Recently in Sciences Category

Dr. W. Yashiro (University of Tokyo, Japan) and his colleagues recently reported an interesting application of X-ray Talbot interferometry, which usually gives absorption and differential-phase images.  As micro-structures of the sample distort X-ray wave fronts, the research group quantitatively discusses how visibility reduction is caused and influenced.  They also experimentally demonstrate that this new type of experimental method using visibility contrast is feasible for imaging micro-structures, which have been studied by ultra small angle X-ray scattering so far.  For more information, see the paper, "On the origin of visibility contrast in x-ray Talbot interferometry", W. Yashiro et al., Optics Express, 18, 16890 (2010).  For more information on visibility contrast, see the paper, "Hard x-ray dark-field imaging using a grating interferometer", F. Pfeiffer et al., Nature Materials, 7, 134 (2008).

NASA's Mars Exploration Rover Spirit has obtained some significant data on the detailed chemical composition of the rock exposed on the ground surface of the Columbia Hills of the Gusev crater.  It was found that the rock is a Mg-Fe carbonate (Mc_0.62Sd_0.25Cc_0.11Rh_0.02, where Mc = magnesite, Sd = siderite, Cc = calcite, and Rh = rhodochrosite) and a forsteritic olivine (Fo_0.72Fa_0.28, where Fo = forsterite and Fa = fayalite).  This could suggest extensive aqueous activity under near-neutral pH conditions that would be conducive to habitable environments on early Mars.  On this occasion, in addition to a X-ray spectrometer, a Mossbauer (MB) spectrometer and Miniature Thermal Emission Spectrometer (Mini-TES) greatly contributed to the findings.  For more information, see the paper, "Identification of Carbonate-Rich Outcrops on Mars by the Spirit Rover", R. V. Morris et al., Science 329, 421 (2010).

Short period, high field undulators can enable short wavelength free electron lasers (FELs) at low beam energy.  A research group led by Professor J. Rosenzweig (University of California, Los Angeles, USA) has recently unveiled a new design based on an approach that utilizes cryogenic materials.  For more information, see the paper, "Short period, high field cryogenic undulator for extreme performance x-ray free electron lasers", F. H. O'Shea et al., Phys. Rev. ST Accel. Beams 13, 070702 (2010).

A Brazilian research group recently discussed the thermal influence of soft X-ray free-electron-laser (FEL) pulses on silicon substrate.  Such analysis is important, because the peak power of a single FEL pulse is roughly four orders of magnitude higher than that in conventional synchrotron light facilities.  Their detailed time-evolution analysis indicates that in a worst case scenario, the second pulse could be adversely affected by dynamic thermal distortion induced by the preceding pulse. For more information, see the paper, "Thermoelastic analysis of a silicon surface under x-ray free-electron-laser irradiation", A. R. B. de Castro et al., Rev. Sci. Instrum. 81, 073102 (2010).

What happens when an atom is excited by extremely strong X-ray photons such as an X-ray laser?  A Stanford research group recently published a very exciting report on the ionization of neon  (Z=10) by X-ray laser at the Linac Coherent Light Source (LCLS) housed at the SLAC National Accelerator Laboratory in California, USA.  The laser used in this experiment is extremely powerful (10¹⁸ W/cm², 10⁵ X-ray photons/Ų), and the research group scanned the X-ray photon energy from 800 eV to 2,000 eV, as well as the pulse width from 80 fs to 230 fs.  As the K absorption edge of neon is around 867 eV, below this energy, X-rays can strip some of the eight weakly bound electrons from the outer L shell of the neon atom.  Such a process of peeling electrons from atoms would come as no surprise for readers of X-ray spectrometry.  Above the absorption edge, K shell electrons are preferentially ejected, creating 1s vacancies that are refilled by electrons from the L shell.  Before the relaxation occurs, the remaining K shell electron is even more tightly bound to the neon nucleus than in the ground state.  Therefore, the K absorption edge for the system with a 1s vacancy is higher than usual.  When the research team raised the X-ray photon energy to 993 eV, both electrons from the inner K shell were knocked out, ionizing the atom from the inside out - in other words, coring the atom.  With this "hollow" neon then, a completely empty K shell has been created for the first time by X-ray photons, though similar phenomena may be possible by means of ultra-high temperature plasma, extremely high-energy collision processes etc.  For more information, see the paper, "Femtosecond electronic response of atoms to ultra-intense X-rays", L. Young et al., Nature 466, 56 (2010).  In the same issue, there is an interesting account by Justin Wark, "X-ray laser peels and cores atoms", Nature 466, 35 (2010).

A research group led by Professor C-U. Ro (Inha University, Korea) has recently reported the combined use of two techniques, attenuated total reflectance FT-IR (ATR-FT-IR) imaging and a quantitative energy-dispersive electron probe X-ray microanalysis, low-Z particle EPMA, for the speciation of mineral particles.  For more information, see the paper, "Speciation of Individual Mineral Particles of Micrometer Size by the Combined Use of Attenuated Total Reflectance-Fourier Transform-Infrared Imaging and Quantitative Energy-Dispersive Electron Probe X-ray Microanalysis Techniques", H-J. Jung et al., Anal. Chem. 82, 6193 (2010).

A Chinese group led by Professor J. Zhang (President of Shanghai Jiao Tong University) recently published a report on the generation of X-ray pulses of around 3 keV by using an Ar clustering gas jet target (~3mm dia.) and a Ti:sapphire laser (power 800 mJ, pulse width 28 fs, wavelength 800 nm, frequency 10 Hz).  The intensity of the Ar K-shell emissions in the forward direction was found to be around 10⁴ photons/mrad²/pulse.  The group emphasized the significance of laser contrast, which is a ratio of the main pulse and pre-pulse, and found that X-ray flux is reduced by 2 orders of magnitude if the laser pulse contrast decreases from 10⁹ to 10⁷ with constant laser pulse energy. For more information, see the paper, "Intense High-Contrast Femtosecond K-Shell X-Ray Source from Laser-Driven Ar Clusters", L. M. Chen et al., Phys. Rev. Lett. 104, 215004 (2010).

An international team of paleontologists, geochemists and physicists led by Dr. R. A. Wogelius (University of Manchester, UK) recently employed X-ray fluorescence imaging to analyze a 150 million year old fossil of Archaeopteryx, which had dinosaur-like teeth and bird-like feathers.  For many years, it was believed that the fossil contained nothing but bone and rock.  However, the use of a brilliant synchrotron X-ray beam enabled the detection of chemical elements hidden within.  It was found that the fossil still had elemental compositions that were completely different from the embedding geological matrix.  The researchers completed the chemical map of the dinobird for 12 elements for the first time.  Some phosphor and sulfur were found in soft tissue, as well as trace zinc and copper in bone.  The experiment was done at wiggler beam line 6-2 at Stanford Synchrotron Radiation Lightsource (SSRL, California, USA).  For more information, see the paper, "Archaeopteryx feathers and bone chemistry fully revealed via synchrotron imaging", U. Bergmann et al., Proc. Nat. Aca. Sci., 107, 9060 (2010).

Professor P. Dutta (University of Chicago) and his colleagues recently clarified that the surface density profile acquires layered structures at 0.2 T_c; T_c is the liquid-gas critical temperature.  The present research was for dielectric liquids, pentaphenyl trimethyl trisiloxane, and pentavinyl pentamethyl cyclopentasiloxane. The X-ray reflectivity technique was employed to determine the surface profile experimentally.  The research group had previously found similar phenomena for other liquid dielectric liquids as well as liquid metals.  The present studies could strengthen their series of work.  For more information, see the paper, "Surface order in cold liquids: X-ray reflectivity studies of dielectric liquids and comparison to liquid metals", S. Chattopadhyay et al., Phys. Rev, B81, 184206 (2010).

Combinatorial materials synthesis is a promising new way of developing and finding novel functional materials.  By the use of sophisticated thin film technology, it is possible to create compositionally graded samples on the same single substrate.  To analyze this combinatorial library, some novel technique is required.  A UK research group led by Professor K. D. Rogers (Cranfield University, UK) recently reported on high-throughput data collection and analysis using an X-ray diffraction (XRD) probe.  In the research, an extended X-ray beam was used to illuminate the libraries, and a large area detector was used to collect the data.  A new algorithm was employed to analyze the collected data and extract the crystallographic information.  For more information, see the paper, "High Throughput X-ray Diffraction Analysis of Combinatorial Polycrystalline Thin Film Libraries", S. Roncallo et al., Anal. Chem., 82, 4564 (2010).

Coherent X-ray diffraction imaging is one of the hottest research topics in advanced X-ray physics.  The method reconstructs a real-space image from an oversampled diffraction signal by using computer algorithms instead of lenses.  So far, its application has been limited to fairly strong phase objects, mainly due to parasitic scattering from the optics used for limiting the beam.  Korean researchers recently published an interesting report on its application to a nonisolated weak phase object, a one-dimensional trench structure fabricated on a Si substrate.  In their discussion, the authors reported that such work was enabled by employing a special aperture with a very high aspect ratio of nearly 100 made of tantalum (1.7 μm × 2.2 μm aperture with a thickness of 130 μm).  For more information, see the paper, "Coherent hard x-ray diffractive imaging of nonisolated objects confined by an aperture", S. Kim et al., Phys. Rev. B81, 165437 (2010).

A group led by Professor Ch. David (Paul Scherrer Institute, Switzerland) recently developed a synchrotron-based full-field microscope, which can work with hard X-rays, typically 10 keV.  The instrument supports tomographic absorption and phase contrast imaging with a spatial resolution of 144 nm.  The researchers demonstrated phase-contrast 3D imaging of a melanocortin-3 preosteoblast cell.  For more information, see the paper, "Phase-contrast tomography at the nanoscale using hard x rays", M. Stampanoni et al., Phys. Rev. B 81, 140105(R) (2010).

Lens-less microscopy is now widely acknowledged to be an elegant solution to the so-called phase problem in X-ray crystallography.  The method is based on the digital retrieval of the phase from the object's coherently diffracted intensity patterns, with the inversion being achieved through the use of time-consuming iterative algorithms.  Fourier transform holography is a similar technique, but is essentially very quick and straightforward.  Dr. V. Chamard (IM2NP, CNRS, Aix-Marseille Universite, France) and her colleagues recently demonstrated 3D imaging of a SiGe nanocrystal with Fourier transform holography.  One unique point of the research is that they employed Bragg geometry, rather than forward scattering geometry, to obtain full 3D information.  The technique requires that a reference crystal is placed near the object crystal to be imaged, and that the two crystals need to have comparable lattice parameters.  They were successful in determining the electron density and the displacement field in 3D without suffering convergence problems, which are often the case with lens-less imaging iterative algorithms.  For more information, see the paper, "Three-Dimensional X-Ray Fourier Transform Holography: The Bragg Case", V. Chamard et al., Phys. Rev. Lett. 104, 165501 (2010).

Dr. M. Giorgetti (University of Bologna and Unita di Ricerca INSTM di Bologna) and his colleagues recently reported the successful application of the chemometric approach to a series of in-situ near edge X-ray absorption spectra of a Cu_0.1V₂O₅ xerogel/Li ion battery.  The research group discusses how the multivariate curve resolution (MCR) technique and also fixed size windows evolving factor analysis (FSWEFA) are useful in determining the number of species and the ratio.  It was found that three different species co-exist during battery charging. For more information, see the paper, "Multivariate Curve Resolution Analysis for Interpretation of Dynamic Cu K-Edge X-ray Absorption Spectroscopy Spectra for a Cu Doped V₂O₅ Lithium", P. Conti et al., Anal. Chem., Article ASAP (DOI: 10.1021/ac902865h)

A German group at BESSY II recently succeeded in studying the evolution of both the spin (S) and orbital angular (L) momentum of a thin Ni film during ultrafast demagnetization, by means of X-ray magnetic circular dichroism (XMCD).  It was found that both S and L components decrease by irradiating a femtosecond laser pulse, and the time constant is 130±40 fs.  For more information, see the paper, "Femtosecond x-ray absorption spectroscopy of spin and orbital angular momentum in photoexcited Ni films during ultrafast demagnetization", C. Stamm et al., Phys. Rev, B81, 104425 (2010).

A Dutch neutron research group at Delft University of Technology, Netherlands, recently published a paper describing the extension of their coherence theory on neutron scattering to X-ray reflectivity.  For more information, see the paper, "Coherence approach in neutron, x-ray, and neutron spin-echo reflectometry", V. O. de Haan et al., Phys. Rev. B81, 094112 (2010).

Professor L. J. Allen (University of Melbourne, Australia) and his colleagues have recently demonstrated atomic-resolution chemical mapping in a scanning transmission electron microscope (STEM).  They obtained Sr and Ti images for SrTiO₃.  Such images are directly interpretable mainly because the effective ionization interaction is localized.  For more information, see the paper, "Atomic-resolution chemical mapping using energy-dispersive x-ray spectroscopy", A. J. D'Alfonso et al., Phys. Rev. B81, 100101(R) (2010).

With linac-based light sources, the electron beam has a high peak current and small energy spread, and this can be used to drive a seeded single pass free electron laser.  On the other hand, the beams in a storage ring usually have a relatively low current and large energy spread.  To generate ultrashort coherent radiation, the coherent harmonic generation (CHG) technique is a promising candidate.  Dr. D. Xiang (SLAC National Accelerator Laboratory, USA) and Dr. W. Wan (Lawrence Berkeley National Laboratory, USA) have recently proposed a scheme to extend the harmonic number of the CHG technique by an order of magnitude using angular-modulated electron beams in the storage ring.  The technique has the potential of generating femtosecond coherent soft X-ray radiation directly from an infrared seed laser.  For more information, see the paper, "Generating Ultrashort Coherent Soft X-Ray Radiation in Storage Rings Using Angular-Modulated Electron Beams", D. Xiang et al., Phys. Rev. Lett. 104, 084803 (2010).

An Australian research group has recently published experimentally obtained X-ray mass attenuation coefficients of zinc for 7.2- 15.2 keV X-rays with an absolute accuracy of 0.044% and 0.197%.  For more information, see the paper, "X-ray mass attenuation coefficients and imaginary components of the atomic form factor of zinc over the energy range of 7.2.15.2 keV", N. A. Rae et al., Phys. Rev. A81, 022904 (2010).

Dr. H. Yan (National Synchrotron Light Source II, Brookhaven National Laboratory, USA) has recently reported a comparative study on various kinoform lenses for X-ray nanofocusing.  He employed the geometrical theory, the dynamical diffraction theory, and the beam propagation method, and showed that the geometrical theory becomes invalid.  The influence of the edge diffraction effect from the individual lens element was studied in view of the limit of the focus size.  It was also shown that the length of the lenses can be optimized to reduce the wave field distortion.  For more information, see the paper, "X-ray nanofocusing by kinoform lenses: A comparative study using different modeling approaches", H. Yan, Phys. Rev. B81, 075402 (2010).

So far, it has been understood that the only way to realize hard-X-ray mirrors with near 100% reflectivity is the use of total external reflection at grazing incidence to a surface.  Dr. Y. V. Shvyd'ko (Argonne National Lab, USA) and his colleagues have recently proposed to use Bragg reflections from synthetic diamond crystal.  They discussed how it shows an unprecedented reflecting power at normal incidence with meV order narrow bandwidths for hard X-rays.  The optics might be a good candidate for X-ray free-electron laser oscillators (X-FELO).  For more information, see the paper, "High-reflectivity high-resolution X-ray crystal optics with diamonds", Y. V. Shvyd'ko et al., Nature Physics, doi:10.1038/nphys1506; published online, 17 January 2010.

A research group led by Professors Y. Takanishi (Kyoto University, Japan) and A. Iida (Photon Factory, KEK, Japan) has recently published its successful investigation into the local layer structure of bent-core liquid crystal, 4-Br-14-O-PIMB, which includes Br atoms.  The group employed a monochromatic X-ray microbeam (3 μm × 4 μm), and observed X-ray scattering from the cell near the Br K absorption edge.  They were able to discover some satellite peaks reflecting the superlattices.  For more information, see the paper, "Microbeam resonant x-ray scattering from bromine-substituted bent-core liquid crystals", Y. Takanishi et al., Phys. Rev. E81, 011701 (2010).

X-ray Photon Correlation Spectroscopy (XPCS) is a novel technique which reveals the slow dynamics of equilibrium and non-equilibrium processes in condensed matter systems.  A group led by Professor N. P. Balsara (University of California, Berkeley, USA) has recently published research on a polystyrene-polyisoprene block copolymer melt in the vicinity of the order-disorder transition.  The group combined several techniques in addition to XPCS; time-resolved small angle X-ray scattering and rheology.  During their studies of ordering kinetics, it was found that two qualitatively different regimes exist, i.e., shallow and deep quench regimes, respectively.  For more information, see the paper, "Dynamic signatures of microphase separation in a block copolymer melt determined by X-ray photon correlation spectroscopy and rheology", A. J. Patel et al., Macromolecules, Article ASAP (DOI: 10.1021/ma902343m).

Dr. G. J. Havrilla (Los Alamos National Lab., USA; one of the associate editors of X-Ray Spectrometry journal) and his colleague recently published a very interesting report on the analysis of picoliter droplets, which can be now accurately prepared using Hewlett-Packard's extremely sophisticated technology.  The research targets application to analytical science, although the instrument is basically designed for inkjet printing and other similar purposes.  It has been shown that dried deposits of single and multielemental solutions generated in picoliter volumes are able to be used as references for micro X-ray fluorescence.  Evaporation can have a strong influence on extremely small amounts at the picoliter level, but the research group successfully devised the optimal instrumental conditions by monitoring X-ray fluorescence intensity.   For more information, see the paper, "Picoliter droplet deposition using a prototype picoliter pipette: Control parameters and application in micro X-ray fluorescence", U. E. A. Fittschen et al., Anal. Chem., 82, 297 (2010).

For many years, substantial effort has been devoted to developing a good mirror for preparing a small X-ray beam.  Professor K. Yamauchi (Osaka University, Japan) and his colleagues have recently reported the breaking of the 10 nm barrier for hard X-rays.  They employed a combination of two mirrors; the surface of the first mirror is deformable, in order to compensate for figure error of the second mirror.  By such an adaptive optical system, the research group attained a beam size of 7 nm at 20 keV.  The experiments were done at BL29XUL, SPring-8.  For more information, see the papers, "Breaking the 10 nm barrier in hard-X-ray focusing", H. Mimura et al., Nature Physics doi:10.1038/nphys1457; published online: 22 November 2009; corrected online: 2 December 2009.

Foamlike, cellular structures of the monolayer of organic capped nanoparticles can sometimes be observed on liquid surfaces.  Professor M. K. Sanyal (Saha Institute of Nuclear Physics, India) and his lab members studied the time evolution in the structure and morphology of transferred monolayers of gold-thiol nanoparticles, formed at the air-water interface at different surface pressure, on to a silicon surface.  The research group employed two complementary techniques, X-ray reflectivity and atomic force microscopy (AFM), to see the whole drying-mediated self-assembly of nanoparticles.  For more information, see the paper, "Nanopattern formation in self-assembled monolayers of thiol-capped Au nanocrystals", R. Banerjee et al., Phys. Rev. E80, 056204 (2009).

So far, X-ray microscopy with many types of lens has achieved great success in the observation of biological cells.  In order to extend the limits of spatial resolution and efficiency, X-ray diffraction microscopy (also called coherent X-ray diffraction imaging), which uses coherent X-rays and some image reconstruction algorithms instead of an optical lens system, is now considered as a promising procedure to see whole cells at once and pick out much smaller features, down to around 10 nm or even less.  A research group led by Professor C. Jacobsen (Stony Brook University, USA) recently reported the results for yeast cells with 520 eV soft X-rays at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory, USA.  Dr. A. Madsen (European Synchrotron Radiation Facility (ESRF), Grenoble, France) and his colleagues observed the cells of the bacteria D. radioduran with 8 keV X-rays.  The advantage of using hard X-rays is the ease of sample handling, and the validity of thin sample approximation for future 3D reconstructions through phasing a diffraction volume.  In both cases, a rapid freezing technique (instead of previously used freeze-drying) was used to avoid the effects of radiation damage from synchrotron X-ray photons.  The Stony Brook group plunged cells in their natural wet state into liquid ethane and maintained them at below -170 ^oC, leading to the reduction of artifacts due to damage from dehydration, ice crystallization, and radiation.  In the ESRF setup, as absorption in air of 8 keV X rays is small, a nonvacuum environment was implemented for ease of sample handling.  Similar to the system for macromolecular crystallography applications, they based the samples in a continuous cryogenic nitrogen gas jet at around -165 ^oC.  The spatial resolution was 25 nm and 30-50 nm, for soft and hard X-rays cases, respectively.  For more information, see the papers, "Soft X-ray diffraction microscopy of a frozen hydrated yeast cell", X. Huang et al., Phys. Rev. Lett., 103, 198101 (2009), and "Cryogenic X-ray diffraction microscopy for biological samples", E. Lima et al., Phys. Rev. Lett., 103, 198102 (2009)

X-ray absorption microscopy is simple, but has low sensitivity in biological samples that are made of light elements.  X-ray phase contrast imaging can provide contrast that is 3 orders of magnitude greater than X-ray absorption.  However, phase contrast imaging has not been that widely used so far mainly because of the unusual requirements of the experimental setup.  Dr. W. Yashiro (The University of Tokyo, Japan) and his colleagues have recently proposed a novel setup that is feasible.  The research group simply added a transmission grating to the setup for conventional X-ray absorption microscopy with a Fresnel Zone Plate (FZP) objective lens.  Because of the self-imaging phenomenon in Talbot effects, a phase difference image can be produced by the transmission grating placed at the downstream of the back focus of the FZP.  The experiment was done at beamline BL20XU, SPring-8.  For more information, see the paper, "Hard-X-Ray Phase-Difference Microscopy Using a Fresnel Zone Plate and a Transmission Grating", W. Yashiro et al., Phys. Rev. Lett. 103, 180801 (2009).

It is known that sulfide sometimes play a significant role in the geochemistry of arsenic under reducing conditions.  So far, it has been assumed that sulfide primarily reduced the solubility and mobility of arsenic by precipitation of arsenic-sulfide minerals, As₂S₃, but recent studies indicate that under certain conditions, significant concentrations of soluble As-S compounds can exist in sulfidic waters.  Thus, the question is whether they are As(III)-S species ("thioarsenites") or As(V)-S species ("thioarsenates").  A research group led by Dr. B. Planer-Friedrich (University of Bayreuth, Germany) has recently reported that use of X-ray absorption spectroscopy (XANES and EXAFS) can determine the concentration ratio of each species.  The experiment was done at beamline BM20, ESRF.  For more information, see the paper, "Discrimination of Thioarsenites and Thioarsenates by X-ray Absorption Spectroscopy", E. Suess et al., Anal. Chem., Article ASAP (2009), DOI: 10.1021/ac901094b

Dr. C. T. Chanter and his colleagues have published a paper on the unresolved quantitative discrepancies between experimental and theoretical Cu Kα spectra.  For more information, see the paper, "Theoretical Determination of Characteristic X-Ray Lines and the Copper Kα Spectrum", C. T. Chantler et al., Phys. Rev. Lett. 103, 123002 (2009).

So far, diffusion in solids has been investigated by profiling the depth dependence of tracer atoms diffused into the sample.  Although one can obtain the diffusion constant from this, the question is how diffusion takes place on the atomic scale, rather than on the micron scale.  Sometimes quasielastic neutron scattering as well as Mobauer spectroscopy can be used in a very limited number of fortunate cases.  A research group led by Professor G. Vogl (University of Vienna, Austria) recently reported the use of X-ray photon correlation spectroscopy (XPCS) to observe the dynamics of diffusing atoms.  The research was done for intermetallic alloy Cu₉₀Au₁₀, at temperatures of around 540 K, where the system is a substitutional solid solution, that is, the Au atoms statistically occupy sites in the Cu fcc lattice.  The research gives the dynamical behavior of single atoms as a function of their neighborhood, and confirms quantitatively that Au atoms have a tendency to locally order on a certain set of sites in the crystal.  Photon correlation spectroscopy is based on analysis of 'speckle' patterns, which are fine-scale diffraction patterns that appear in the scattering of coherent light from a disordered system.  Speckle patterns are sensitive to the exact spatial arrangement of the disorder.  By observing the intensity fluctuations in the speckle pattern, the characteristic times of fluctuations in the system can be determined.  For more information, see the paper, "Atomic diffusion studied with coherent X-rays", M. Leitner et al., Nature Materials,8, 717 (2009).

When a strong laser beam hits the surface of a material, plasma is produced there, subsequently leading to the emission of a short burst of X-rays.  It is believed that the electrons in the surface plasma are accelerated by the strong electric field of the laser and then penetrate the solid behind. There, they knock out electrons from inner electronic shells, which subsequently undergo inner-shell recombination, leading to characteristic line emissions such as Kα and Kβ spectra.  A research group led by Professor U. Teubner (University of Applied Sciences, Emden, Germany) has reported detailed experimental results on copper and titanium K X-rays.  Particular attention has  been paid to the interplay between the angle of incidence of the laser beam on the target, as well as the influence of prepulses.  For more information, see the paper, "Optimized K x-ray flashes from femtosecond-laser-irradiated foils", W. Lu et al., Phys. Rev. E 80, 026404 (2009).

In X-ray diffraction experiments, one measures the intensity (amplitude) of the diffracted X-rays as a function of position in the reciprocal space, and the information on the phase is always missing.  For many years, this so-called phase problem has been thought as one of the biggest problems in X-ray crystallography.  Professor E. Wolf (University of Rochester, New York) has recently published a very interesting and inspirational paper.  He is famous for several important textbooks on optics and also for his presidency of the Optical Society of America.  The present paper is theoretical, and starts with a criticism of basic understanding of the problem. The author says that trying to measure the phase is rather meaningless.  Almost all scientists assume that the incident X-ray beam is monochromatic in the data analysis, but the author points out that a monochromatic beam is not possible in reality.  Any beam that can be produced in a laboratory is, at best, quasimonochromatic and, therefore, even if both the amplitudes and the phases are given, it is still not possible to solve the problem.  Alternatively, the author proposes the measurement of certain correlation functions, with the use of spatially coherent beams.  While it is extremely important to think about a future strategy regarding the final solution of the phase problem as discussed in the paper, the author makes no mention of the recent significant strides in coherent X-ray scattering.  For more information, see the paper, "Solution of the Phase Problem in the Theory of Structure Determination of Crystals from X-Ray Diffraction Experimentst", E. Wolf, Phys. Rev. Lett. 103, 075501 (2009).

X-ray phase-contrast imaging is extremely powerful for visualizing internal structures with low-Z matrices, which are most likely in bio-medical specimens.  The use of an X-ray interferometer is one of the most promising ways forward for this imaging technology, but resolution has been limited to the micrometer scale so far.  A research group led by Dr. A. Snigirev (European Synchrotron Radiation Facility, Grenoble, France) has recently developed a novel type of X-ray interferometer employing a bilens system with two parallel arrays of compound refractive lenses, each of which creates a diffraction limited beam under coherent illumination.  The energy of the X-rays is 10-20 keV and the material used in the refractive lenses is silicon.  When the two beams overlap, they produce an interference pattern with fringe spacing ranging from tens of nanometers to tens of micrometers.  Readers may notice that the system is similar to the model of a Billet split lens in classical optics (See Fig.7.8, page 263 in "Principle of Optics", M. Born and E. Wolf, 6^th Ed, Pergamon Press (1988)).  The use of a modern synchrotron source and this novel optical device thus opens up a new field and could revive old theorems.  Coherent moiré imaging or radiography are promising straightforward applications.  For more information, see the paper, "X-Ray Nanointerferometer Based on Si Refractive Bilenses", A. Snigirev et al., Phys. Rev. Lett., 103, 064801 (2009).

It is well known that the physical properties of semiconductor nanostructures, which have been grown in most cases by the Stranski-Krastanow (SK) mechanism, depend on their size, shape, strain and composition.  In the case of the growth of Ge on Si(001), where the 2D-3D transition is driven by the 4.16% lattice mismatch between Ge and Si, the increase of Ge coverage above a critical thickness of around 4 ML can make coherent islands.  First, square pyramids appear, and then dome-shaped islands are formed.  At about 9 ML, the misfit strain can no longer be accommodated coherently and larger islands called superdomes are present. This raises detailed questions as to dependence on the growth rate, temperature etc.  To provide answers to such questions, in-situ X-ray studies are extremely important.  Professor G. Bauer (Institute of Semiconductor and Solid State Physics, Johannes Kepler University in Linz, Austria) and his colleagues recently performed grazing-incidence small angle X-ray scattering (GISAXS) and diffraction (GID) experiments with a UHV-MBE chamber.  They clarified the kinetics of the growth of Ge superdomes and their facets on Si(001) surfaces, as a function of deposited Ge thickness for different growth temperatures at a low growth rate, by in situ grazing-incidence small-angle x-ray scattering in combination with in situ grazing-incidence x-ray diffraction. At a low growth rate, intermixing is found to be enhanced and superdomes are formed already at lower coverages than previously reported. In addition, the research team observed that at the dome-to-superdome transition, a large amount of material is transferred into dislocated islands, either by dome coalescence or by anomalous coarsening. Once dislocated islands are formed, island coalescence is a rare event and introduction of dislocations is preferred. The superdome growth is thus stabilized by the insertion of dislocations during growth.  For more information, see the paper,   "In situ X-ray scattering study on the evolution of Ge island morphology and relaxation for low growth rate: Advanced transition to superdomes", M.-I. Richard et al., Phys. Rev. B 80, 045313 (2009).

Since 1984, laboratory-scale X-ray lasers have been extensively studied.  The shortest wavelength achieved so far is 3.6 nm, with a weak intensity.  On the other hand, X-ray free-electron lasers (XFEL) based on self-amplified spontaneous emission (SASE) from a long undulator in the linear electron accelerator will be available in near future.  The next idea is the use of XFEL to pump a photoionization inner-shell X-ray laser in an atomic gas.  Dr. R. London (Lawrence Livermore National Lab) and a colleague have recently published their theoretical calculations.  For more information, see the paper, "Atomic inner-shell X-ray laser pumped by an x-ray free-electron laser", N. Rohringer et al., Phys. Rev. A 80, 013809 (2009).

Imaging individual objects of several nanometer resolution in space and several femtosecond resolution in time, is now one of the most exciting experiments in X-ray physics.  Over the past decade, coherent X-ray diffraction has overcome a lot of limits in imaging noncrystalline objects at a resolution in the order of X-ray wavelength.  So far, X-ray free electron lasers (or, in the mean time, 3^rd generation synchrotron sources) have been considered as a promising source, but the table-top source is no doubt extremely important for many new sciences.  Recently, Dr. H. Merdji (CEA Saclay, France) and his colleagues reported the feasibility of a laser-driven soft X-ray source, which uses the 25^th harmonics (32 nm wavelength, 20 fs pulse width) of a Ti:sapphire laser.  They succeeded in observing diffraction patterns from isolated nano-objects with a single 20 fs pulse.  Images were reconstructed with a spatial resolution of 119 nm from the single shot and 62 nm from multiple shots.  For more information, see the paper, "Single-Shot Diffractive Imaging with a Table-Top Femtosecond Soft X-Ray Laser-Harmonics Source", A. Ravasio et al., Phys. Rev. Lett. 103, 028104 (2009).

Dr. P. Glatzel (European Synchrotron Radiation Facility (ESRF), Grenoble, France) and his colleagues recently published an interesting paper reporting systematic studies on both X-ray absorption and Kα emission spectra from sulfur compounds.  The compounds' spectra were compared with quantum chemical calculations using density functional, multiple-scattering, and atomic multiplet theory.  It was found that the near-edge absorption spectra are mainly determined by the geometry of the first coordination sphere in the case of the sulfates and sulfite, while strong orbital hybridization in the case of sulfides results in a much more complex analysis.  On the other hand, the spectral shape of the Kα fluorescence lines shows little influence of the chemical environment, but its energy position is correlated with the valence-shell electron population.  The experiments were done at beamline ID26, ESRF.  The spectrometer used for Kα emission is a combination of a Johansson Si(111) crystal and a CCD camera.  The energy resolution was 0.44 eV for S Kα.  For more information, see the paper, "Electronic Structure of Sulfur Studied by X-ray Absorption and Emission Spectroscopy", R. A. Mori et al., Anal. Chem., Article ASAP, DOI: 10.1021/ac900970z

In January 2006, NASA's Stardust spacecraft brought comet coma particles and interstellar grains from Comet 81P/Wild2.  Synchrotron facilities all over the world have been used for extensive analysis of the chemical composition and crystal structures of the matter.  Recently, Professor L. Vincze (X-ray Microspectroscopy and Imaging Group, Ghent University, Belgium) and his colleagues reported the results of 3D X-ray imaging based on X-ray fluorescence (XRF) tomography.  In the present research, a 200 nm beam was employed, because the typical size of the particles from space was 2 microns.  The measurement consisted of 2D scanning XRF maps for each rotation angle of the sample.  In the XRF spectra, many peaks were found; Ca, Cr, Mn, Fe, Cu, Se etc. For more information, see the paper, "X-ray Fluorescence Nanotomography on Cometary Matter from Comet 81P/Wild2 Returned by Stardust", G. Silversmit et al., Anal. Chem., Article ASAP, DOI: 10.1021/ac900507x  For related work on the same 'star dust' by other groups, for example, see "Chondrulelike Objects in Short-Period Comet 81P/Wild 2", Tomoki Nakamura et al., Science, 321, 1664-1667 (2008) and "Mixing Fraction of Inner Solar System Material in Comet 81P/Wild2", A. J. Westphal et al, The Astrophysical Journal, 694, 18-28 (2009).

As reported here previously, in April this year, the first 1.5 Å wavelength laser light was generated at Stanford, USA.  An interesting account of the hard X-ray laser was published in Nature Photonics.  See the article, "Free electron lasers: First light from hard X-ray laser", B. McNeil, Nature Photonics, 3, 375-377 (2009).

Confocal X-ray micro fluorescence is a method of 3D analysis, and uses the formation of confocal volume (probing microvolume) defined through the intersection of a focused excitation beam and the sensitive volume of a polycapillary lens placed in front of the detector.  Because of increasing demands, the technique has been widely used at both synchrotron and laboratory sources.  However, some essential problems in quantitative analysis have remained so far.  Dr. A-G. Karydas (Institute of Nuclear Physics, N.C.S.R. "Demokritos", Greece) and his colleagues recently published a paper on the influence of the secondary fluorescence enhancement in this technique.  For more information, see the paper, "Secondary Fluorescence Enhancement in Confocal X-ray Microscopy Analysis", D. Sokaras et al., Anal. Chem., Article ASAP, DOI: 10.1021/ac900688n

The use of short pulses of extremely bright synchrotron X-rays has opened up a new world.  In Japan, Dr. S. Adachi (KEK, Tsukuba Japan) and his colleagues recently succeeded in recording movies during changes in the molecule structures of myoglobin.  The samples used are frozen myoglobin crystals that had CO (carbon monoxide) stored inside before the start of the experiments.  Even at 100K, irradiating pulsed laser light gave the trigger for the migration of CO molecules.  To see changes in atomic scale, time-resolved X-ray diffraction measurements were performed.  The obtained movie tells us that the CO molecules penetrate into a number of cavities in the crystal and even expand their size.  The research group has obtained an important result suggesting some self-opening mechanism in the ligand migration channel.  For more information, see the paper, "Visualizing breathing motion of internal cavities in concert with ligand migration in myoglobin", A. Tomita et al., Proceedings of National Academy of Science, 106, 2612-2616 (2009) Published online before print February 9, 2009, doi: 10.1073/pnas.0807774106

In classical metallurgy, there exists a very famous rule known as Hume-Rothery's rule, which describes the conditions necessary for the formation of a solid solution from two independent metals.  In order to have a substitutional crystalline solid solution in which the atoms of one element randomly substitute for atoms of another element in a crystal structure, the components must have an atomic size within 15% and electronegativity within 0.4 of each other.  According to this rule, a Ce-Al solid solution cannot be obtained.  Recently, a research team led by Professor H.K. Mao (Carnegie Institution of Washington) and Professor R. Ahuja (Uppsala University) found during high pressure research on the intermetallic compound of Ce₃Al that a solid solution is formed in a Ce-Al system.  The differences in radii and electronegativity of Ce and Al were diminished by applying pressure.  Both synchrotron X-ray studies (XRD and X-ray absorption spectroscopy) and ab initio calculations showed the same cause for bringing the two elements closer in radii and electronegativity, resulting in the new alloy phase.  Even after the release of pressure, this substitutional alloy remained.  During in-situ X-ray absorption measurements at the Ce LIII edge, conspicuous changes in the sharpness of the absorption, correlated to delocalization of 4f electrons, were observed.  For more information, see the paper, "Substitutional alloy of Ce and Al", Q-S.Zeng et al., Proceedings of National Academy of Science, 106, 2515-2518 (2009) Published online before print February 2, 2009, doi: 10.1073/pnas.0813328106

Eu is one of the most interesting lanthanides, compounds of which often exhibit remarkable optical, electrical, and magnetic properties.  Therefore, it is extremely important to develop a technique for chemical state analysis.  The X-ray emission spectra of Eu had not been thought to exhibit significant chemical effects.  A research group led by Professor H. Hayashi (Japan Women's Univ) firstly found a large chemical shift (~5 eV) in Eu Lγ₄ emission line, depending on the valence state.  They discussed the feasibility of using this as a probe for spin- and valence-selective X-ray absorption fine structure spectroscopy.  For more information, see the paper, "Probe for spin- and valence-selective X-ray absorption fine structure spectroscopy: EuLγ₄ emission", H. Hayashi et al., Anal. Chem., 81, 1522 (2009).

X-ray absorption spectroscopy is one of the most powerful probes of molecular structures.  So far, applications have been limited to the steady state and/or quite slowly changing systems.  Recently, Professor M. Chergui (Ecole Polytechnique Federale de Lausanne, (EPFL), Switzerland) and his colleagues reported a very impressive ultrafast X-ray absorption experiment.  There is a large class of Fe(II)-based molecular complexes that show two electronic states closely spaced in energy: a low-spin (LS) singlet and a high-spin (HS) quintet state. They therefore exhibit spin crossover (SCO) behavior, wherein conversion from a LS ground state to a HS excited state (or the reverse) can be induced by small changes in temperature and pressure or by light absorption.  The studies were done for an aqueous solution of [Fe^II(bpy)₃]²⁺, which serves as a model system for the family of Fe(II)-based SCO complexes.  A 100-mm-thick free-flowing liquid jet of an aqueous solution of 50 mM [Fe^II(bpy)₃]²⁺ was excited by an intense 400-nm laser pulse (115-fs pulse width, repetition rate 1 kHz), and a tunable femtosecond hard X-ray pulse from the slicing source was used to probe the system in transmission mode at 2 kHz.  The X-ray flux was about 10 photons/pulse at 7 keV.  The time resolution was under 250 fs.  By recording the intensity of a characteristic near edge absorption spectral feature as a function of laser pump/X-ray probe time delay, the very early stages of photo excitation in Fe(II)-based complexes were clarified.  For more information, see the paper, "Femtosecond XANES Study of the Light-Induced Spin Crossover Dynamics in an Iron(II) Complex", Ch. Bressler et al., Science, 323, 489 (2009).

Laser generation in the X-ray region has become realistic because of the construction of free electron laser facilities, which will be available in the near future (Linac Coherent Light Source (LCLS) at Stanford in 2009; European XFEL in 2014).  Another significant route is the extension of existing laser technologies such as high-order harmonic generation (HOHG), particularly from relativistically oscillating plasma mirror-like surfaces.  Professor M. Zepf (Queens University Belfast, UK) and his colleagues recently published an interesting paper showing that it is possible to achieve a near-diffraction-limited focal spot size that is also controllable.  For more information, see the paper, "Diffraction-limited performance and focusing of high harmonics from relativistic plasmas", B. Drome et al., Nature Physics, advanced online publication doi:10.1038/nphys1158

Professor T. Rayment (School of Chemistry, University of Birmingham, UK) and his colleagues have developed a channel-flow cell to study electrochemical reactions on electrodes by time-resolved X-ray absorption spectroscopy.  During the studies with the model system, it was found that a flowing solution is essential to remove any products of beam damage.  For more information, see the paper, "Channel-Flow Cell for X-ray Absorption Spectroelectrochemistry", R. J. K. Wiltshire et al., J. Phys. Chem., C 113, 308 (2009)

Diffractive imaging is a technique for so-called lens-less microscopy, and uses diffraction intensity (image) and phase retrieval calculations rather than focusing systems such as lenses, which are not free from aberrations.  The spatial resolution is basically limited only by the amount of high-angle scattering.  Therefore, the technique has been considered as having the potential to achieve atomic resolution for hard X-rays or other short-wavelength particle beams.  However, so far, the reported results have been still at the level of several nanometers.  Recently, a research group at the University of Illinois, USA proposed a method of improving the resolution.  One of the biggest technical reasons limiting the spatial resolution of diffractive imaging is the difficulty of recording weak coherent scattering signals.  The research group proposes the combined use of low-resolution imaging, which provides the starting phase, real-space constraint, missing information in the central beam and essential marks for aligning the diffraction pattern.  The group used an electron microscope to see a single CdS quantum dot with sub-angstrom resolution and noted that it is possible to use the same procedure in the case of coherent X-ray scattering.  For more information, see the paper, "Sub-angstrom-resolution diffractive imaging of single nanocrystals", W. J. Huang et al., Nature Physics, advanced online publication doi:10.1038/nphys1161

Professor L. Natarajan (University of Mumbai, India) recently published a paper calculating the energies and electric dipole rates of X-rays from the empty K shells of atoms in the range of Z=12 to 56.  For more information, see the paper, "Relativistic fluorescence yields for hollow atoms in the range 12<Z<56", L. Natarajan, Phys. Rev. A78, 052505 (2008).

Some of the most well known self-assembled monolayers (SAMs) are alkyl sulfides on gold surfaces.  They have many potential applications in molecular electronics, biosensors, and nanopatterning.  However, there have still been unsolved problems in basic research regarding Au-S interaction.  Recently, Professor A. Morgante (Universita' di Trieste, Italy) and his colleagues published the results of grazing incidence X-ray diffraction and density functional theory-based molecular dynamics simulations for hexanethiol and methylthiol.  The research group demonstrated surface complexes wherein two S atoms are joined by an intermediate Au adatom (RS-Au-SR) for longer chain cases. It was found that the sulfur atoms of the molecules bind at two distinct surface sites, and that the first surface layer contains vacancies as well as gold adatoms that are laterally bound to two sulfur atoms.  Competition between SAM ordering and disordering of interfacial Au atoms takes an important role in the system.  For more information, see the paper, "X-ray Diffraction and Computation Yield the Structure of Alkanethiols on Gold(111)", A. Cossaro et al., Science, 321, 943-946 (2008).

X-ray Bragg diffraction can determine crystal structures.  So far, however, distinguishing between right- and left-handed crystals has not been done by ordinary X-ray diffraction.  Japanese scientists led by Professor S. Shin (RIKEN & The University of Tokyo) recently succeeded in revealing the chirality of crystals by measuring Bragg diffraction near the absorption edge, using circular polarization of synchrotron X-rays at the SPring-8.  Reflections only allowed at resonant conditions have been well interpreted for the α-quartz case.  For more information, see the paper, "Right Handed or Left Handed? Forbidden X-Ray Diffraction Reveals Chirality", Y. Tanaka et al., Phys. Rev. Lett., 100, 145502 (2008).

As an X-ray free-electron laser (X-FEL) provides extremely strong pulses, it is necessary to understand the photon-induced damage processes for biological samples.  A research group led by Dr. Chapman (DESY, Germany and Lawrence Livermore National Lab, USA) has discussed how several aspects of existing continuum damage models can be tested during early operation of X-FEL at lower X-ray energies in the range of 0.8-5 keV and low fluences, focusing particularly on macroscopic collective effects such as particle charging, expansion, and average ionization of nanospheres.  For more information, see the paper, "Modeling of the damage dynamics of nanospheres exposed to x-ray free-electron-laser radiation", S. P. Hau-Riege et al., Phys. Rev. E77, 041902 (2008).