Modelling of damage in Ru thin films induced by femtosecond XUV laser pulses

Survivability of optical elements exposed to high doses of XUV laser radiation is an important issue in the context of rapidly developing x-ray free-electron laser (XFEL) light sources. In order to prevent optics from being damaged, the fundamental mechanisms governing the material response to ultrashort high peak power XFEL pulses must be identified and studied.
We present computational study of the interaction of femtosecond XUV (13.5 nm wavelength) laser pulses with 50 nm thin Ru films. With our calculations we model the damage experiments that was performed at Free-Electron LASer in Hamburg (FLASH) [1]–[3]. Ru is chosen as optically favorable material for grazing incidence reflective mirrors.
The performed simulations consist of two parts. First, the effect of electron cascading induced after absorption of XUV photons is studied using an event-by-event Monte Carlo code XCASCADE [4]. Time of cascading and ballistic range of non-thermalized electrons are calculated. Second, the evolution of electron and lattice temperatures in the regime of thermal non-equilibrium together with atomic motion in irradiated Ru are modeled with a combination of two temperature hydrodynamics [5] and molecular dynamics [6]. Our calculations showed that the mechanism responsible for the ablation of Ru observed in the experiment is spallation in the stress confinement regime. The processes of melting, cavitation, spallation and recrystallization are modeled. The results show good agreement with the experimental observations.