A Rate Model of Electron Populations for Non-linear High-Fluence X-ray Absorption Near-Edge Spectra
arXiv:2211.17144 · doi:10.1063/4.0000206
Abstract
Absorbing a focused, femtosecond X-ray pulse from a Free-Electron Laser (FEL) can lead to extreme electronic excitations in solids. This excitation drives changes of the electronic system over the course of the pulse duration and the overall absorption of the pulse becomes fluence-dependent. Thus, fluence-dependent non-linear X-ray Absorption Near Edge Spectroscopy (XANES) is sensitive to the valence excitation dynamics around the Fermi level on the few-femtosecond timescale. Here we present a simplified rate model based on well-established physical mechanisms to describe the evolution of the electronic system. We construct temporal and spatial differentials for the processes of resonant absorption, stimulated emission, non-resonant absorption, Auger decay, valence band thermalization and scattering cascades of free electrons. The phenomenological rate model approach provides a direct understanding how each physical process contributes to the fluence-dependent changes observed in XANES measurements. Without accounting for fluence-dependent changes to the density of states, the model shows good agreement with experimental results on metallic nickel over more than three orders of magnitude in fluence, establishing electron redistribution as the main driver of non-linear absorption changes at high fluences. Although in the closest vicinity of the resonance, more complex approaches are necessary to describe remaining discrepancies of the fluence-dependence changes, the demonstrated capability to describe spectral changes up to extreme fluences yields fundamental insights into the complex dynamics after intense core excitation and provides an important tool for the design and evaluation of future FEL experiments, in particular for the development of non-linear X-ray spectroscopy.
arXiv commentTotal of 10 pages, 7 figures, 1 table. Also see the simulatneously submitted letter for a detailed report on the experiment and its interpretation. Changes to previous version: - Updated reference 32 to point to the simultaneously submitted arXiv article. - Amended a missing "+1" in the Fermi-Dirac distribution in section II.B.4
References in corpus (18)
- scikit-image: Image processing in Python
- Free-electron properties of metals under ultrafast laser-induced electron-phonon nonequilibrium: A first-principles study
- Attosecond screening dynamics mediated by electron-localization
- Various damage mechanisms in carbon and silicon materials under femtosecond x-ray irradiation
- Separation of ultrafast spin currents and spin-flip scattering in Co/Cu(001) driven by femtosecond laser excitation via the complex MOKE
- Influence of laser-excited electron distributions on the x-ray magnetic circular dichroism spectra: Implications for femtosecond demagnetization in Ni
- Electron Thermalization and Relaxation in Laser-Heated Nickel by Few-Femtosecond Core-Level Transient Absorption Spectroscopy
- Ultrafast X-Ray Induced Changes of the Electronic and Magnetic Response of Solids Due to Valence Electron Redistribution
- Multiple Core-Hole Coherence in X-Ray Four-Wave-Mixing Spectroscopies
- Microscopic non-equilibrium energy transfer dynamics in a photoexcited metal/insulator heterostructure
- Mapping the Electronic Structure of Warm Dense Nickel via Resonant Inelastic X-ray Scattering
- The interplay of local electron correlations and ultrafast spin dynamics in fcc Ni
- Ab-initio simulations and measurements of the free-free opacity in Aluminum
- Statistical properties of the radiation from VUV FEL at DESY operating at 30 nm wavelength in the femtosecond regime
- Photon shot-noise limited transient absorption soft X-ray spectroscopy at the European XFEL
- Towards novel probes for valence charges via X-ray optical wave mixing
- Characterization of the ePix100a and the FastCCD Semiconductor Detectors for the European XFEL
- Balanced Detection in Femtosecond X-ray Absorption Spectroscopy to Reach the Ultimate Sensitivity Limit