Development of Attosecond Two-Color Pump/Probe Techniques with X-ray Free-Electron Lasers*

The natural time scale of quantum mechanical motion of electrons in molecules and solids is on the order of hundreds of attoseconds to few femtoseconds. Probing dynamics on such timescales requires the generation of attosecond pulse pairs with variable delay and synchronization down to the sub-femtosecond level. We report the generation of GW-level attosecond pump/probe pulse pairs with tunable sub-femtosecond delays at the Linac Coherent Light Source (LCLS). Two pulses are generated in a harmonic configuration to access sub-femtosecond delays. We use the angular streaking technique to measure temporal delays between pulse pairs and demonstrate control of our pump/probe setup with attosecond accuracy. The measured minimal achievable delay is 200~300 attoseconds, with a corresponding delay jitter that is shorter than the duration of the pulse.