (USA), University of Bern (Switzerland), German Electron Synchrotron DESY (Germany), University of Kassel (Germany), Technical University of Munich (Germany), European X-ray free electron laser, Lomonosov Moscow State University, Stanford University (USA), University of Gothenburg (Sweden), Qamcom Research & Technology AB (Sweden), University of Colorado at Boulder (USA), University of the Basque Country (Spain), Donostia International Physics Center (Spain), Ikerpbasque Fund (Spain), Max Planck Institute of Quantum Optics (Germany), and Ludwig Maximilian University of Munich (Germany).
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Journal
Nature Photonics