Femtosecond X-ray pulse length characterization at the LCLS free electron laser

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. 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 yield X-ray pulse shapes of 120 ± 20 fs FWHM and an upper limit of 40 ± 20 fs FWHM, respectively. Simulations of the SASE process using the GENESIS code agree well with the experimental results.