Ultrafast charge and energy transfer dynamics in solids and at interfaces revealed by time-resolved x-ray absorption spectroscopy.

The excitation of solid materials with femtosecond optical pulses induces non-equilibrium states that are interesting for both fundamental and application-related reasons, as their properties can be quite different from those generated by e.g. thermal excitation or chemical doping. However, once several degrees of freedom such as electronic and lattice excitations, or multiple components in a heterostructure, couple strongly, analyzing the resulting ultrafast dynamics on femto- to picoseconds requires element-sensitive spectroscopy on these timescales. This Topical Review discusses how new insights into ultrafast charge and energy transfer dynamics in solids and at interfaces can be gained from time-resolved soft x-ray absorption spectroscopy (XAS). The state-of-the-art of the pump-probe XAS method at large scale facilities, namely synchrotron Femtoslicing sources and x-ray free electron lasers, is reviewed, with an emphasis on the opportunities and challenges of the respective facilities that motivate new methodological developments. For three selected material systems, it is then shown how x-ray absorption spectroscopy with femtosecond time resolution, in conjunction with theoretical calculations, is able to reveal and characterize non-equilibrium states by analyzing the transient electronic structure as well as lattice excitation via characteristic, time-dependent spectral features. Finally, future opportunities through the further development of laboratory-based soft x-ray sources and the combination of time-resolution with non-linear XAS are presented.