FLASH and the FLASH2020+ project—current status and upgrades for the free-electron laser in Hamburg at DESY

Abstract

The free-electron laser in Hamburg, FLASH, is the first extreme ultra-violet and soft X-ray free-electron laser (FEL) user facility and has been continuously upgraded since its start in 2005. Further major works are currently underway within the FLASH2020+ project that pioneeringly implements full repetition rate external seeding at a superconducting accelerator facility. With fully tunable undulators providing variable polarization FEL pulses, we expect FLASH to turn into the ideal spectroscopy machine for ultrafast processes within the coming years.

Inverse Problems in Pump–Probe Spectroscopy

“…After sorting out the basic model for the pump-probe dynamics given by Equation (60) with basis functions described in Equation ( 67), we are ready to account for deviations of the dynamics from delta-shaped pump-probe measurements. There are two main reasons why real experimental observations do not exactly follow the trends described in Section 3.2 [41,[48][49][50]]:…”

“…The last additional function, describing a transient coherent oscillation ("to"), can be taken from Equation (57): 61)-( 66), we can describe any pump-probe observable using expression (60). In the previous discussion, we assumed that the pump pulse only initialized the dynamics and that the probe pulse only changed the species produced with the pump.…”

Unraveling the ultrafast dynamics of thermal-energy chemical reactions

“…Alternatively, one could use free-electron laser sources, such as specific IR and THz facilities 93,94 or THz-undulator sources at X-ray free-electron lasers. 95 This could be especially interesting to extend the proposed studies to lower-energy-photon excitations, at the cost of limited accessibility to beamtime at facilities.…”

“…134,[136][137][138] CDI was also demonstrated for small gas-phase molecules, 9,139,140 based on access to the molecular frame through the imaging of laser-aligned species. 9,[141][142][143] Assuming that analyzable gas-phase diffraction patterns can be captured in hours, if not minutes, 36,144 and the recent drive to develop and integrate THz/mid-IR sources at FEL experimental endstations to deliver thermal-energy-scale pump pulses, 95 coupled with Stark deflectors for separating out specific molecular species, 9 the coherent-diffractive imaging of thermal-energy induced reactions at FELs is closer than ever.…”

Time-Resolved X-ray Photoelectron Spectroscopy: Ultrafast Dynamics in CS2Probed at the S 2p Edge

“…In the case of CS 2 , the spectral overlap and varying ionization cross sections of different fragments present some challenges for TRXPS in separating their individual contributions. This motivates the use of channel-resolved analysis techniques such as electron–ion covariance to correlate weaker photoelectron spectral signatures to particular ionic fragments in a background-free manner. ,, TRXPS will also benefit greatly from the ongoing developments in seeded and high-repetition rate FELs, allowing large volumes of data to be recorded at high energy resolution in a short acquisition period. This will further increase the applicability of TRXPS, for instance, by enabling one to scan the probe photon energy and thus probe various core sites within a molecule during a single experiment.…”