Two-Temperature Warm Dense Matter Produced by Ultrashort Extreme Vacuum Ultraviolet-Free Electron Laser (EUV-FEL) Pulse

Ultrashort X-ray laser pulse acts onto electron subsystem of dielectrics such as LiF and transfers matter into a two-temperature state with hot electrons excited by the pulse from valence to conduction band. Because of the small heat conduction, the hydrodynamic motion proceeds in adiabatic regime keeping the radiation attenuation depth Datt as the only scale of the spatial heat distribution. Hydrodynamic motion qualitatively changes with absorbed energy increase. At low fluences F ∼ 10mJ/cm 2 a spallative removal of LiF remaining in solid state takes place. This is a reason for the low ablation threshold. The paper presents new experimental findings supporting this conclusion. For the first time these findings are obtained using ultrashort extreme vacuum ultraviolet-free electron laser (EUV-FEL). In the case of high fluence, also achieved in our experiments at EUV-FEL, material removal happens as result of outflow of matter transferred into the gaseous state. This explains the slow growth of the amount of ablated mass with significant fluence increase.