Femtosecond time-resolved XPS spectra of the W 4f and Pt 4f photolines as a function of pump-probe delay (vertical) and binding energy (horizontal). (Credit: L. Wenthaus (DESY) and F. Roth (TU Bergakademie Freiberg))
The recent advancements in high-intensity ultrafast X-ray science have paved the way for a new era of time-resolved pump-probe experiments: They have the potential to reveal previously inaccessible information about the interactions of photons with surfaces and the electronic dynamics they induce. This study presents an innovative investigation of the laser-assisted photoelectric effect (LAPE) from metallic surfaces, offering novel insights into the interaction of intense laser fields with electrons in solid-state materials. By employing femtosecond time-resolved X-ray photoelectron spectroscopy (tr-XPS), the team of researchers from TU Bergakademie Freiberg, DESY, European XFEL, University Hamburg (all Germany) and Lawrence Berkeley National Laboratory (U.S.) investigated the dynamic dielectric responses of tungsten and platinum surfaces when subjected to synchronised infrared and X-ray laser pulses at the FLASH beamline PG2 at DESY.
When X-ray and optical laser pulses are in temporal and spatial overlap, the intense optical laser field accelerates the photo-ejected electrons. This results in the generation of sidebands in the photoelectron spectrum which corresponds to the absorption and stimulated emission of photons in this laser field. So far, the appearance of sidebands in XPS spectra from solids has previously been used to determine the temporal overlap of X-ray and IR lasers in time-resolved photoemission experiments. However, a fundamental understanding of the two-photon processes that could be exploited for a new class of attosecond photoemission experiments of solid surfaces has been lacking until now. This work represents a scientific and technical breakthrough on the route to enabling a deeper understanding of the ultrafast pump-probe experiments.
The researchers observed the material-specific sideband generation in similar materials, with tungsten surprisingly producing a greater number of photoelectron sidebands compared to platinum under identical conditions. These unexpected observations can be explained on a semi-quantitative level by analysing the dynamic dielectric responses of the two metals (single crystals: W(110) and Pt(111)). Utilising the strong-field approximation, it has been demonstrated that the intrinsic, near-surface IR laser field enhancement by a factor of four is the underlying cause of the observed variation in the number of sidebands between tungsten and platinum. This discovery provides novel insights into the actual time-dependent IR field strength in the surface region of these metals and the dynamics of the light-matter interaction.
Based on these findings a new standard could be established for the quality and detail with which LAPE spectra from solid-state materials can be recorded with leading ultrafast X-ray light sources. This advance promises to expand our understanding of the capabilities of LAPE measurements on solid surfaces and potentially lead to revolutionary advances in attosecond science, similar to the technique 'Reconstruction of attosecond beating by interference of two-photon transitions' (RABBITT).
Furthermore, the findings underscore the sensitivity of femtosecond tr-XPS to the dielectric properties of solids in a sub-nm thick surface layer. The development of new techniques for monitoring electronic and lattice dynamics in this surface region is important and may contribute to a deeper understanding of surface electromagnetic fields, including local fields at a chemisorbed atom or molecule. A detailed understanding of LAPE contributions in high-resolution pump-probe photoemission spectroscopy is a prerequisite for the accurate interpretation of an increasing number of pump-probe experiments at next-generation light sources with high repetition-rates.
Moreover, this work advances the knowledge of light-matter interactions and also provides new tools for monitoring ultrafast processes in next-generation technologies such as photocatalysis, optoelectronics and quantum materials. It sets the stage for high-precision diagnostics in complex systems and could revolutionise how we study electron dynamics at interfaces.
By combining experimental and theoretical innovations, this research offers insights into the influence of intense laser fields on electron behaviour at the femtosecond scale.
Reference:
Insights into the laser-assisted photoelectric effect from solid-state surfaces, L. Wenthaus, N. M. Kabachnik, M. Borgwardt, S. Palutke, D. Kutnyakhov, F. Pressacco, M. Scholz, D. Potorochin, N. Wind, S. Düsterer, G. Brenner, O. Gessner, S. Molodtsov, W. Eberhardt, and F. Roth, Phys. Rev. B 110 (2024) DOI: 10.1103/PhysRevB.110.235406
