March 2010 Archives

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).

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)

There are still many unknown problems related to the structure of amorphous materials, because the X-ray diffraction technique has some limitations in the case of disordered systems. A research team led by Dr. A. L. Goodwin (Oxford University, UK) recently reported a new elegant general scheme to solve the structure by successfully demonstrating its application to molecular C₆₀, a-Si, and a-SiO₂. The team proposes to employ the information gained in spectroscopic experiments (such as EXAFS, Raman, NMR etc) regarding the number and distribution of atomic environments. The idea is that such information can be used as a valuable constraint in the refinement of the atomic-scale structures of nanostructured or amorphous materials from the pair distribution function (PDF), which is obtained by Fourier transform of the X-ray diffraction pattern. Although a conventional reverse Monte Carlo (RMC) approach is not always successful in obtaining the correct structure solution, the team showed that such difficulties can be removed by including the above variance term. For more information, see the paper, "Structure Determination of Disordered Materials from Diffraction Data", M. J. Cliffe et al., Phys. Rev. Lett. 104, 125501 (2010).

Professor S. Techert (Max-Planck-Institute for Biophysical Chemistry, Goettingen, Germany) and his colleagues have reported on Bragg diffraction experiments with a soft X-ray laser (wavelength 8 nm, pulse width 30 fs, power 4×10¹¹ photons/pulse) from the free electron laser at FLASH, Deutsche Elektronen-Synchrotron (DESY) in Hamburg. The research group studied Bragg diffraction patterns of single nano-crystal (20 nm×20 nm×20 μm) and powder with grain sizes smaller than 200 nm of silver behenate (AgC₂₂H₄₃O₂, chain length 5.8 nm). So far, many coherent X-ray diffraction studies have been done even with soft X-ray wavelengths, but the present research aims at the analysis of periodic structures that are usually targets of X-ray diffraction with hard X-rays. They showed an interesting comparison between the single nano crystal and the powder, and also discussed the influence of the extremely high peak power of laser pulses. For more information, see the paper, "Diffraction Properties of Periodic Lattices under Free Electron Laser Radiation", I. Rajkovic et al., Phys. Rev. Lett. 104, 125503 (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).

Demand for learning analytical techniques for surfaces and interfaces appears to be on the increase. In Tokyo, Japan, the 4th tutorial course on the analysis of thin films and multilayers by X-ray reflectivity was held on March 16. The first Japanese textbook that serves as an introduction to X-ray reflectivity was published in 2009 (also translated into Korean in 2010), and the 8 authors gave lectures as part of the course. Further information is available at http://www.nims.go.jp/xray/ref/ (in Japanese only).

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).