Dynamics deep from the core.

In van der Veen et al., [Struct. Dyn. 2, 024302 (2015)], femtosecond and nanosecond electron energy loss spectroscopy of deep core-levels are demonstrated. These results pave the way to the investigation of materials and molecules with combined energy, time, and spatial resolution in a transmission electron microscope. Furthermore, the authors elucidate the role of the electron phonon coupling in the band-gap renormalization that takes place in graphite upon photo-excitation.

If today Prometheus were to ask condensed matter physicists “What would you want me to risk my liver for?” he would be answered “An element specific technique capable of correlating atomic motions with changes in the orbital occupancy, local spin states, and angular momenta.”

This is achievable today by ultrafast core-level spectroscopy available at modern X-ray Free Electron Lasers (XFEL) sources. However, a lab-based technique, such as femtosecond-Electron Energy Loss Spectroscopy (FEELS) delivers very similar information and is complementary to X-ray spectroscopy, as it allows the investigation of very tiny samples, such as nanostructures, thin films, and of samples consisting of very light elements. Furthermore, the very high spatial resolution reached by electron-based techniques can be combined with EELS in energy filtered electron microscopy and/or via scanning techniques, giving access to inhomogeneities and phase separation effects typically found near the critical point of phase transitions.

The challenge of these experiments resides in the inherent tendency of electrons to repel each other (the so-called space-charge effect), causing temporal broadening of the pulses, chromatic aberrations, and a loss in energy resolution. This is currently circumvented by operating the ultrafast transmission electron microscope (TEM) with one electron only per pulse, thus limiting the signal-to-noise ratio achievable in this kind of experiments.

In the article “Ultrafast Core-Loss Spectroscopy in 4D Electron Microscopy,” van der Veen et al. report on a detailed investigation of the photoinduced structural dynamics in graphite by means of femtosecond and nanosecond EELS. In particular, the achievement of ultrafast time-resolution at the Carbon K-edge absorption is a milestone in modern ultrafast technology.

The interplay between crystal distortions and the material's electronic structure is usually investigated by means of ultrafast transient optical spectroscopy and/or photoemission. However, real-time visualization of the nuclear dynamics of ions has been achieved via time-resolved X-ray or electron diffraction. The ability to observe both the nuclear and the electronic dynamics has only recently been possible via ultrafast X-ray absorption spectroscopy and ultrafast EELS, with the latter being limited to the low-loss plasmon region and the shallower core levels. Probing deeper core-levels offers the unique advantage of chemical selectivity, as opposed to lower-energy experiments such as optical spectroscopy that are sensitive to highly hybridized valence orbitals.

The authors demonstrate this capability on photoexcited graphite by probing the FEELS signal at the carbon K-edge. It is important to note as well that the machine used to perform these pioneering experiments is the first ever capable of achieving ultrafast EELS of core levels. Certainly, the technology has not reached its limits with this very first design, and ideas are already proposed to take the instrumentation to the next level. The key advance will be an increment in the signal-to-noise ratio, which would allow the observation by ultrafast EELS of more subtle effects excited by a smaller laser fluence.

In their experiment, van der Veen et al. excite a graphite thin film with a rather high laser fluence (tens of mJ/cm2). A rich variety of problems can be addressed in this regime, such as structural phase transitions and metal-insulator transitions, among others. A consequence of the high-fluence excitation is the observation, for the first time, of the Photon-Induced Near-Field Electron Microscopy effect (PINEM) at the carbon K-edge. Briefly, when an intense laser pulse strikes an object, transient electromagnetic fields can be induced on its surface. These fields can accelerate/decelerate electrons passing in their vicinity, resulting in weaker replicas of the elastic electron beam shifted in energy by multiple integers of the driving laser photons. The efficiency of this effect strongly depends on the excitation, shape, and dielectric properties of the surface and is therefore, rather insensitive to the bulk properties of the specimen under investigation. For this reason, the authors rightfully warrant attention in the interpretation of the first ps dynamics of the FEELS spectra. An open question, however, remains: Is the temporal evolution of the surface plasmonic field induced by the laser pulses completely insensitive to the material properties, for example, its metallicity or the electron-hole pairs recombination dynamics? So far, to the best of my knowledge, only systems characterized by a rather fast re-equilibration of their electronic structure have been investigated via PINEM: carbon nanotubes, silver nanowires, and a few other examples, including biological materials. Would the observed dynamics be different on the surface of a solid in which the electron-hole recombination time is very slow? In this respect, even the PINEM signal near time-zero may reveal dynamical properties of the system.

Besides the technological breakthrough that these results represent, they deliver great insight into the photoexcited dynamics of graphite. The evolution of photoexcited electron-hole pairs in graphite is a long-standing subject of debate. Hard to believe, a system whose electronic structure can be calculated with a pen and a sheet of paper, there are still open questions regarding the relaxation mechanism of light-induced electron-hole pairs and its impact on the out-of-equilibrium structural properties of the material. In particular, the ps dynamics of the transient optical absorption is still under debate. Originally, such a long-lasting transient optical signal was interpreted as resulting from band gap renormalization (BGR) caused by a long recombination time of the photoexcited electron-hole pairs. Later, ultrafast optical absorption experiments in the ultraviolet show that the recombination of the photoexcited carriers and the consequent BGR are much faster, lasting few hundreds of fs. An alternative scenario was proposed in which the recombination of the electron-hole pairs leaves the crystal structure out of equilibrium through the excitation of specific phonon modes, leading to a long-lasting dynamics of the optical constants. van der Veen et al. succeed in unraveling the precise microscopic mechanism of this effect, showing that the shifts of the electronic bands, responsible for the transient optical signal, are mainly due to a temperature-dependent electron-phonon coupling, combined with a band-shift due to the c-axis distortion, which is a consequence of the anharmonic couplings between the initially excited in-plane phonon modes and the out-of-plane vibrations.

The great news is that this is just the beginning. Further developments of the technique will allow reaching even deeper core levels, in the region of the transition metals (TM) L-edges, where most of the physics of TM oxides of current interest for microelectronics, spintronics, water splitting, bio-sensing, and solar energy conversion resides. Furthermore, the reported results open the path to extend to the time-domain other TEM-specificities such as element mapping, momentum-resolved EELS, and possibly even dichroic experiments with the newly discovered electron vortex beams. The development of these tools in parallel with the emerging new XFEL technology will offer a unique window of observation on virtually all types of materials: organic, inorganic, nanostructured, or bulk.