Femtosecond x-ray pulse length characterization at the Linac Coherent Light Source free-electron laser

S Düsterer ,Adrian L Cavalieri ,Yuantao Ding ,Wolfram Helml ,F Grüner ,Reinhard Kienberger ,Wolfgang Schweinberger ,Gilles Doumy ,Vincent Richardson ,C Roedig ,Thomas Tschentscher ,J T Costello ,Louis F Dimauro ,Andreas Maier ,D Cubaynes ,Christoph Bostedt ,Ryan Coffee ,John D Bozek ,P Radcliffe ,Marc Messerschmidt ,Michael Meyer

doi:10.1088/1367-2630/13/9/093024

Abstract

Two-color, single-shot time-of-flight electron spectroscopy of atomic neon was employed at the Linac Coherent Light Source (LCLS) to measure laser-assisted Auger decay in the x-ray regime. This x-ray-optical cross-correlation technique provides a straightforward, non-invasive and on-line means of determining the duration of femtosecond (>40 fs) x-ray pulses. In combination with a theoretical model of the process based on the soft-photon approximation, we were able to obtain the LCLS pulse duration and to extract a mean value of the temporal jitter between the optical pulses from a synchronized Ti-sapphire laser and x-ray pulses from the LCLS. We find that the experimentally determined values are systematically smaller than the length of the electron bunches. Nominal electron pulse durations of 175 and 75 fs, as provided by the LCLS control system, yield x-ray pulse shapes of 120±20 fs full-width at half-maximum (FWHM) and an upper limit of 40±20 fs FWHM, respectively. Simulations of the free-electron laser agree well with the experimental results.

Highlights

The Linear Coherent Light Source (LCLS) started lasing in the X-ray regime (0.8 – 8 keV) in the summer of 2009 with an unprecedented flux in the range of 1013 photons per pulse and controllable duration from a few-fs up to 300 fs [1]
Two-color, single-shot time-of-flight electron spectroscopy of atomic neon was employed at the Linear Coherent Light Source (LCLS) to measure the laser-assisted Auger decay in the X-ray regime
Each spectrum represents the acquisition of many single-shot spectra (∼ 103), which were sorted a posteriori into 50 fs time bins, according to the LCLS phase cavity electron arrival time monitor, and averaged [6]

Summary

Introduction

The Linear Coherent Light Source (LCLS) started lasing in the X-ray regime (0.8 – 8 keV) in the summer of 2009 with an unprecedented flux in the range of 1013 photons per pulse and controllable duration from a few-fs up to 300 fs [1]. Combined with appropriate focusing optics, peak irradiance levels of more than 1017 W/cm are possible, extending multi-photon [2, 3, 4] and non-linear [5] spectroscopy into the X-ray region of the electromagnetic spectrum. With temporally synchronized ultra-fast lasers, new pump-probe experiments involving intense laser fields of radically different photon energies can be performed [6, 7]. In all experiments, utilizing both single color pulses and pump-and-probe methodologies, the X-ray pulse duration is a pivotal parameter, and so the continuous development of reliable techniques to determine it are crucial for the full exploitation of ultra-fast X-ray free electron lasers (FEL). In addition to the pulse length, the technique reported here can reveal and quantify the timing jitter of the FEL with respect to an external synchronized optical laser, a parameter especially important for time-resolved pump-and-probe experiments

Methods

Results

Conclusion