Measuring sub-femtosecond x-ray pulses with angular streaking*

The recent development of sub-femtosecond x-ray pulses from free-electron lasers has called for a high resolution measurement scheme to characterize such short pulses and the electronic dynamics they induce. The angular streaking technique exploits the phase-dependent momentum shift experienced by the photoelectrons ionized by x-ray pulses in the presence of a circularly polarized streaking laser field. We present a method to extract the temporal and spectral profiles of an electronic wavepacket produced by x-ray ionization from the photoelectron momentum distribution. We use this method to demonstrate the attosecond operation of a soft x-ray FEL, and study attosecond electron dynamics in x-ray photoionization.