Experimental Demonstration of Attosecond Pump-Probe Spectroscopy with an X-ray Free-Electron Laser

Pump-probe experiments with sub-femtosecond resolution are the key tounderstanding electronic dynamics in quantum systems. Here we demonstrate thegeneration and control of sub-femtosecond pulse pairs from a two-colour X-rayfree-electron laser (XFEL). By measuring the delay between the two pulses withan angular streaking diagnostic, we characterise the group velocity of the XFELand demonstrate control of the pulse delay down to 270 as. We demonstrate theapplication of this technique to a pump-probe measurement in core-excitedpara-aminophenol. These results demonstrate the ability to perform pump-probeexperiments with sub-femtosecond resolution and atomic site specificity.