Ultrafast X-ray Science @ 25: Capturing Dynamics in Molecules and Materials using Ultrafast Coherent X-Rays

The discovery of x-rays made possible not-only new medical technologies, but also allowed man to “see” for the first time at the atomic scale, revolutionizing our understanding of matter. Using element-specific absorption edges, soft x-rays can provide elemental, spin and chemical site-specific information. Efforts to bring x-ray techniques into the realm of femtosecond time-scales—the fundamental “atomic” time scale— have now been ongoing for approximately the past 1⁄4 century, and have brought new insight into the behavior of matter, at the truly fundamental time-scale for atomic interactions. Ultrafast high harmonic (HHG) x-rays can probe the fastest charge, spin and energy transport in molecular and materials systems relevant to function at the nanoscale. Understanding the correlated motions of electrons and nuclei lies at the heart of chemistry. In molecular systems, the sudden removal of an electron after irradiating a molecule with a short burst of x-rays will initiate radiationinduced femtochemistry, by creating super-excited states of the molecular ion. In recent work, we used few-fs HHG and laser pulses to initiate and probe the explosion of triatomic molecular ions (ozone and N2O) in real time, in a regime dominated by coupled electron-electron and electron-nuclear dynamics where a simple Born-Oppenheimer approximation breaks down.[1, 2] We observe ultrafast multi-electron dynamics as a result of the competition between autoionization and dissociation. We also show that the branching ratio for bond breaking can be altered in the presence of the laser field. Finally, in other work we probe what information can be extracted from angle dependent photoelectron holography (Fig. 1).[3] In materials systems, HHG can probe the dynamics of the fundamental quantum mechanical exchange interaction in magnetic materials such as layered systems and alloys, relevant to our understanding of correlated matter, and also to magnetic switching for next-generation data storage technologies.[5] HHG also enables record spatial resolution tabletop nano-imaging, and makes it possible to understand the limiting physics of energy flow at the nanoscale.[6, 7]