X-ray induced electron and ion fragmentation dynamics in IBr

Characterization of the inner-shell decay processes in molecules containing heavy elements is key to understanding x-ray damage of molecules and materials and for medical applications with Auger-electron-emitting radionuclides. The 1s hole states of heavy atoms can be produced by absorption of tunable x-rays and the resulting vacancy decays characterized by recording emitted photons, electrons, and ions. The 1s hole states in heavy elements have large x-ray fluorescence yields that transfer the hole to intermediate electron shells that then decay by sequential Auger-electron transitions that increase the ion's charge state until the final state is reached. In molecules the charge is spread across the atomic sites, resulting in dissociation to energetic atomic ions. We have used x-ray/ion coincidence spectroscopy to measure charge states and energies of I$^{q+}$ and Br$^{q'+}$ atomic ions following 1s ionization at the I and Br \textit{K}-edges of IBr. We present the charge states and kinetic energies of the two correlated fragment ions associated with core-excited states produced during the various steps of the cascades. To understand the dynamics leading to the ion data, we develop a computational model that combines Monte-Carlo/Molecular Dynamics simulations with a classical over-the-barrier model to track inner-shell cascades and redistribution of electrons in valence orbitals and nuclear motion of fragments

Spin-state studies with XES and RIXS: From static to ultrafast

We report on extending hard X-ray emission spectroscopy (XES) along with resonant inelastic X-ray scattering (RIXS) to study ultrafast phenomena in a pump-probe scheme at MHz repetition rates. The investigated systems include low-spin (LS) Fe-II complex compounds, where optical pulses induce a spin-state transition to their (sub)nanosecond-lived high-spin (HS) state. Time-resolved XES clearly reflects the spin-state variations with very high signal-to-noise ratio, in agreement with HS-LS difference spectra measured at thermal spin crossover, and reference HS-LS systems in static experiments, next to multiplet calculations. The 1s2p RIXS, measured at the Fe Is pre-edge region, shows variations after laser excitation, which are consistent with the formation of the HS state. Our results demonstrate that X-ray spectroscopy experiments with overall rather weak signals, such as RIXS, can now be reliably exploited to study chemical and physical transformations on ultrafast time scales. (C) 2012 Elsevier B.V. All rights reserved

DETERMINATION OF THE MOLECULAR STRUCTURE OF SEVERAL CARBO-CATIONS VIA THE ``COULOMB EXPLOSION'' TECHNIQUE

$^{1}$ E. P. Kanter, Z. Vager, G. Both, and D. Zajman, J. Chem. Phys. 85, 7487 (1986). $^{2}$ G. P. Raine and H. F. Schaefer, J. Chem. Phys. 81, 4034 (1984). $^{3}$ J. Pople, private communication.Author Institution: Argonne National Laboratory, 9700 S. Cass Ave.; Argonne National Laboratory, The Weizmann Institute of ScienceSeveral important carbo-cations such as $C_{2}H_{3}{^{+1}}$ and $CH_{5}^{+}$ have been studied by this radically new technique which obtains direct three-dimensional images of small molecules. Protonated acetylene has been a molecule of extreme interest because of the possibility of it assuming nonclassical ""bridged"" and classical ""nonbridged"" configurations. Our data is consistent with having the proton in the bridged position; the spatial geometry obtained is in quite reasonable agreement with ab $initio^{2,3}$ calculations. This work supported by the U. S. Department of Energy, Office of Basic Energy Sciences, under Contract W-31-109-ENG-38

STEREOCHEMICAL INFORMATION ON CH3+CH_{3}^{+} AND NH3+NH_{3}^{+} DERIVED USING THE COULOMB EXPLOSION TECHNIQUE

$^{(1)}$ Z. Vager et al., Phys. Rev. Lett. 57 (1986), p. 2793. $^{(2)}$ E. P. Kanter et al., J. Chem. Phys. 85 (12), 1986, p. 7487.Author Institution: Physics Department, Weizmann Institute, Rehovot; Physics Department, Weizmann Institute, Argonne National LaboratoryStructure parameters for $CH_{3}^{+}$ and $NH_{3}^{+}$ molecular ions are extracted from Coulomb-explosion measurements utilizing 4.5-MeV ion beams. A typical experiment determines the relative velocities acquired by the atomic fragments as the molecular ion dissociates. The relative positions of the constituent nuclei, at the moment the dissociation starts, determine the total kinetic energy of the system and the way it is distributed among the atomic fragments. We are in the process of accumulating this type of information for several small molecular ions (total mass smaller than 50 a.m.u.) and have already succeeded to produce information not available through classical $methods^{(1,2)}$. The $CH_{3}^{+}$ and $NH_{3}^{+}$ ions data reveal structure parameters of these ions as well as correlations between the different internal degrees of freedom