Ultrafast X-ray photoelectron spectroscopy and photoelectron diffraction

by Dr Philip Hofmann (Department of Physics and Astronomy, Aarhus University, Denmark)

Tuesday 4 Jul 2023, 13:00 → 14:00 Europe/Berlin

XHQ / E1.173 (EuXFEL / Schenefeld)

X-ray photoelectron spectroscopy (XPS) is an experimental tool capable of accurately determining the core level binding energies of atoms. This energy is not only element-specific but also provides detailed information on the atoms' oxidation state and chemical environment, making XPS an essential tool for studying catalytic processes. Indeed, the technique is so important that recent experimental developments have pushed it into regimes for which it was long deemed unsuitable, e.g., the presence of near-ambient-pressure gases or liquids. Due to the relative simplicity of the core level photoemission process, XPS lineshapes have attracted considerable theoretical interest and many-body theories to study the solid’s  electronic and vibrational many-body response to the removal of the photoelectron have been developed since the 1970s. Finally, the core level photoemission intensity can be interpreted as a diffraction pattern that gives access to the emitting atom’s local geometrical environment in an experimental approach called X-ray photoelectron diffraction (XPD).

The increasing availability of ultrafast X-ray sources at free electron lasers now opens the opportunity to take XPS and XPD into the ultrafast time domain and this talks will give two recent examples, based on data collected at FLASH. For graphene, it is shown that a study of the time-dependent XPS line shape can reveal detailed insight into the excitation of the system, directly giving access to parameters such as the electronic temperature. Indeed, a high electronic temperature can lead to inelastic final state scattering that becomes the dominant broadening mechanism of the XPS line shape. Surprisingly, the XPD pattern from graphene is only affected little by these effects, such that performing XPD remains possible, even at electronic temperatures of several thousand Kelvin. A demonstration of a quantitative structural determination by XPD is given, in which the motion of surface atoms of the topological insulator Bi2Se3 is tracked after the excitation of a coherent optical phonon.

 

The seminar will be given in person in XHQ/E1.173. Online participants can join via Zoom:

 

Join Zoom Meeting

https://xfel.zoom.us/j/98343016092?pwd=c2xPdzJ5YWZJOHJ1bytlUHB4cms2dz09

Meeting ID: 983 4301 6092

Passcode: 207337