Imaging atom-clusters by hard x-ray free electron lasers

The ingenious idea of single molecule imaging by hard x-ray Free Electron Laser (X-FEL) pulses was recently proposed by Neutze et al. [Nature,406,752(2000)]. However, in their numerical modelling of the Coulomb explosion several interactions were neglected and no reconstruction of the atomic structure was given. In this work we carried out improved molecular dynamics calculations including all quantum processes which affect the explosion. Based on this time evolution we generated composite elastic scattering patterns, and by using Fienup's algorithm successfully reconstructed the original atomic structure. The critical evaluation of these results gives guidelines and sets important conditions for future experiments aiming single molecule structure solution.Comment: 8 pages, 4 figures, submitted to Europhysics Letter

One-dimensional Model of a Gamma Klystron

A new scheme for amplification of coherent gamma rays is proposed. The key elements are crystalline undulators - single crystals with periodically bent crystallographic planes exposed to a high energy beam of charged particles undergoing channeling inside the crystals. The scheme consists of two such crystals separated by a vacuum gap. The beam passes the crystals successively. The particles perform undulator motion inside the crystals following the periodic shape of the crystallographic planes. Gamma rays passing the crystals parallel to the beam get amplified due to interaction with the particles inside the crystals. The term `gamma klystron' is proposed for the scheme because its operational principles are similar to those of the optical klystron. A more simple one-crystal scheme is considered as well for the sake of comparison. It is shown that the gamma ray amplification in the klystron scheme can be reached at considerably lower particle densities than in the one-crystal scheme, provided that the gap between the crystals is sufficiently large.Comment: RevTeX4, 22 pages, 4 figure

Shape Transition in the Epitaxial Growth of Gold Silicide in Au Thin Films on Si(111)

Growth of epitaxial gold silicide islands on bromine-passivated Si(111) substrates has been studied by optical and electron microscopy, electron probe micro analysis and helium ion backscattering. The islands grow in the shape of equilateral triangles up to a critical size beyond which the symmetry of the structure is broken, resulting in a shape transition from triangle to trapezoid. The island edges are aligned along $Si[110]$ directions. We have observed elongated islands with aspect ratios as large as 8:1. These islands, instead of growing along three equivalent [110] directions on the Si(111) substrate, grow only along one preferential direction. This has been attributed to the vicinality of the substrate surface.Comment: revtex version 3.0, 11 pages 4 figures available on request from [email protected] - IP/BBSR/93-6

First Observation of Self-Amplified Spontaneous Emission in a Free-Electron Laser at 109 nm Wavelength

We present the first observation of Self-Amplified Spontaneous Emission (SASE) in a free-electron laser (FEL) in the Vacuum Ultraviolet regime at 109 nm wavelength (11 eV). The observed free-electron laser gain (approx. 3000) and the radiation characteristics, such as dependency on bunch charge, angular distribution, spectral width and intensity fluctuations all corroborate the existing models for SASE FELs.Comment: 6 pages including 6 figures; e-mail: [email protected]

Resonant X-ray Scattering in Manganites - Study of Orbital Degree of Freedom -

Orbital degree of freedom of electrons and its interplay with spin, charge and lattice degrees of freedom are one of the central issues in colossal magnetoresistive manganites. The orbital degree of freedom has until recently remained hidden, since it does not couple directly to most of experimental probes. Development of synchrotron light sources has changed the situation; by the resonant x-ray scattering (RXS) technique the orbital ordering has successfully been observed . In this article, we review progress in the recent studies of RXS in manganites. We start with a detailed review of the RXS experiments applied to the orbital ordered manganites and other correlated electron systems. We derive the scattering cross section of RXS where the tensor character of the atomic scattering factor (ASF) with respect to the x-ray polarization is stressed. Microscopic mechanisms of the anisotropic tensor character of ASF is introduced and numerical results of ASF and the scattering intensity are presented. The azimuthal angle scan is a unique experimental method to identify RXS from the orbital degree of freedom. A theory of the azimuthal angle and polarization dependence of the RXS intensity is presented. The theoretical results show good agreement with the experiments in manganites. Apart from the microscopic description of ASF, a theoretical framework of RXS to relate directly to the 3d orbital is presented. The scattering cross section is represented by the correlation function of the pseudo-spin operator for the orbital degree of freedom. A theory is extended to the resonant inelastic x-ray scattering and methods to observe excitations of the orbital degree of freedom are proposed.Comment: 47 pages, 24 figures, submitted to Rep. Prog. Phy

Combining CXMD and XSW to study magnetic and geometric properties of thin films: Gd/Fe(100)

The magnetic and geometric structure of 1-5 monolayers (ML) of Gd on Fe(100) single crystals has been investigated using synchrotron X-rays between 4 and 8 keV. Choosing a high-flux wiggler beamline made it possible to analyse the magnetic properties by studying the circular X-ray magnetic dichroism (CXMD) and, in addition, to apply the X-ray standing-wave (XSW) technique to elucidate the geometric structure of the overlayers. CXMD reveals that Gd couples antiferromagnetically to the Fe substrate and that 2 ML Gd at 243 K yield a magnetization of ~22% with respect to the saturation value of a thick Gd foil at 123 K. A magnetic polarization of the Gd pertains even at 308 K. While LEED investigations do not show any long-range lateral order of the Gd film, XSW measurements reveal an ordering of Gd perpendicular to the (100) surface of Fe

Calculated and measured performance of an X-ray wiggler beamline at DORIS III

The new BW1 beamline at the DORIS III storage ring at HASYLAB in Hamburg was characterized using ray tracing and finite element analysis programs. The angular distribution of the beam, power densities at different locations and spectral distributions were calculated. The main aspect for the calculations was the thermal deformation of the optical elements, especially the distortion of the first monochromator crystal resulting from the high power emitted by the wiggler.In the present setup a 4 m long X-ray wiggler is used as the source, imposing a heat load of up to 2.5 kW on a directly cooled Si(1 1 1) crystal. The bending of this crystal was analyzed by means of X-ray diffractometry. The experimental results were in good agreement with theoretical calculations and reveal the basic characteristics of the directly cooled crystal. The present limitations of the design indicate ways for optimization which are clearly needed for the available high powers

The LCLS: A fourth generation light source using the SLAC linac

Recent technological developments make it possible to consider use of the Stanford linear accelerator to drive a linac coherent light source (LCLS)—a laser operating at hard x‐ray wavelengths. In the LCLS, stimulated emission of radiation would be achieved in a single pass of a high‐energy, extremely bright electron beam through an undulator, without the optical cavity resonator normally used in storage ring‐based free‐electron lasers. The x‐ray laser beam would be nearly diffraction limited with very high transverse coherence, and would exhibit unprecedented peak intensity and peak brightness, and sub‐picosecond pulse length. Such an x‐ray source offers unique capabilities for a large number of scientific applications