title: Ultrafast core-loss spectroscopy in four-dimensional electron microscopy.
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.
authors.forename: Renske M
authors.initials: RM
authors.lastname: van der Veen
authors.affiliation: SwissFEL, Paul Scherrer Institut , 5232 Villigen PSI, Switzerland.
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Penfold
authors.affiliation: Physical Biology Center for Ultrafast Science and Technology, Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology , Pasadena, California 91125, USA.
authors.forename: Ahmed H
authors.initials: AH
authors.lastname: Zewail