Time-resolved pump-probe experiments beyond the jitter limitations at FLASH

Abstract

Using a noninvasive, electro-optically based electron bunch arrival time measurement at FLASH ͑free electron laser in Hamburg͒ the temporal resolution of two-color pump-probe experiments has been significantly improved. The system determines the relative arrival time of the extended ultraviolet pulse of FLASH and an amplified Ti:sapphire femtosecond-laser pulse at the interaction region better than 90 fs rms. In a benchmarking pump-probe experiment using two-color above threshold ionization of noble gases, an enhancement in the timing resolution by a factor of 4 compared to the uncorrected data is obtained. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3111789͔Progress in generation and application of ultrashort laser pulses over the past two decades has been tremendous. Efforts toward decreasing the pulse duration and the concomitant widening of the spectral range are steadily increasing, thereby enabling the investigation of dynamical phenomena on faster and faster timescales. At FLASH ͑free electron laser in Hamburg͒, 1 in particular, investigations on the phenomena in the extended ultraviolet ͑XUV͒ regime with pulses of only few tens of femtoseconds are now possible, with a worldwide unrivalled high power such that a whole new class of experiments is accessible. The pump-probe excitation scheme is the most promising concept to study dynamical processes on the femtosecond level. Two ultrashort pulses are required for this kind of experiment, first, initiating a reaction and the second permitting the observation of the induced changes. The temporal resolution of this scheme is determined by precise knowledge of the delay between both pulses and, ultimately, by their duration. At FLASH the XUV-free electron laser ͑XUV-FEL͒ pulse can be combined with an optical laser pulse. This scheme has already been utilized from the beginning of its operation as a user facility in 2005 and has led to several exciting results, e.g., 2-6 two complementary laser systems are available. First, a system delivering a high energy pulse ͓Ͼ10 mJ, 120 fs full width at half maximum ͑FWHM͔͒, albeit the repetition rate is limited to 10 pulses per second. The second system provides trains of 20 J pulses ͑120 fs FWHM͒ that map exactly onto the complex timing pattern of the FEL, delivering up to 4000 pulses per second. Since the FEL and the optical laser are independent sources of femtosecond pulses the synchronization between both is of vital importance to perform well defined pump-probe experiments. However, due to technical limitations the synchronization is compromised by the inherent timing jitter reducing the effective temporal resolution from the pulse duration limit of 120 fs ͑FWHMӍ 50 fs͒ root mean square ͑rms͒ width ͑since the FEL pulse has a pulse duration of only few tens of femtoseconds͒ to about 250 fs rms. This jitter is dominated by the accelerator itself. Our approach to increase the temporal resolution is to measure the arrival time of the pulses during a pump-probe experiment independently and shot by shot without ...

Time-resolved resonant soft x-ray diffraction with free-electron lasers: Femtosecond dynamics across the Verwey transition in magnetite

“…2, we fitted the data with a double exponential decay convoluted by a Gaussian taking the FLASH temporal resolution into account, which is mainly determined by the arrival time jitter between the two pulses. 20 This procedure yields that for all laser fluences the initial fast decay is essentially resolution limited which implies a time constant Ͻ270 fs. In all cases, a subsequent slower decay with smaller amplitudes of ϳ20% and ϳ10% follows, the timescale of which we estimate to several ten ps indicating its relationship with a lattice rearrangement.…”

Exploring few-photon, few-electron reactions at FLASH: from ion yield and momentum measurements to time-resolved and kinematically complete experiments

“…Several two-color studies employing optical [125][126][127] and terahertz [124,128] radiation synchronized with EUV pulses have been recently reported, and the development of the beam-splitter setups [122,123] XFELs, multiphoton and time-resolved experiments discussed here can be extended deep into the X-ray domain. As mentioned in the Introduction, this will open the way for a number of novel imaging techniques, including the possibility of coherent diffraction imaging of nanoscale objects [132,133] and recording "molecular movies" of light-induced chemical reactions via time-resolved X-ray diffraction or photoelectron holography [76].…”

Extending electro-optic detection to ultrashort electron beams

“…Recently, this has been adapted to the longitudinal profiling of accelerated charged particle beams. EOS has been used to characterize beams either directly, by measuring their self-fields [2][3][4][5][6][7][8][9][10][11], or indirectly by measuring the THz fields generated from transition radiation diagnostics [12,13]. In the case of the former, the particle beam's longitudinal profile can be deduced from its self-fields due to the fact that for sufficiently relativistic beams the temporal structure of the selffields matches the temporal structure of the beam [14].…”