Characterization of Non-Thermal Phase Transitions in Ionic Compounds with Two-color X-ray Pulses.

High resolution crystallography has benefited from the availability of x-ray Free Electron Lasers (FEL). It has been possible to resolve hydrogen atoms and water molecules. [2] Intense x-ray FEL pulses interact with samples changing their electronic and atomic structure. To date, the experiments studying the x-ray FEL-matter interaction have predominantly examined semiconductors, such as diamond and silicon [4-5]. There is little known about how x-ray induced bond breaking occurs in a solid with more than one element or in a solid with ionic bonding[HP1] . A recent calculation has predicted a crystalline to disordered phase transition in the case of the high-intensity x-ray interaction with sodium chloride, an ionic solid [6]. With FEL x-ray pump and x-ray probe pulses, non-thermal phase transitions are predicted and new material phases can be detected. We have investigated the time-dependent intensity of diffraction peaks in sodium chloride (NaCl) and magnesium oxide (MgO). We will discuss the observed ultrafast responses of the materials through analysis of the observed diffraction peak intensities.