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Scientists used terahertz radiation for measurements of strongly excited material
In-depth understanding of the electrical conductivity of matter is the key to many cutting-edge research and applications, ranging from phase-change memory in microelectronics to magnetospheres rooted in planetary interiors due to the motion of the conductive fluid. Unique states of material created by ultrafast table-top lasers or free-electron lasers (FEL) allow us to gain insight into atomic levels. However, it also requires sub-picosecond resolution to capture the details on the timescale of atomic motion.
An international research team, led by scientists from the SLAC National Accelerator Laboratory and DESY, have recently measured the electrical conductivity of strongly heated material using the THz FEL radiation at FLASH. In this study, gold nano-foil samples were heated by the FLASH extreme ultraviolet (XUV) FEL pulses to electron temperatures up to 16,000 °C. As the thermal energy transfers from the electrons to the ions, the sample transits from cold to superheated solid and eventually melts into warm dense liquid. The researchers have determined the DC electrical conductivity by measuring the transmitted THz electric field through the heated samples. The multi-cycle THz pulses from FLASH provide continuous measurements with temporal resolutions better than 500 femtoseconds.
Image: The FLASH XUV pulse heats the gold nanofoil samples to electron temperature above 16,000 °C while the multi-cycle THz pulse enables measuring the electrical conductivity of the nanofoil samples as thermal energy transfers from electrons to ions (Credit: Z. Chen, SLAC).
DOI:10.1038/s41467-021-21756-6
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Study at FLASH with femtosecond time-resolved X-ray photoemission spectroscopy (TR-XPS)
Molecular heterojunctions receive significant attention due to their key role in a wide variety of emerging organic semiconductor applications, such as organic light-emitting diodes, field-effect transistors, spintronic devices, and photovoltaic cells. Understanding ultrafast dynamics of photon-to-charge conversion is paramount for optimising novel light-harvesting systems. By using the femtosecond long X-ray pulses of the free-electron laser FLASH at the plane-grating monochromator beamline PG2 and the wide-angle electron spectrometer (WESPE) end station, the team of researchers has access to specific charge separation sites and monitor free charge formation in a model donor-acceptor system on their natural timescale.
Image: Left: 2D false-colour map of the measured time-dependent C 1s signal of the CuPc:C60 heterojunction as a function of time-delay and kinetic energy and schematic representation of the time-resolved XPS experiment and the observed kinetics. (Figures from original publication F. Roth et al., licensed under a Creative Commons Attribution 4.0)
DOI:10.1038/s41467-021-21454-3
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New method displays accelerating field in plasma accelerator with unparalleled precision
The technology of plasma-based acceleration promises to deliver a new generation of powerful and compact particle accelerators. Prior to applying this new technology, however, various obstacles must be overcome. In particular precise control of the acceleration process itself must be achieved. Using an innovative technique researchers at DESY have now succeeded in measuring the accelerating plasma wake with previously unattained precision. Their method allows the shape of the effective accelerating field to be determined with a resolution on the order of femtoseconds (trillionths of a millionth of a second) so that the acceleration process can be studied in great detail, thereby paving the way for the controlled and optimised operation of future plasma accelerators, as the team led by DESY’s Jens Osterhoff explains in the journal Nature Communications.
Image: View of the FLASHForward accelerator module. The plasma is generated in the narrow channel in the centre by a high voltage. Credit: DESY, Alexander Knetsch
DOI:10.1038/s41467-020-19811-9
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Free electron laser study sheds light on the mechanism and timescale of the photocatalytic oxidation of carbon monoxide on titanium oxide
An international team of scientists have studied, for the first time, the light induced oxidation of carbon monoxide (CO) to carbon dioxide (CO2) on the surface of an oxide photocatalyst in real-time using the soft X-ray free-electron laser FLASH combined with theoretical calculations. According to their findings, which are published in the journal ACS Catalysis, the photoconversion of CO to CO2 takes place between 1.2 and 2.8 picoseconds (ps) after the reaction is triggered by an ultrafast optical laser pulse.
Image: Using FLASH, the research team was able to precisely resolve that between 1.2 and 2.8 picoseconds after triggering, the carbon monoxide oxidised to carbon dioxide.
DOI:10.1021/acscatal.0c04098
Kai Siegbahn was awarded the 1981 Nobel Prize in physics for “his contribution to the development of high-resolution electron spectroscopy”.
Albert Einstein received the 1921 Nobel Prize in physics for “his services to theoretical physics, and especially for his discovery of the law of the photoelectric effect”.
10 years of SASE at FLASH
Back in 2005, early in the morning of January 14th, first SASE has been observed at DESY's newly installed VUV free-electron laser. The electron beam has been accelerated to 445 MeV corresponding to a wavelength of 32 nm. In summer 2005, the VUV-FEL turned into a user facility named FLASH.
The image shows the Spectrum of the first SASE signal measured in the early morning of January 14th, 2005.
SASE FEL radiation observed on a Ce:YAG screen of the FLASH2 photon beamline.
Fifth user period started 24-Feb-2014 with its first beam time block
Coming out of a long shutdown to finish up the construction of the new beamline FLASH2, the fifth user period for beamline FLASH1 has started end of February with its first user block. Until April 2015, more than 5000 hours of user experiments are scheduled. Beam time will also be available for accelerator and photon beam line studies as well as for FLASH2 commissioning. FLASH2 saw first beam in March 2014 pushing the beam to the dump for the first time May, 23. Since then, FLASH2 is operated in parallel to FLASH1 whenever possible to finish up the commissioning of beam diagnostics and to refine beam optics. First SASE radiation at 40 nm has been seen on Aug 20, 2014. The next goal is to characterize the SASE radiation, to measure gain length for example for as many other wavelength.
The image shows a schematic layout of FLASH. Not to scale. The second beamline, FLASH2, is being commissioned.
Installation of the new switch yard and extraction beamline to FLASH2 ready to go. The extraction beamline turns to the right through the new opening of the FLASH tunnel towards the FLASH2 beamline hall.
