Molecular Dynamics Study of the Temperature Effects on Damage Mechanisms of Ni Films Irradiated by Extreme Ultraviolet Free- electron Laser

Since the appearance of Extreme Ultraviolet (EUV) and X-Ray Free-Electron Lasers (XFEL) , there has been a significant interest in studying radiation damage on thin-film optical components. The mechanisms of laser-induced damage on thin films are closely related to various factors such as wavelength, pulse duration, material type, film thickness, and notably, the initial temperature of film. In this study, a combined approach using the two-temperature model and molecular dynamics was employed to investigate the influence of initial temperature on the melting damage mechanism of nickel films irradiated by EUV free-electron laser with a wavelength of 13.5 nm. The melting damage thresholds of Ni films at various initial temperatures have been determined, revealing a decrease in thresholds with increasing initial temperature and the presence of two distinct linear intervals. Specifically, within the temperature range of 300 to 900 K, the linear regression slope is -0.125 J/cm2/K, whereas the slope shifts to -0.200 J/cm2/K between 900 K and 1 300 K. Two distinct melting behaviors were observed by analyzing the atomic snapshots of irradiated films: the homogeneous melting inside the films with the initial temperature of 300--900 K, and the heterogeneous melting on the surface of films with the initial temperature of 900--1 300 K. It may be the reason for these two linear intervals existing in the relationship between the melting damage absorption fluence and the initial temperature. By analyzing the temperature and the stress variations over time and space, it was found that the different damage behaviors are mainly related to the lattice overheating, the thermoelastic stress induced by FEL and the lattice average heating rate. For Ni films with initial temperatures below 900 K, the melting damage under FEL irradiation induces significant tensile stress within the films, which leads to the reduction of equilibrium melting temperature and causes localized overheating. Furthermore, the lattice stability is compromised under high stress and the increased lattice average heating rate enhances the probability of internal liquid phase nucleation. Conversely, for Ni films with initial temperatures above 900 K, the internal stress remains relatively low when the melting damage occurs, which results in a lower probability of liquid phase nucleation within the films. Nevertheless, on the film surface, the formation of liquid phase nuclei is more likely due to a lower surface free energy.