Two-level ablation and damage morphology of Ru films under femtosecond extreme UV irradiation

[Display omitted] •New model is developed that simulates full timescale of laser induced damage in Ru.•Hybrid model combines Monte Carlo, hydrodynamics and molecular dynamics approaches.•Fast energy deposition in a subsurface layer of Ru leads to a two-level ablation.•The second thin liquid ablating layer is unstable and disintegrates into droplets.•Simulations explain fluence dependent morphology outside and inside damage craters. The dynamics of a thin ruthenium film irradiated by femtosecond extreme UV laser pulses is studied with a hybrid computational approach, which includes Monte Carlo, two-temperature hydrodynamics and molecular dynamics models. This approach is capable of accurate simulations of all stages of material evolution induced by extreme UV or X-ray photons: from nonequilibrium electron kinetics till complete lattice relaxation. We found that fast energy deposition in a subsurface layer leads to a two-level ablation: the top thin layer is ablated as a gas–liquid mixture due to expansion of overheated material at near and above critical conditions, whereas a thicker liquid layer below is ablated via a cavitation process. The latter occurs due to a thermo-mechanically induced tensile pressure wave. The liquid ablating layer exhibits unstable behaviour and disintegrates into droplets soon after detachment from the rest of the target. Our simulations reveal basic processes leading to formation of specific surface morphologies outside and inside the damage craters. The calculated ablation threshold, crater depth and morphological features are in quantitative agreement with the experimental data, which justifies the applicability of our hybrid model to study laser-induced material damage.