An atomic-scale investigation of structural transitions in ruthenium induced by x-ray free-electron lasers

The surface thermodynamic properties of ruthenium (Ru) targets were systematically investigated under different laser energy fluences to elucidate the irradiation damage mechanisms of the grazing incidence mirror in x-ray free-electron laser (XFEL) facilities, by combining the two-temperature model and molecular dynamics method. Initially, the 50 nm Ru thin film was irradiated by a 20 fs laser pulse at an absorbed laser fluence of 40 mJ cm−2, and surface dynamic processes with rapid heating and homogeneous melting lasting picosecond-timescale were observed. However, the ultra-short laser-induced surface dynamic responses in 600 nm Ru thick film were lasting much longer time. With the increasing energy fluence of the laser pulse, the intensified irradiation effects characterized by the surface morphology changes were observed, followed by homogeneous melting of the Ru thick film. As the laser fluence approaches the damage threshold, an unconventional ablation effect was observed in the irradiated thick film, featuring the formation of point defects, voids, and subsequent nanometer-scale splitting in the front surface (approximately 5 nm). It indicates that, at laser fluences ranging from 20 to 320 mJ cm−2, the surface morphology undergoes expansion at angstrom to nanometer scales, accompanied by nanoscale splitting. The results provide valuable insights into the irradiation damage mechanism of the high-Z coating materials for x-ray optics in XFELs.