Timing methodologies and studies at the FERMI free-electron laser

Abstract

Time-resolved investigations have begun a new era of chemistry and physics, enabling the monitoring in real time of the dynamics of chemical reactions and matter. Induced transient optical absorption is a basic ultrafast electronic effect, originated by a partial depletion of the valence band, that can be triggered by exposing insulators and semiconductors to sub-picosecond extreme-ultraviolet pulses. Besides its scientific and fundamental implications, this process is very important as it is routinely applied in free-electron laser (FEL) facilities to achieve the temporal superposition between FEL and optical laser pulses with tens of femtoseconds accuracy. Here, a set of methodologies developed at the FERMI facility based on ultrafast effects in condensed materials and employed to effectively determine the FEL/laser cross correlation are presented.

Design of a soft x-ray free-electron lasers arrival time diagnostic device

“…Pump-probe experiments rely on stable spatial superposition and well-defined time delay between pump and probe beams. The time instant (time zero, 𝑡 0 ) when the pulses are temporary overlapped has to be determined 4,5 . When X-ray FEL (especially based on the self-amplified spontaneous emission (SASE) mode) is used as the probe pulse for pump-probe experiments, there is a femtosecond-scale timing jitter between the arrival times of each FEL pulse.…”

Ultrafast Dynamics of Plasmon-Mediated Charge Transfer in Ag@CeO2 Studied by Free Electron Laser Time-Resolved X-ray Absorption Spectroscopy

“…In order to account for possible nonuniformities of the sample, we have excluded the uppermost and the lowermost 5% of the distribution of the measured changes in transmission for each energy and delay. Time zero was calibrated using Si 3 N 4 as usually performed for FEL/Vis pump cross-correlation . It is possible that slight variations in the zero delay from run to run (e.g., changing the photon energy of the FEL) can result in a variation of the delay by up to 100 fs.…”

“…Time zero was calibrated using Si 3 N 4 as usually performed for FEL/Vis pump cross-correlation. 34 It is possible that slight samples. The spectra of Ce 4+ (dashed green line) and Ce 3+ (dashed blue line) reference samples taken from the literature (30) are also reported.…”