Visualization of atomic and electronic motion in 4D

"On a fundamental level, chemical reactions and condensed-matter transformations are defined by the motion of atoms and electrons from initial to final conformations, typically along a complex reaction path involving ultrasmall and ultrafast dimensions. Here we report on our progress towards a full visualization of structural dynamics in space and time. After laser excitation, ultrashort electron pulses at 30-100 keV are diffracted with time delays and provide a pump-probe sequence of structural snapshots with atomic resolution. We solve the most essential problem for time resolution, Coulomb repulsion, by using single-electron pulses in combination with a microwave compressor. The so achieved 12-fs electron pulses (rms) are among the shortest worldwide and now provide access to the fastest phonons or molecular modes with atomic resolution. In order to further advance towards the regime of purely electronic motion, we apply the microwave compressor’s time-dependent fields for reshaping the single-electron phase space from the temporal into the energetic domain. The achievable pulse durations are shorter than optical light cycles, promising direct diffraction access to electronic motion with a resolution of picometers and attoseconds. We report our first proof-of-principle results and reflect on what discoveries we may expect to see."