Ultrashort Free-Electron Laser X-ray Pulses

Abstract

For the investigation of processes happening on the time scale of the motion of bound electrons, well-controlled X-ray pulses with durations in the few-femtosecond and even sub-femtosecond range are a necessary prerequisite. Novel free-electron lasers sources provide these ultrashort, high-brightness X-ray pulses, but their unique aspects open up concomitant challenges for their characterization on a suitable time scale. In this review paper we describe progress and results of recent work on ultrafast pulse characterization at soft and hard X-ray free-electron lasers. We report on different approaches to laser-assisted time-domain measurements, with specific focus on single-shot characterization of ultrashort X-ray pulses from self-amplified spontaneous emission-based and seeded free-electron lasers. The method relying on the sideband measurement of X-ray electron ionization in the presence of a dressing optical laser field is described first. When the X-ray pulse duration is shorter than half the oscillation period of the streaking field, few-femtosecond characterization becomes feasible via linear streaking spectroscopy. Finally, using terahertz fields alleviates the issue of arrival time jitter between streaking laser and X-ray pulse, but compromises the achievable temporal resolution. Possible solutions to these remaining challenges for single-shot, full time-energy characterization of X-ray free-electron laser pulses are proposed in the outlook at the end of the review.

Time-resolved photoelectron angular distributions from nonadiabatically aligned CO2 molecules with SX-FEL at SACLA

“…In figure 4(c), one can see considerable change of the LFPAD, for the molecules aligned parallel to the polarization vector, from those in figures 4(a) and (b). This change is reasonably reproduced by the simulated LFPAD according to equation (4), which is the average of the MFPAD over the molecular orientation, as can be seen in the figure.…”

“…Recent developments of x-ray free electron lasers (XFELs) [1][2][3][4] and table-top laser sources [5] are bringing off ultrafast imaging of molecules with femtosecond temporal and sub-Ångstrom spatial resolutions [6][7][8][9]. The first imaging of atomic motions during photochemical processes has been achieved by the ultrafast electron diffraction (UED) [10][11][12] and the ultrafast x-ray diffraction (UXD) [13][14][15][16].…”

“…Polarization vector of the NIR laser is shown by the double-headed arrows. The distribution functions are used for the theoretical simulations of equation(4). The statistical errors of the experimental results are±0.5%.…”

Femtosecond profiling of shaped x-ray pulses

“…For the interested reader, a comprehensive review of x-ray FEL pulse characterization techniques can be found in [33]. Otherwise, it should be emphasized that the novel aspect of the current work is not in x-ray diagnostics.…”

X-ray Free-Electron Laser

“…The present issue also includes XFEL-pulse characterizations and instrumentations. Helml et al review the progress on ultrashort pulse characterization of XEFLs [9], while Inbushi et al…”