3.3 Transient Structures and Imaging with X-rays

Imaging with soft X-rays combines high spatial resolution (< 50 nm) with unique contrast mechanisms, such as spectroscopic and magnetic contrast, and high penetration depth. In addition, the time structure of the X-ray sources enables time-resolved imaging with picosecond down to femtosecond temporal resolution. The key issue of X-ray imaging is to efficiently solve the phase problem, i. e., the loss of the scattered wave's phase information upon detection. We particularly employ (and further develop) three different methods: numerical iterative phase retrieval from a coherent diffraction pattern, interference with a reference wave (holography), and full-field microscopy using a diffractive X-ray lens.

In a pump-probe approach, photo-induced structure changes in the femtosecond time domain are studied by diffracting ultrashort hard X-ray pulses from the excited sample. Both changes of atomic arrangements, i.e. lattice geometries, and of electronic charge density are addressed. The measurement of transient electron density maps has been pioneered by MBI and requires the simultaneous detection of many diffraction peaks. Femtosecond powder diffraction and the rotation method for single crystals have been implemented to fulfill this requirement.