Abstract
We perform a computational study of damage formation in extreme ultraviolet (XUV)-irradiated ruthenium thin films by means of combining the Monte Carlo approach with the two-temperature model. The model predicts that the damage formation is most affected by ultrafast heating of the lattice by hot electrons, and is not very sensitive to the initial stage of the material excitation. Numerical parameters of the model were analyzed, as well as different approximations for the thermal parameters, showing the importance of the temperature dependence of the electron thermal conductivity and the electron–phonon coupling factor. Our analysis reveals that the details of photoabsorption and ultrafast non-equilibrium electron kinetics play only a minor role in the XUV irradiation regime.
Highlights
Survivability of optical elements exposed to ultrafast high-peak-power free-electron laser (FEL) pulses becomes more and more important in the context of rapidly developing extreme ultraviolet (XUV) and x-ray Free-electron lasers (FELs) light sources [1,2,3,4,5]
We describe the different approximations for the following model parameters of Ru: photoelectron velocity distribution, electron cutoff energy, electron heat capacity, electron thermal conductivity, and electron–phonon coupling factor
We demonstrate the influence of thermal parameters on the melting dynamics in the example of varying electron thermal conductivity, since we showed that this parameter has the strongest impact on the temperature behavior
Summary
Survivability of optical elements exposed to ultrafast (femtosecond) high-peak-power free-electron laser (FEL) pulses becomes more and more important in the context of rapidly developing extreme ultraviolet (XUV) and x-ray FEL light sources [1,2,3,4,5]. The free electron–electron scattering is neglected, so the cascades develop independently Such an approximation is valid if the density of high-energy electrons participating in the cascading process is significantly smaller than the atomic density, thereby making impact ionization and elastic scattering the dominant processes of electron interaction [33,34]. In other words, it means that the fluence of an incident laser pulse must not be too high to produce a density of excited electrons comparable to or higher than atomic density of a target. The units of energy density, eV/atom, are used for the vacuum region to be compared with the energy density inside the material, there are no atoms in vacuum; this should only be used to estimate the total emitted energy, and can be converted into the energy density units of, e.g., eV∕cm by multiplying with the target atomic density under normal conditions
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