X-ray--optical cross correlator for gas-phase experiments at the LCLS free-electron laser

X-ray--optical pump--probe experiments at the Linac Coherent Light Source (LCLS) have so far been limited to a time resolution of 280\,fs fwhm due to timing jitter between the accelerator-based free-electron laser (FEL) and optical lasers. We have implemented a single-shot cross-correlator for femtosecond x-ray and infrared pulses. An independent reference experiment relying only on the pulse arrival time information from the cross-correlator shows a time resolution better than 50\,fs fwhm (22\,fs rms) and also yields a direct measurement of the maximal x-ray pulse length. The improved time resolution enables ultrafast pump--probe experiments with x-ray pulses from LCLS and other FEL sources. Reference: S. Schorb et al., Appl. Phys. Lett. 2012 in press