August 2005 Archives

Some very interesting structural studies have been performed recently at the European Synchrotron Radiation Facility (ESRF), Grenoble, France, on photo-chemically generated, short-lived (<10^-6 sec) iodo radicals. The research team dissolved a molecule of C₂H₄I₂ in liquid methanol and then subjected it to a short laser pulse. This excited the molecule, which then cooled down while releasing heat into the surrounding liquid. As a consequence, the temperature rose and the liquid started to expand in response to the increase in temperature. The absorption of light triggered a chemical reaction, which the researchers studied with picosecond time resolution. The research team measured the change in shape and composition as early as 100 picoseconds after the initial explosion, then at an interval of 10 nanoseconds, then 1 microsecond and so on. From these measurements, the team obtained direct structural evidence of the bridged radical (CH₂ICH₂) in a polar solution. This transient intermediate has long been hypothesized to explain stereo-chemical control in many association and/or dissociation reactions involving haloalkanes. For more information, see the paper, "Ultrafast X-ray Diffraction of Transient Molecular Structures in Solution", H. Ihee et al., Science, 309, 1223-1227, (2005).

Professor E. Weber's team at Berkeley, California, US has recently succeeded in finding a new technique to handle metal defects in low-grade silicon, which could dramatically reduce the cost of solar cells. At present, around 90 % of solar cells in the world are made from a refined, highly purified form of silicon. This is because solar cells made from cheaper forms of silicon do not perform well and also because removing impurities is expensive. The new idea is to manipulate the impurities in a way that reduces their detrimental impact on the solar cell, instead of purifying the material. The team analyzed how metal contaminants in silicon respond to different types of processing using a synchrotron X-ray microprobe capable of detecting metal clusters as small as 30 nanometers. In addition to micro-XRF and micro-XAFS, they employed a new method based on a spectrally resolved X-ray-beam-induced current, which generates a map of the minority-carrier diffusion length, revealing the precise impacts of metal impurity clusters on local material performance. They found that they were able to manipulate the distribution of the metal impurities by varying the cooling rate of the silicon. When the material is cooled quickly, the metal defects are quickly locked in a scattered distribution. For more information, see the paper, "Engineering metal-impurity nanodefects for low-cost solar cells", T. Buonassisi et al., Nature Materials, 4, 676-679 (2005).

It is known that the colours of many flowers are produced by anthocyanin, which has 6 different types of structure; a cyanidin-type anthocyanin is responsible for the red in roses, while most blue flowers have delphinidin-type anthocyanin. However, the same cyanidin-type anthocyanin makes roses red but cornflowers blue. The phenomenon has so far not been entirely explained. A Japanese group led by Professor K. Takeda (Tokyo Gakugei University, Koganei, Tokyo) recently carried out detailed X-ray analysis and clarified that a complex of six molecules each of anthocyanin and flavone, with one ferric iron, one magnesium and two calcium ions is responsible for the blue in cornflowers. For more information, see the paper, "Phytochemistry: Structure of the blue cornflower pigment", M. Shiono et al., Nature, 436, 791 (2005).

The mineral silica (SiO₂) is a common substance that is a constituent of all of the planets in our solar system. At SPring-8, Harima, Japan, Dr. K. Hirose (Tokyo Institute of Technology; Japan Agency for Marine-Earth Science and Technology) and his co-workers recently found that, above 268 GPa and 1800 K, silica exhibits a novel stable high-pressure form with a pyrite-type structure, which is much denser than other known silica phases. This form of silica could be one of the main constituents of the core of a gas-giant planet such as Uranus or Neptune. For more information, see the paper, "The Pyrite-Type High-Pressure Form of Silica", Y. Kuwayama et al., Science, 309, 923-925 (2005).

The following awards were presented during the plenary session of the 54th Annual Denver X-Ray Conference:

The 2005 Barrett Award in X-ray Diffraction to D. Keith Bowen - Bede Scientific Instruments, Ltd., Durham, UK and Brian Tanner - University of Durham, Durham, UK
The 2005 Jenkins Award for Lifetime Achievement in the Advancement of the Use of X-rays for Materials Analysis to Victor E. Buhrke - Consultant, Portola Valley, CA

The first user operation of the VUV free-electron laser (FEL) at DESY, Hamburg in Germany is now under way. German Chancellor Gerhard Schroeder paid a visit to the facility to join the celebrations. The VUV-FEL employs the new technology developed at DESY from 1992 to 2004 by the international team as part of the TESLA Collaboration. Electrons are brought to high energies by a superconducting linear accelerator, and then race through an undulator, which is a periodic arrangement of magnets that forces the electrons to follow a slalom course and thereby radiate flashes of light. According to self-amplified spontaneous emission (SASE), the process finally generates intense flashes of short-wavelength laser light. Its peak brilliance surpasses that of the most modern synchrotron radiation sources by a factor of ten million. Its radiation is coherent, and its wavelength is tunable within a range of 6 to 30 nm. The very intense radiation pulses have an extremely short duration of 10~50 femto seconds. Five experimental stations have been constructed at the facility. For more information, visit the Web page, http://www.desy.de