The application of the Rietveld refinement technique to synchrotron X-ray data collected from a capillary sample of AI20 3 in Debye-Scherrer geometry is described. The data were obtained at the Cornell High Energy Synchrotron Source (CHESS) with an Si(111) double-crystal monochromator and a Ge(111) crystal analyzer. Fits to a number of well resolved individual peaks demonstrate that the peak shapes are very well described by the pseudo-Voigt function, which is a simple approximation to the convolution of Gaussian and Lorentzian functions. The variation of the Gaussian and Lorentzian half widths, F~ and Ft+, with Bragg angle can be approximated quite closely by the functions V tan 0 and X/cos 0 which represent the contributions from instrumental resolution and particle-size broadening respectively. Rietveld refinement based on this model yields generally satisfactory results. The refined values of V and X are consistent with the expected vertical divergence (~_ 0-1 mrad) and the nominal particle size (~_ 0-3 I.tm). In particular, the use of a capillary specimen virtually eliminates preferred orientation effects, which are highly significant in flat-plate samples of this material.

The motion of atoms on interatomic potential energy surfaces is fundamental to the dynamics of liquids and solids. An accelerator-based source of femtosecond x-ray pulses allowed us to follow directly atomic displacements on an optically modified energy landscape, leading eventually to the transition from crystalline solid to disordered liquid. We show that, to first order in time, the dynamics are inertial, and we place constraints on the shape and curvature of the transition-state potential energy surface. Our measurements point toward analogies between this nonequilibrium phase transition and the short-time dynamics intrinsic to equilibrium liquids.

An rf photocathode electron gun is used as an electron source for ultrafast time-resolved pump-probe electron diffraction. We observed single-shot diffraction patterns from a 160 nm Al foil using the 5.4 MeV electron beam from the Gun Test Facility at the Stanford Linear Accelerator. Excellent agreement with simulations suggests that single-shot diffraction experiments with a time resolution approaching 100 fs are possible. SLAC-PUB-12162 Submitted to Applied Physics Letters 2Our understanding about dynamical processes in chemistry, materials science and biology on the picosecond and sub-picosecond time scale stems almost exclusively from time-resolved spectroscopy. Structural changes, on atomic length scales, can only be inferred indirectly from the analysis of spectra. Both x-ray and electron diffraction share the goal of 'imaging' molecular structures with a time resolution that captures the motions as systems evolve, whether they be solids, liquids or gases. Lab scale experiments in both electron diffraction 1,2 and x-ray scattering 3 have produced impressive results. Recently, in anticipation of the construction of the Linac Coherent Light Source (LCLS) at the Stanford Linear Accelerator Center (SLAC), an experiment using the electron bunch from the SLAC Linac to produce spontaneous undulator radiation 4 has shown the possibilities for ultrafast x-ray scattering from condensed systems with 100 fs time resolution. 5 This has encouraged us to approach ultrafast electron diffraction (UED) using experimental techniques based on electron sources developed for particle accelerators, with the aim of obtaining single-shot diffraction patterns on a 100 fs time scale.Electron diffraction is complementary to x-ray scattering, but features much larger cross sections that allow the study of surface phenomena, the bulk structures of thin foils and membranes, as well as molecular structures of gas phase samples. 6 As with linac based x-ray sources there has been significant development of electron sources for UED based on the use of photocathodes. 7 Unfortunately, the space-charge interactions of the electrons within a pulse, and the initial kinetic energy distribution with which the electrons are generated, have made it difficult to obtain pulses much shorter than 1 ps 8,9,10 ,in 'conventional' UED experiments using ≈30 keV electron beams. To improve the time resolution one could use fewer electrons per pulse, but that requires longer data acquisition times to obtain the necessary signal-to-noise ratio. 11 Alternatively, it is possible to increase the electric field inside the electron gun, while reducing the flight distance between the gun and the target. 12 Both tend to reduce the time of flight of the electron pulse, thereby giving the electron pulse less time to spread. Even so, this 3 approach is limited because the maximum DC and pulsed electric fields are 12 MV/m and 25 MV/m, respectively. 13,14 In the present work we take a fresh approach to ultrafast time-resolved pump-probe diffraction by using MeV electron be...

The application of synchrotron X-ray radiation to powder diffraction is described. A perfect Si doublecrystal monochromator at the Cornell High Energy

No abstract

The coherent nuclear reemission in the forward direction has been measured following excitation of the Mossbauer transitions in polycrystalline ' Fe foils by synchrotron radiation. The excited resonances are sufficiently narrow in energy that the time dependence of their decay can be straightforwardly observed, and in particular the details of the collective response to single-photon excitation can be studied.The collective nuclear scattering conditions were varied directly over a large range by using foil thicknesses between 0.5 and 28.5 pm. The envelope of the time evolution showed significant changes with increasing foil thickness. For thin foils (up to 3 pm) the envelope was exponential in the observed time window, with a decay time decreasing with increasing foil thickness. For thicker foils an oscillatory modulation of the envelope and a significant shift in the phase of the beat pattern were observed. A simple optical model can explain all the observed phenomena.

No abstract

Rotation of the plane of polarization following transmission of a synchrotron x-ray beam through a thin sample has been directly observed. Within a few eV of the cobalt Zf-absorption edge, rotations of up to 2 mrad were obtained in cubic Coo.9Feo.i, and somewhat less in a cobalt metallic glass and in chiral organometallic samples. These effects were observed using a high-extinction tunable perfect-crystal Bragg-reflection x-ray polarimeter of novel design which can detect optical rotations as small as 70 /irad. PACS numbers: 78.70.Dm, 78.20.Ek, 78.20.LsTo date, attempts to observe optical activity in the xray region 1 " 4 have met with little success. Interest in such phenomena has been revived recently due to the discovery of resonantly enhanced magnetic x-ray scattering 5 " 7 and the observation of magnetic x-ray circular dichroism. 8 The previous experiments designed to measure x-ray optical rotations, magnetic or otherwise, were all single-wavelength instruments. Only one took advantage of synchrotron radiation to operate near an absorption edge, 4 but at a fixed wavelength some 30 eV away from the edge. It is therefore not surprising that no significant effect was found. This Letter describes successful observations of both the Faraday effect in ferromagnetic materials and optical activity in chiral compounds.Hart and Rodrigues 9 demonstrated that a highperformance Bragg-reflection polarizer for x rays could be made which was able to operate over a broad spectral range. This development provided the possibility of studying resonant phenomena using synchrotronradiation sources, i.e., of tuning the probe beam energy near an absorption edge. The measurements of the Faraday effect and structurally induced optical activity in cobalt-containing alloys and compounds described here use a polarimeter comprising two such devices as the polarizer and analyzer in an arrangement similar to that used in visible-light optics, i.e., crossed polaroids (see Fig. 1). Its extinction ratio is about 10 6 near the cobalt K edge and exceeds 10 4 throughout the energy range from 7.0 to 9.2 keV, which allows measurements to be made near a large number of absorption edges in the transition metals and rare earths. A different choice of Bragg reflection in the polarimeter would provide a different operating range and Ref. 9 gives designs suitable for use in the range 4-15 keV.The polarizer and analyzer are similar crystals. Each is a monolithic channel-cut 422-Bragg-reflection crystal providing four consecutive Bragg reflections, two on each side of the channel. The special polarizing properties arise because the angle between the two sides of the channel may be offset by a small amount 9 using a controlled elastic deformation of the monolith. This offset is smaller than the angular width of the perfect-crystal Bragg reflection for the a polarization but larger than that for n polarization. Thus, consecutive reflections across the channel are imperfectly aligned and the npolarized radiation is selectively suppressed at each reflection. ...