A new technique for visualizing the rapidly changing electronic structures of atomic-scale materials as they twist, tumble and traipse across the nanoworld is taking shape at the California Institute of Technology. There, researchers have for the first time successfully combined two existing methods to visualize the structural dynamics of a thin film of graphite. Described this week in the journal Structural Dynamics, from AIP Publishing and the American Crystallographic Association, their approach integrated a highly specific structural analysis technique known as "core-loss spectroscopy" with another approach known as ultrafast four-dimensional (4-D) electron microscopy -- a technique pioneered by the Caltech laboratory, which is headed by Nobel laureate Ahmed Zewail.

In core-loss spectroscopy, the high-speed probing electrons can selectively excite core electrons of a specific atom in a material (core electrons are those bound most tightly to the atomic nucleus). The amount of energy that the core electrons gain gives insight into the local electronic structure, but the technique is limited in the time resolution it can achieve -- traditionally too slow for fast catalytic reactions. 4-D electron microscopy also reveals the structural dynamics of materials over time by using short pulses of high-energy electrons to probe samples, and it is engineered for ultrafast time resolution.

Combining these two techniques allowed the team to precisely track local changes in electronic structure over time with ultrafast time resolution.

"In this work, we demonstrate for the first time that we can probe deep core electrons with rather high binding energies exceeding 100 eV," said Renske van der Veen, one of the authors of the new study. "We are equipped with an ultrafast probing tool that can investigate, for example, the relaxation processes in photocatalytic nanoparticles, photoinduced phase transitions in nanoscale materials or the charge transfer dynamics at interfaces."