November 2011 Archives

Recently, a very stimulating paper has been published discussing experimentally the fundamental processes of photo-absorption and excitation of electrons by using extremely high-fluence, ultra-short X-ray pulses. The research was done for the electron system in inert Ne gas at LCLS (Linac Coherent Light Source), Stanford, USA, which is the world's first hard X-ray free-electron laser facility. The scheme is as follows: an intense single X-ray pulse of sub-10-fs duration at 848 eV first strips a 2p electron from Ne and, at this stage, since the X-ray energy is below the binding energy of a 1s electron in neutral neon, 870 eV, a 1s hole cannot be produced, but because of the above 2p hole, the next pulse can excite the 1s electron, leading to 1s-2p resonance in the Ne⁺ ion and, finally, stimulated emission (2p-1s) competes with Auger decay to refill the 1s hole. The results have indicated that intense X-ray pulses of sub-10-fs duration can modify and even control the Auger decay process. For more information, see the paper, "Unveiling and Driving Hidden Resonances with High-Fluence, High-Intensity X-Ray Pulses", E. P. Kanter et al., Phys. Rev. Lett. 107, 233001 (2011).

BaTiO₃ is a promising candidate ferroelectric material for magnetoelectric composites and layered film structures. Recently, some interesting soft X-ray absorption spectra at Ti-L_{II, III} , O-K, and Ba-M_{IV, V} edges have been discussed mainly from a theoretical point of view by a German group. For more information, see the paper, "High-resolution x-ray absorption spectroscopy of BaTiO₃: Experiment and first-principles calculations", A. Chassֺé et al., Phys. Rev. B84, 195135 (2011).

Multi-wavelength anomalous diffraction (MAD) has been widely employed to determine phase information in X-ray crystallography. The method uses the contrast of the scattering power of heavy atoms at the absorption edges. However, when the X-ray source becomes extremely brilliant, the sample encounters severe electronic radiation damage, especially to heavy atoms, which makes the interpretation of MAD rather difficult. Recently, a theoretical paper discussing this problem has been published. The theory uses a Karle-Hendrickson-type equation in the high-intensity regime, and demonstrates the calculation of relevant coefficients with detailed electronic damage dynamics of heavy atoms. For more information, see the paper, "Multiwavelength Anomalous Diffraction at High X-Ray Intensity", S-K.Son et al., Phys. Rev. Lett. 107, 218102 (2011).

Scientists at Argonne National Laboratory, USA have recently reported a novel set of optics for X-ray monochromators, which combine the effect of angular dispersion and anomalous transmission of X-rays in Bragg reflection from asymmetrically cut crystals. The optics employ a five-reflection, three-crystal arrangement, and it was found that the spectral contrast, the bandwidth and the angular acceptance are approximately 500, 0.5 meV, and 0.1 mrad, respectively, for 9.1 keV X-rays. The new optics could be a foundation for next-generation inelastic X-ray scattering spectrometers. For more information, see the paper, "Using angular dispersion and anomalous transmission to shape ultramonochromatic x rays", Y. Shvyd'ko et al., Phys. Rev. A84, 053823 (2011).

One of the key aspects of progress in X-ray microscopy is the advent of coherent diffractive imaging, which basically does not use any lenses. Ptychography is one improved version of a series of techniques using coherent X-ray beams and allows reconstructions of both strongly and weakly scattering samples. A further extension of this method has recently been published by a German group led by Professor T. Salditt (Georg-August-University Göttingen). The research introduced chemical contrasts based on near-edge X-ray absorption fine structures. The group demonstrated that two different molecules in a biological system are distinguished visually by using the contrasts near the oxygen K edge. For more information, see the paper, "Chemical Contrast in Soft X-Ray Ptychography", M. Beckers et al., Phys. Rev. Lett. 107, 208101 (2011).

W. Rontgen took the world's first X-ray photo on November 8, 1895, thereby creating the very famous X-ray image of his wife's fingers. Ker Than has written a short article in National Geographic News on this X-ray photo. For more information, see the article, " 115-Year-Old X-Ray", http://news.nationalgeographic.com/news/2010/11/photogalleries/101108-x-rays-google-doodle-115th-anniversary-years-science-pictures/?now=2010-11-08-00:01

A U-shaped design for rotating anode X-ray sources is one method for enabling high brilliance, and was first proposed by Professor N. Sakabe (KEK, Tsukuba, Japan) in 1995. Unlike ordinary rotating anode X-ray sources, the electron beam goes beyond the outside surface of the rotating anode and then reverses its direction so that it can hit the inside surface. In this case, because of the centrifugal force of the rotating anode, the surface can be much smoother than usual even near the melting point, enabling the production of more X-ray photons. A KEK research group has published a report on recent progress with this type of X-ray source. According to their simulation, by optimizing both the bending and the steering magnets, the beam size can be 0.45 mm (horizontal) × 0.05 mm (vertical) for a 120 keV/75 mA beam. The effective brilliance is about 500 kW/mm². For more information, see the paper, "Research and development of an electron beam focusing system for a high-brightness X-ray generator", T. Sakai et al., J. Synchrotron Rad. 18, (2011) (Published online, DOI:10.1107/S0909049510029948).

Professor P. Blaha (Technische Universität Wien, Austria) and his colleagues have recently calculated the X-ray absorption spectra at the L_{II, III} edges of the early 3d elements by solving the Bethe-Salpeter equation (BSE). Under the independent particle approximation (IPA), X-ray absorption spectra are usually considered as proportional to the unoccupied part of the projected density of states weighted by the momentum matrix elements between the core and the conduction states. However, due to the localization of the core wave function, the interaction of the exited electron with its hole is not negligible. In particular, the branching ratio between the L_II and L_III edges cannot be explained. Though the IPA gives 1:2, the observed ratios are much closer to 1:1 or even higher for K, Ca, Sc, Ti, and V. They detailed this problem based on their BSE calculation. For more information, see the paper, "Understanding the L2,3 x-ray absorption spectra of early 3d transition elements", R. Laskowski et al., Phys. Rev. B 82, 205104 (2010).