Angular streaking with attosecond x-ray pulses

The fundamental process of interaction between light and matter involves ultrafast electron motions in atoms and molecules. The pursuit of understanding electron motion drives the development of attosecond x-ray pulses. Together with the unique properties of a free-electron laser, we can extend the attosecond x-ray pulse to the soft x-ray regime, and also to reach high enough intensity to drive interesting processes including nonlinear interactions. At the Linac Coherent Light Source (LCLS), where a relativistic electron bunch travels through an undulator, coherent x-ray laser pulses are generated. We manipulate the electron beam to alter the electron distribution in the bunch, resulting in the emission of an attosecond x-ray pulse in the undulator. Subsequently, a velocity map imaging spectrometer collects photoelectrons emitted from gas molecules interacting with the x-ray photons. The momentum distribution of these photoelectrons is measured and decoded using a reconstruction algorithm to characterize the attosecond x-ray pulses. Beyond attosecond pulse characterization, angular streaking serves as a time-resolved measurement technique for studying ultrafast electron motions, including Auger Meitner decay and photoemission delay. The presentation will delve into the exciting results of measuring and controlling electronic coherences in core-excited states, which paves the way for a wide range of applications in ultrafast science.