Kim, K.H. Ihee, H. Kim, J.G. FRA02 2015 2015-12 Bond breaking and bond making are essential processes in chemical reactions. While ultrafast dynamics of bond breaking have been studied intensively using time time-resolved techniques, it is challenging to keep track of structural dynamics of bond making due to its bimolecular nature. Time-resolved X-ray liquidography (TRXL) has been developed by using the X-ray pulse, instead of optical pulse, as a probe and gives the time-resolved scattering response sensitive to global molecular structure.(*, **) The time resolution of 3rd generation source is 100 ps and due to this limited time resolution, ultrafast structural dynamics involved in ultrafast excited-state dynamics and direct observation of bond-breaking and -making processes cannot yet be accomplished with TRXL. This limit is improved with X-ray free electron lasers (XFELs), which deliver 100 fs long X-ray pulses with ~10¹³ photons per pulse. With the advent of X-ray free electron laser (XFEL) generating ultrashort X-ray pulses, the exploration of chemical processes occurring on femtosecond time scale with pump-probe X-ray solution scattering is possible. With the aid of 4th generation X-ray sources (SACLA, Japan), ultrafast tight Au-Au bond formation process of [Au(CN)2-]3 oligomer upon 267 nm laser excitation was directly observed using TRXL.