Proceedings Article | 14 May 2019
KEYWORDS: Ruthenium, Monte Carlo methods, X-rays, Data modeling, Extreme ultraviolet, Absorption, Ionization, Scattering, Chemical species, Hard x-rays
Modern free-electron lasers produce femtosecond X-ray pulses sufficiently intense to induce modifications of material properties. Energetic photoelectrons created upon the X-ray absorption and Auger electrons emitted after relaxation of core-hole states trigger secondary electron cascades, which increase the transient electron density. In this talk, we present and apply the XCASCADE(3D) code [1], based on classical event-by-event individual-particle Monte-Carlo approach, which is valid for relatively low absorbed fluences. The code takes into account photoabsorption, electron impact ionization, electron elastic scattering, and Auger decays of core holes. It uses atomistic cross sections and atomistic ionization potentials. Such approximation is justified for high-energy X-ray absorption and for high-energy electron-atom collisions, since they both excite electrons from the deeply lying core levels. At the same time, it makes the approach very efficient and flexible, since the corresponding cross sections can be found in the literature for many elements. In this approximation, one can treat any practical material assuming that the total cross section is the sum of cross sections for each individual atom. Elastic and inelastic anisotropic scatterings of photo- and secondary electrons on atoms are taken into account. The validity of the model is confirmed by comparing the calculated electron ranges with published data for silicon and gold.
We apply the code to study the consequences of the electron cascading in ruthenium (Ru). Ru, which is a promising material for coating of X-ray mirrors [2], is simulated at different incoming photon energies, from extreme ultraviolet (XUV) to hard x-ray, in a grazing incidence geometry. The generated data then serve as an input to Two-Temperature Model, enabling analysis of the temperature evolution and damage thresholds [2].
According to the simulations, much larger electron ranges for hard x-rays case result in spread of absorbed laser energy into a larger volume compared to the XUV case and, consequently, into smaller temperature gradients. The latter is the source of thermo-induced stresses that eventually lead to material damage. As a consequence of the electron transport, one should expect higher damage thresholds with increasing energy of incident photons, even for the same photon penetration depth, which can be achieved by adjusting the incident angle.
[1] V. Lipp, N. Medvedev, B. Ziaja, Proc. SPIE 10236, 102360H (2017).
[2] Igor Milov, Igor A. Makhotkin, Ryszard Sobierajski, Nikita Medvedev, Vladimir Lipp et al., Opt. Express 26, 19665-19685 (2018).
