ABSTRACT
Experimental facilities for picosecond X-ray spectroscopy and scattering based on RF deflection of stored electron beams face a series of optical design challenges. Beamlines designed around such a source enable time-resolved diffraction, spectroscopy and imaging studies in chemical, condensed matter and nanoscale materials science using few-picosecond-duration pulses possessing the stability, high repetition rate and spectral range of synchrotron light sources. The RF-deflected chirped electron beam produces a vertical fan of undulator radiation with a correlation between angle and time. The duration of the X-ray pulses delivered to experiments is selected by a vertical aperture. In addition to the radiation at the fundamental photon energy in the central cone, the undulator also emits the same photon energy in concentric rings around the central cone, which can potentially compromise the time resolution of experiments. A detailed analysis of this issue is presented for the proposed SPXSS beamline for the Advanced Photon Source. An optical design that minimizes the effects of off-axis radiation in lengthening the duration of pulses and provides variable X-ray pulse duration between 2.4 and 16â ps is presented.
ABSTRACT
The short pulse x-ray imaging and microscopy beamline is one of the two x-ray beamlines that will take full advantage of the short pulse x-ray source in the Advanced Photon Source (APS) upgrade. A horizontally diffracting double crystal monochromator which includes a sagittally focusing second crystal will collect most of the photons generated when the chirped electron beam traverses the undulator. A Kirkpatrick-Baez mirror system after the monochromator will deliver to the sample a beam which has an approximately linear correlation between time and vertical beam angle. The correlation at the sample position has a slope of 0.052 ps/µrad extending over an angular range of 800 µrad for a cavity deflection voltage of 2 MV. The expected time resolution of the whole system is 2.6 ps. The total flux expected at the sample position at 10 keV with a 0.9 eV energy resolution is 5.7 × 10(12) photons/s at a spot having horizontal and vertical full width at half maximum of 33 µm horizontal by 14 µm vertical. This new beamline will enable novel time-dispersed diffraction experiments on small samples using the full repetition rate of the APS.
ABSTRACT
The single-crystal structure of Mn(CO)(3)(C(7)H(11)) is the first to be solved by direct methods based on time-of-flight neutron diffraction data obtained at the Argonne Intense Pulsed Neutron Source. The molecule contains an unusual three-center, two-electron manganese-hydrogen-carbon interaction.
ABSTRACT
A new class of coordinately unsaturated polynuclear rhodium and iridium alkyl, benzyl, and aryl derivatives of the form [RM(1,5-cyclooctadiene)](x) have been prepared by the reaction of the organolithium reagent with the cyclooctadienemetal chlorides at -78 degrees C. The x-ray crystal structure of [mu-CH(3)Rh (1,5-cyclooctadiene)](2) is reported. The analogous iridium dimer decomposes by an initial sequence of alpha-hydrogen abstraction and then reductive elimination of hydrogen to give [mu-CH(2)Ir(1,5-cyclooctadiene)](2). Formed in high yield by the decomposition of the ethylrhodium complex was [HRh(C(8)H(11))](4), a tetrahedral cluster with face-bridging hydride ligands. Also discussed are the reactivities of the benzyl and phenyl derivatives. Unique reaction pathways for C-H bond activation and scission in this chemistry are delineated.
ABSTRACT
Molecular metal clusters with carbide carbon atoms of low coordination number have been prepared; they are the anionic [HFe(4)C(CO)(12) (-)] and [Fe(4)C(CO)(12) (2-)] clusters. An x-ray crystallographic analysis of a tetraaminozinc salt of the latter has established a butterfly array of iron atoms with the carbide carbon atom centered above the wings of the Fe(4) core. Each iron atom was bonded to three peripheral carbonyl ligands. The distances from the carbide carbon to iron were relatively short, particularly those to the apical iron atoms (1.80 A average). Protonation of the anionic carbide clusters reversibly yielded HFe(4)(CH)(CO)(12), and methylation of the dianion gave {Fe(4)[CC(O)CH(3)](CO)(12) (-)}. Oxidation of [Fe(4)C(CO)(12) (2-)] yielded the coordinately unsaturated Fe(4)C(CO)(12) cluster, which was extremely reactive. Hydrogen addition to this iron cluster was rapid below 0 degrees C, and a C-H bond was formed in this transformation.
