  Arbitrary manipulation of the temporal and spectral properties of X-ray
pulses at free-electron lasers (FELs) would revolutionize many experimental
applications. At the Linac Coherent Light Source at Stanford National
Accelerator Laboratory, the momentum phase-space of the FEL driving electron
bunch can be tuned to emit a pair of X-ray pulses with independently variable
photon energy and femtosecond delay. However, while accelerator parameters can
easily be adjusted to tune the electron bunch phase-space, the final impact of
these actuators on the X-ray pulse cannot be predicted with sufficient
precision. Furthermore, shot-to-shot instabilities that distort the pulse shape
unpredictably cannot be fully suppressed. Therefore, the ability to directly
characterize the X-rays is essential to ensure precise and consistent control.
In this work, we have generated X-ray pulse pairs and characterized them on a
single-shot basis with femtosecond resolution through time-resolved
photoelectron streaking spectroscopy. This achievement completes an important
step toward future X-ray pulse shaping techniques.
