Jitter-correction for IR/UV-XUV pump-probe experiments at the FLASH free-electron laser

Abstract

In pump-probe experiments employing a free-electron laser (FEL) in combination with a synchronized optical femtosecond laser, the arrival-time jitter between the FEL pulse and the optical laser pulse often severely limits the temporal resolution that can be achieved. Here, we present a pump-probe experiment on the UV-induced dissociation of 2,6-difluoroiodobenzene (C 6 H 3 F 2 I) molecules performed at the FLASH FEL that takes advantage of recent upgrades of the FLASH timing and synchronization system to obtain high-quality data that are not limited by the FEL arrival-time jitter. We discuss in detail the necessary data analysis steps and describe the origin of the timedependent effects in the yields and kinetic energies of the fragment ions that we observe in the experiment.

FLASH free-electron laser single-shot temporal diagnostic: terahertz-field-driven streaking

“…for most of the settings investigated so far. This result tells us that one can indeed significantly improve the time resolution of optical-XUV pump-probe experiments by sorting the acquired data according to the electron bunch arrival times measured by the BAM (Savelyev et al, 2017). Moreover, this diagnostic can in future be used to characterize and improve the performance of the FLASH FEL synchronization system and the arrival time detector.…”

Time-resolved ion imaging at free-electron lasers using TimepixCam

“…Both the FLASH FEL as well as the pumpprobe laser operated at a repetition rate of 10 Hz. From a measurement of the electron bunch duration in the FLASH accelerator (Dü sterer et al, 2014;Savelyev et al, 2017), the XUV pulse length was estimated to be 120 fs (FWHM), while Sketch of the experimental setup showing the molecular beam, the path of the UV, NIR and XUV laser beams, and the double-sided velocity-map imaging spectrometer. Ions were detected with an MCP and a fast P47 phosphor screen detector coupled with TimepixCam.…”

“…Electron images, which are discussed in a separate publication (Brauße et al, 2018), were recorded at 10 Hz using a commercial CCD camera (Allied Vision Pike F-145B), while the ion images were recorded with TimepixCam, which is described in detail in the following section. Further details on the data analysis and sorting procedure for pump-probe experiments using the FLASH FEL in combination with the FLASH pump-probe laser, such as the normalization of the data to the fluctuating XUV pulse energy and the correction for the arrival-time jitter between the XUV and the UV/NIR pulses, are given by Savelyev et al (2017).…”

“…The current version of TimepixCam, which employs the UNO readout system for the data acquisition (Imatek, 2016), allows a continuous readout at approximately 10 Hz, limited by the USB connection to the computer and the camera software. The FLASH 'bunch ID', which is an identification number for each FEL pulse, was stored along with the data from each readout cycle and was used to establish the correspondence of the TimepixCam frames and the FLASH bunches so that various intensity and pump-probe timing corrections could be applied (Savelyev et al, 2017). This association was made by the readout software, which recorded the bunch ID in the header of each camera frame.…”

“…The increased availability of short-pulse extreme ultraviolet (XUV) and (soft) X-ray sources such as free-electron lasers (FELs) (Ackermann et al, 2007;Shintake et al, 2008;Feldhaus, 2010;Emma et al, 2010;Allaria et al, 2012Allaria et al, , 2013Ishikawa et al, 2012;Ullrich et al, 2012) and high-order harmonic generation (HHG) sources (Popmintchev et al, 2010;Chini et al, 2014;Hä drich et al, 2014), accompanied by continuing advances in femtosecond laser technology, promises a new era for femtosecond pump-probe experiments studying the dynamics of photochemical reactions in gas-phase molecules. Many of these experiments are performed using the velocitymap imaging (VMI) technique (Eppink & Parker, 1997), where the photoelectrons or fragment ions that are created by the ionizing light pulse are accelerated by an electric field and projected onto phosphor screen detectors that are commonly read out by CCD or CMOS cameras.…”