Characterization of transient molecular structures formed during chemical and biological processes is essential for understanding their mechanisms and functions. Over the last decade, time-resolved X-ray liquidography (TRXL) and time-resolved X-ray absorption spectroscopy (TRXAS) have emerged as powerful techniques for molecular and electronic structural analysis of photoinduced reactions in the solution phase. Both techniques make use of a pump-probe scheme that consists of (1) an optical pump pulse to initiate a photoinduced process and (2) an X-ray probe pulse to monitor changes in the molecular structure as a function of time delay between pump and probe pulses. TRXL is sensitive to changes in the global molecular structure and therefore can be used to elucidate structural changes of reacting solute molecules as well as the collective response of solvent molecules. On the other hand, TRXAS can be used to probe changes in both local geometrical and electronic structures of specific X-ray-absorbing atoms due to the element-specific nature of core-level transitions. These techniques are complementary to each other and a combination of the two methods will enhance the capability of accurately obtaining structural changes induced by photoexcitation. Here we review the principles of TRXL and TRXAS and present recent application examples of the two methods for studying chemical and biological processes in solution. Furthermore, we briefly discuss the prospect of using X-ray free electron lasers for the two techniques, which will allow us to keep track of structural dynamics on femtosecond time scales in various solution-phase molecular reactions.
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We greatly appreciate our co-workers listed in many references of this article. We acknowledge other research groups who have made significant contributions to the advance of TRXL and TRXAS as well as other related X-ray techniques. This work was supported by IBS-R004-G2. This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2014R1A1A1002511, 2013R1A1A2009575, 2014R1A4A1001690, and 2011-0031558). Fig. 12 (A) Time-resolved X-ray absorption spectra of the iodoform photolysis measured at the L1 edge of the iodine atom and at the time delays of 100 ps (black) and 300 ps (red). (inset) Temporal change of the difference absorption intensity at a peak position of 5.1857 keV (green dashed line) in the time range from 0.4 to 0.6 ns. (B) The fitting analysis of the L1-edge difference XANES spectrum measured at 100 ps for two candidate reaction pathways, (i) CHI2 radical + I and (ii) CHI2-I isomer formation. Experimental (black) and theoretical (red) difference XANES spectra are compared with each other. Feature Article ChemComm 3746 | Chem. Commun., 2016, 52, 3734-3749 This journal is ? The Royal Society of Chemistry 2016 This research has been supported by the TJ Park Science Fellowship of POSCO TJ Park Foundation.
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Ceramics and Composites
- Surfaces, Coatings and Films
- Metals and Alloys
- Materials Chemistry