Synchrotron-Based Time-Resolved X-ray Solution Scattering (Liquidography)

Visualizing molecular structures in the course of a reaction process is one of the major grand challenges in chemistry, biology and physics. In particular, most chemical and biologically relevant reactions occur in solution, and solution-phase reactions exhibit rich chemistry due to the solute-solvent interplay. Studying photo-induced reactions in the solution phase offers opportunities for understanding fundamental molecular reaction dynamics and interplay between the solute and the solvent, but at the same time the interactions between solutes and solvents make this task challenging. Ultrafast emission, absorption and vibration spectroscopy in ultraviolet, visible and infrared regions have made possible the investigation of fast time-evolving processes. However, such time-resolved optical spectroscopic tools generally do not provide direct and detailed structural information such as bond lengths and angles of reaction intermediates because the spectroscopic signals utilizing light in the ultraviolet to infrared range cannot be directly translated into a molecular structure at the atomic level. In contrast, with the advance of X-ray synchrotron sources that can generate high-flux, ultrashort X-ray pulses, time-resolved X-ray diffraction (scattering) and absorption techniques have become general and powerful tools to explore structural dynamics of matters. Accordingly, the techniques have been successfully applied to studying various dynamics of chemical and biological systems (Coppens, 2003; Coppens et al., 2004; Ihee, 2009; Ihee et al., 2005b; Kim et al., 2002; Schotte et al., 2003; Srajer et al., 1996; Techert et al., 2001; Tomita et al., 2009) and of condensed matters (Cavalieri et al., 2005; Cavalleri et al., 2006; Collet et al., 2003; Fritz et al., 2007; Gaffney et al., 2005; Lee et al., 2005; Lindenberg et al., 2005). On one hand, time-resolved X-ray diffraction enables us to access to the mechanism of structural transformations at the atomic level in crystalline state (Collet et al., 2003; Schotte et al., 2003; Srajer et al., 1996; Techert et al., 2001). On the other hand, timeresolved X-ray absorption fine structure (XAFS) (Chen et al., 2001; Saes et al., 2003; Sato et al., 2009) and time-resolved solution scattering (Davidsson et al., 2005; Ihee, 2009; Ihee et al., 2005a; Plech et al., 2004) can probe structural dynamics in non-crystalline states of materials, complementing the X-ray diffraction technique.