Characterization of Non-Thermal Phase Transitions in MgO and NaCl with Two-color X-ray Pulses*

With x-ray Free Electron Lasers (FEL), intense x-ray FEL pulses interact with samples, which can alter their electronic and atomic structures [1]. Previous experiments on x-ray FEL-matter interactions have mostly focused on semiconductors like diamond and silicon [2-4]. However, little is known about x-ray-induced bond breaking in multi-element solids or ionic solids. Recent calculations have predicted a crystalline-to-disordered phase transition in sodium chloride (NaCl) upon high-intensity x-ray interaction [5]. Using FEL x-ray pump and x-ray probe pulses, we can induce non-thermal phase transitions and detect new material phases. We have investigated the time-dependent intensity of diffraction peaks in NaCl and magnesium oxide (MgO). Our experimental observations are compared with density functional theory and particle-in-cell simulations. Our findings reveal the ultrafast responses of these materials, analyzed through the variations in diffraction peak intensities.