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ntroduction Electron energy-loss spectroscopy (EELS) in the transmission electron microscope has become an invaluable tool for unraveling the chemical compositions and structures of materials, enabling imaging of individual atoms and their bonding states with unprecedented resolutions.1,2 The low-energy (“lowloss,” 0–50 eV) region of the EEL spectrum yields electronic information in the form of valence intraband and interband transitions, as well as plasmon excitations, rendering this part of the spectrum sensitive to changes in the overall electron density of the material. Conversely, the high-energy (“core-loss” >100 eV) region of the EEL spectrum is characterized by excitations of core-level electrons into well-defined higher-lying empty states and into the continuum, providing a technique suitable for studying the chemical state, local geometric structure, and nature of chemical bonding centered around the absorbing atom. When combined with the excellent spatial resolution of the transmission electron microscope, EELS constitutes a powerful technique for the electronic characterization of nanoscale materials. However, if one wishes to study dynamical processes, the temporal resolution has been typically limited by the acquisition time of the detector (∼30 ms). Only recently, ultrafast EELS with temporal resolutions ranging from femtoseconds

(fs) to nanoseconds (ns) has been demonstrated and applied to study laser-induced pre-ablation dynamics3 and bandgap renormalization4 in graphite thin films, electron–phonon coupling and structural dynamics in multiwalled carbon nanotubes,5 imaging of evanescent waves surrounding carbon nanotubes,6 plasmonic nanostructures,7–9 biological structures,10,11 and photo-induced charge-transfer and phase transitions in transitionmetal oxides.12,13 This article discusses some of the latest results, prospects for applications, and new methods in ultrafast EELS at the nanometer scale. Refer to References 14–17 for a detailed discussion of ultrafast and dynamic electron microscopy and References 18–20 for a deeper theoretical understanding of inelastic electron–photon interactions.

Ultrafast core-level electron spectroscopy Apart from relativistic effects in the former,21 electron energyloss and x-ray absorption core-level spectra are essentially equivalent and provide analogous information. Energy losses between 100 and 1500 eV are routinely accessible in transmission electron microscopy-EELS (TEM), which (partly) overlaps with the soft x-ray region. Ultrafast x-ray and extreme ultraviolet spectroscopies have experienced tremendous

Enrico Pomarico, École Polytechnique Fédérale de Lausanne, Switzerland; [email protected] Ye-Jin Kim, Ulsan National Institute of Science and Technology, South Korea; [email protected] F. Javier García de Abajo, Institut de Ciències Fotòniques, Spain; [email protected] Oh-Hoon Kwon, Department of Chemistry, Ulsan National Institute of Science and Technology, South Korea; [email protected] Fabrizio Carbone, École Polytechnique F