Ultrafast 4D core-loss spectroscopy meets graphite

Electron energy-loss spectroscopy (EELS) uses an electron beam shot through a sample to knock a resident atom’s electron into an unoccupied outer shell or out of the atom entirely. In the process, the probing electrons lose energy. When the target is a tightly bound core electron, the beam’s energy losses can easily exceed 100 eV; by analyzing the resultant “core-loss” spectra, researchers can glean information about a sample’s chemical and structural properties much as they could using soft x rays. Unlike x rays, however, electrons interact strongly with the light elements that are prevalent in organic materials. A group at Caltech led by Ahmed Zewail has now incorporated nano- and femtosecond time resolution into EELS to study the chemical and structural dynamics of a 50-nm-thick film of graphite. The researchers first use a laser to excite electronic and lattice motions in a small spot of the sample; after the desired time delay, they probe the spot with ultrashort focused electron pulses from a transmission electron microscope (for more on TEMs, see Physics Today, April 2015, page 32 ). Combining the electron energy-loss spectra with molecular dynamics simulations—to account for vibrations and other thermal disorder in the film—the Caltech group deduced that the transient laser heating caused the in-plane bonds of the carbon lattice to contract even as the carbon–carbon bonds between the planes elongated. Additionally, they found that laser-induced phonons caused the overall energy gap between filled and empty electronic bands to shrink significantly on a subpicosecond time scale. (R. M. van der Veen et al., Struct. Dyn., in press, doi:10.1063/1.4916897 .)