Abstract
We demonstrate ultrafast core-electron energy-loss spectroscopy in four-dimensional electron microscopy as an element-specific probe of nanoscale dynamics. We apply it to the study of photoexcited graphite with femtosecond and nanosecond resolutions. The transient core-loss spectra, in combination with ab initio molecular dynamics simulations, reveal the elongation of the carbon-carbon bonds, even though the overall behavior is a contraction of the crystal lattice. A prompt energy-gap shrinkage is observed on the picosecond time scale, which is caused by local bond length elongation and the direct renormalization of band energies due to temperature-dependent electron–phonon interactions.
Highlights
Electron energy-loss spectroscopy (EELS) in the electron microscope has become an invaluable tool for unraveling the chemical composition and structure of materials, enabling the imaging of individual atoms and their bonding states with unprecedented resolutions.1–4 The low-energy (0–50 eV) region of the EEL spectrum delivers electronic information in the form of valence intra- and interband transitions, and plasmon excitations, rendering this part of the spectrum sensitive to changes in the overall electron density of the material
A prompt energy-gap shrinkage is observed on the picosecond time scale, which is caused by local bond length elongation and the direct renormalization of band energies due to temperature-dependent electron–phonon interactions
We demonstrated here for the first time the feasibility of ultrafast core-loss spectroscopy in 4D-electron microscopy with femtosecond and nanosecond time resolutions, and at deep corelevel ionization edges >100 eV
Summary
Electron energy-loss spectroscopy (EELS) in the electron microscope has become an invaluable tool for unraveling the chemical composition and structure of materials, enabling the imaging of individual atoms and their bonding states with unprecedented resolutions. The low-energy (0–50 eV) region of the EEL spectrum delivers electronic information in the form of valence intra- and interband transitions, and plasmon excitations, rendering this part of the spectrum sensitive to changes in the overall electron density of the material. The high-energy (>100 eV) region of the EEL spectrum is characterized by excitations of coreelectrons into well-defined higher-lying empty states and into the continuum.. The high-energy (>100 eV) region of the EEL spectrum is characterized by excitations of coreelectrons into well-defined higher-lying empty states and into the continuum.5 This core-loss spectroscopy provides a technique suitable for studying the chemical state, local geometric structure, and nature of chemical bonding centred around the absorbing atom. Until now, if one wishes to study dynamical processes, the temporal resolution has been limited by the speed of the acquisition time of the detector ($30 ms). This leaves inaccessible many phenomena that occur on shorter timescales
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