June 2008 Archives

Andrew Lang, Emeritus Professor of Physics at the University of Bristol, has died. Born in 1924 at St Annes-on-Sea in the UK, Professor Lang obtained a First-Class Honours London External BSc in Physics at Exeter in 1944, a London External MSc in 1947 and a Cambridge PhD in 1953. He worked in industrial research in the UK (Lever Brothers and Unilever Ltd) and in the USA (Philips Laboratories, Irvington-on-Hudson, NY). He was Assistant Professor of Physical Metallurgy at Harvard University (1954-1959) before moving to the University of Bristol. He became Professor of Physics in 1979. Professor Lang achieved fame for his pioneering studies in X-ray diffraction physics, especially his original technique of X-ray topography, i.e., the 'Lang method' or 'Lang Camera', which displays the internal imperfections in a crystal, such as dislocations, stacking faults, growth-sector boundaries and ferromagnetic domains. The method has been widely used in the non-destructive assessment of crystals for the electronics and diamond industries, among others. Professor Lang studied many types of X-ray diffraction phenomena, including variations from Bragg's law, X-ray moire patterns and other types of fringes. One of his most important discoveries (in collaboration with Professor N. Kato (1923-2002)) was the presence of interference fringes in wedge-shaped perfect crystals, leading to a precise measure of absolute structure amplitude from a unit cell (See the paper, "A study of pendellosung fringes in X-ray diffraction", Acta Cryst. 12, 787 (1959)). Professor Lang is also known for his research using other techniques, such as electron microscopy and cathode-luminescence. In 1964, he was awarded the Charles Vernon Boys Prize of the Institute of Physics and the Physical Society. He was elected a Fellow of the Royal Society in 1975 and was awarded the Royal Society Hughes Medal in 1997. An obituary by Professor M. Moore can be found in the Journal of Applied Crystallography, 41, 825 (2008). The Independent (August 25, 2008) carried an obituary as well.

Vadim Ivanovitch Nefedov, a member of the Russian Academy of Science (RAS), has died in Moscow due to cancer at the age of 70. Born in Magnitogorsk in the USSR, Professor Nefedov graduated from the Physicochemical Institute of Leipzig University in 1962. At Leipzig, he was one of the first research students of Armin Meisel at the Laboratory for X-Ray Spectroscopy. In 1965, he completed a post-graduate course at the Kurnakov Institute of General and Inorganic Chemistry, RAS, where he continued to work and later became head of a laboratory. Nefedov's main scientific work concerns physical chemistry by electron and X-ray spectroscopy, in particular, chemical binding and the structures of many types of materials and compounds. He published more than 400 papers and 10 monographs, which are very useful as comprehensive handbooks in this field. Professor Nefedov formulated an original theory of electron density transfer between ligands and predicted a cis-effect in compounds of nontransition metals, which was confirmed later in experiments. He developed a method for determining the effective charge of atoms in compounds and Madelung energy, which offered a new way of calculating the energy of chemical bonds. He provided a theoretical basis and developed an experimental procedure for quantitative X-ray photoelectron analysis of the surface of solids and depth profiling. Nefedov was awarded the 1985 USSR State Prize, 1989 RSFSR State Prize, the international title of X-ray Professor (1998), and the 2000 and 2005 Alexander von Humboldt Foundation Prizes. An obituary by Professors R. Szargan, E. Z. Kurmaev and C. E. Fadley can be found in the Journal of Electron Spectroscopy and Related Phenomena, 168, 47 (2008).

The Helmholtz Association and the Humboldt Foundation have announced the 2008 recipients of the Helmholtz Humboldt Research Award; Professors Roberto Bassi (Universita degli Studi di Verona, Italy) and Shigemasa Suga (Osaka University, Japan). The award amounts to 60,000 Euros, and an additional amount of 25,000 Euros is made available by the Helmholtz Association if the awardee accepts the invitation to undertake research in Germany. In the X-ray field, in addition to this year's award winner Professor Suga, Professors Charles S. Fadley and Ian Robinson were previous recipients of this award. For more information, visit the Web page,

http://www.helmholtz.de/en/research/research_awards/helmholtz_humboldt_research_award/

Zeolites are microporous crystalline materials, and in the unit cell, the tetrahedrally coordinated Si and Al atoms occupy the so-called crystallographic T-sites. In addition to their pore size, Al's occupancy in the specific T-sites is extremely important in catalytic activity. So far, however, the distribution of Al has remained an unresolved problem. Recently, Professor J. A. van Bokhoven (ETH Zurich, Switzerland) and his colleagues employed the X-ray standing wave technique to study Al distribution in scolecite (CaAl₂Si₃O₁₀-3H₂O, hydrated calcium aluminum silicate). They measured the intensity of X-ray fluorescence, Al K, Si K and Ca Kα near the Bragg conditions of (040), (002) and (-402) reflections. The experiments were done at beamline ID32, ESRF. For more information, see the paper, "Determining the aluminium occupancy on the active T-sites in zeolites using X-ray standing waves", J. A. van Bokhoven et al., Nature Materials, 7, 551-555 (2008).

Recently, Professor K.-J. Kim (Argonne National Lab., USA) and his colleagues published a very interesting proposal for the world's brightest X-ray source. In most currently on-going X-ray free electron laser (FEL) projects, self-amplified spontaneous emission (SASE) is employed. It is known that SASE-FEL creates extremely brilliant, coherent X-ray pulses of 0.1 ps duration. Due to the low repetition rate, the average brightness is only about 10,000 times compared with existing 3^rd generation synchrotron sources. On the other hand, future X-ray sciences will require other types of X-ray laser source, with an even smaller number of photons in one pulse (to reduce radiation damage to the sample) and with much greater average intensity via a high repetition rate. In Professor Kim's X-ray source based on a FEL oscillator (X-FELO), a pulse of electrons is carried into an undulator as ordinary FEL, but in order to reflect back the generated X-rays into the undulator entrance, there is an optical cavity consisting of two or more Bragg reflectors with low-Z atoms and with low Debye temperature, such as diamond, beryllium oxide and sapphire crystals. In the next step, the X-ray photons connect with the next electron bunch and again travel back along the undulator. This pattern is repeated indefinitely with the X-ray intensity growing each time until equilibrium is reached. As the spectral bandwidth is extremely narrow, at three to four orders of magnitude finer than those produced by SASE-FEL, the intensity of an individual X-ray pulse from an X-FELO is rather low. But the average X-ray intensity is higher than that of SASE-FEL. Over the past 5 years, highly advanced electron beam technologies, which can be used, for example, for a multi-GeV class energy recovery linac (ERL), have become available. One of the key elements of Professor Kim's idea is combination with ERL. This is predicted to produce X-ray pulses with 10⁹ photons at a repetition rate of 1-100 MHz. The pulses are temporarily and transversely coherent, with a rms bandwidth of about 2 meV, and rms pulse length of about 1 ps. To gain an understanding of the original concept of X-FELO, see the paper, "Proposal for a free electron laser in the X-ray region", R. Colella and A. Luccio, Optical Commun., 50, 41-44 (1984). For more information on the proposed X-ray source, see the paper, "A Proposal for an X-Ray Free-Electron Laser Oscillator with an Energy-Recovery Linac", K.-J. Kim et al., Phys. Rev. Lett., 100, 244802 (2008).

The molecular structure of liquid water has been the subject of intense debate for decades. In 1892, German physicist W. C. Röntgen, who became famous for his discovery of X-rays, published a paper proposing a "mixture model" according to which liquid water consists of two kinds of molecules: a tetrahedral ice-like structure, and another more loosely arranged structure. In 1933, J. D. Bernal and R. H. Fowler successfully analyzed early X-ray diffraction data on water in terms of a disordered quartz-like structure, and concluded that the unique properties of water are due to the tetrahedral geometry. Since then, a number of experimental and theoretical studies have been published. Nevertheless, scientists have not yet captured a clear picture of liquid water. The debate is far from settled. Very recently, an international collaborative team led by Dr. A. Nilsson (Stanford Synchrotron Radiation Laboratory) and Professor S. Shin (RIKEN & The University of Tokyo) succeeded in obtaining X-ray spectroscopic evidence to support Röntgen's mixture model. Thanks to the brilliant synchrotron beamline at the SPring-8, the research group obtained some high resolution oxygen K-edge X-ray emission spectra of liquid water. The team found that there are two distinct narrow lone-pair derived peaks assigned, respectively, to tetrahedral and strongly distorted hydrogen-bonded species. For more information, see the paper, "High resolution X-ray emission spectroscopy of liquid water: The observation of two structural motifs", T. Tokushima et al., Chem. Phys. Lett., 460, 387-400 (2008).

Advanced high-intensity laser systems can be used to drive electrons to velocities close to the speed of light. A fair degree of research is now being devoted to the generation of high-energy beams that are extremely brilliant, ultra-short pulses, and have excellent spatial quality as well. The following recently published review paper is useful for those wishing to ascertain the current status of research. "Principles and applications of compact laser-plasma accelerators", V. Malka et al., Nature Physics 4, 447-453 (2008).