Abstract
Bond formation, bond dissociation, and electron/proton transfer (migration) are ubiquitous steps in chemical reactions. In particular, the study of bond formation in the solution phase has been challenging due to the difficulty of initiating and probing such diffusion-limited bimolecular processes with ultrafast time resolution. In this chapter, we describe how femtosecond time-resolved X-ray liquidography (TRXL) is used to visualize in real time the photoinduced birth of a covalently bonded gold trimer complex, [Au(CN)2−]3. The ground (S0) state of the trimer has Au atoms that are weakly bound to each other by van der Waals interaction and aligned in a bent geometry. Upon laser excitation, the S0 state rapidly converts into an excited (S1) state where Au–Au covalent bonds are formed among Au atoms aligned in a linear geometry. Subsequently, the S1 state transforms to a T1 state in 1.6 ps while accompanying further contraction of Au–Au bonds by 0.1 A. Later, the T1 state of the trimer converts to a tetramer on nanosecond time scale. This work demonstrates the possibility of tracking detailed structural changes in solution with sub-picosecond temporal and sub-angstrom spatial resolutions, owing to the advent of X-ray free electron lasers (XFELs) and the advancement of TRXL data analysis.
Published Version
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