X-ray Probes of Molecular and Chemical Dynamics

Photoinduced metal-to-ligand charge transfer and spin crossover in aqueous [Fe(bipy)3]2+

We have implemented a high repetition rate (54 kHz--6.5 MHz), high power (>10 W) laser system at APS beamline 7ID for laser-pump/x-ray-probe studies of photoinduced molecular dynamics. Our end station consists of a liquid jet of solvated molecules, laser transport and focusing optics, and Kirkpatrick-Baez x-ray mirrors to implement a time-resolved x-ray microprobe. Results are shown in Fig. 7 for laser-induced metal-to-ligand charge transfer and spin crossover in aqueous [Fe(bipy)3]2+. Determinations of the spin states, electronic states, and atomic structures of the ground state and laser-excited state were derived from x-ray absorption, x-ray emission, and x-ray diffuse (liquid) scattering measurements. Time-resolved measurements with high signal:noise are obtained by efficient use of the x-ray flux at high repetition rates. Experiments using picosecond x-ray pulses at APS complement experiments using femtosecond x rays at the LCLS and SACLA XFELs
Molecular response to x-ray absorption and vacancy cascades

X-ray absorption by a heavy atom produces an inner-shell hole that decays in a multi-step process with the ejection of fluorescent x rays and Auger electrons that results in high charge states on the atomic ion. In molecules, the inner-shell hole and the holes produced by the first decay steps remain localized, but eventually charge is redistributed to neighboring atoms and the system Coulomb explodes. This process is responsible for x-ray damage in molecules and materials and has been exploited for targeted destruction of malignant cells. To study the physics of core-hole dynamics in small molecules, we use APS x rays to ionize K-shell electrons from a heavy atom such as Br, I, and Xe of the molecules IBr, CH2BrI, and XeF2. A fluorescent x-ray from the K-shell hole is detected and triggers an x-ray/ion coincidence measurement of the fragment ions. Figure 8 shows an example of a scatter plot of ion coincidences in XeF2. The data yield the ion charge state distributions and kinetic energies released to the ions, and comparisons are made with calculated core-hole decays. Femtosecond XFEL pulses are also being used to explore core-hole decay dynamics.