Phys. Rev. Lett. 110, 137403 (2013)

Observing the motion of electrons during chemical or biological processes is of interest as it can provide important insights into the mechanisms involved. Although scattering short X-ray pulses from electronic wave packets is a promising approach for realizing this goal, there are still some questions about the feasibility of achieving such imaging. Now, Gopal Dixit and co-workers in Germany have theoretically proposed a way to image the instantaneous electron density of the wave packet by ultrafast X-ray phase contrast imaging in the far field. The team suggests that the technique can be used to obtain information about dynamical changes in the spatial electron probability distribution at different points in time. The approach may also provide the Laplacian of the instantaneous electron density, which can be used to garner details of the internal structures of the wave packet. This may be useful for understanding complex bonding and topology of charge distributions in complex systems. The researchers emphasize that inelastic scattering processes, which are a problem for far-field ultrafast scattering, do not affect ultrafast X-ray phase-contrast imaging.