December 2012 Archives

Recently a research group led by Okayama University in Japan has reported the successful application of resonant X-ray emission spectroscopy (RXES) to BaTiO₃ nanoparticles of various sizes ranging from a bulk-like 200 nm to a paraelectric 50 nm. While it is well known that the crystal structure changes from tetragonal to cubic as the particle size decreases, some recent reports indicated that a very large enhancement of the dielectric constant was observed at a specific particle size of around 70 nm. The research was done to clarify the above problem. In the X-ray emission spectra measured with monochromatic excitation near the sharp peak of the Ti-K absorption edge, two small Raman peaks were observed between Kβ_2,5 (4962.6 eV) and elastic scattering of (for example, 4983.6 eV) peaks. It was found that the higher energy Raman peak (5.3 eV lower than incident X-ray energy) still exists at a size of 85 nm, even though the intensity basically diminishes for the small particle size BaTiO₃, which corresponds to the extraordinary large crystal structure change. The results suggest that Raman peak intensity is correlated to the large enhancement of the dielectric constant. For more information, see the paper, "Enhancement of dielectric constant of BaTiO₃ nanoparticles studied by resonant x-ray emission spectroscopy", N. Nakajima et al., Phy. Rev. B86, 224114 (2012).

In Japan, a research team led by Professor K. Yamauchi (Osaka University) and Professor T. Ishikawa (Riken, Harima, Japan) has recently succeeded in focusing ultra short X-ray laser pulses from the SPring-8 Angstrom Compact free electron LAser (SACLA). With reflective optics comprising elliptically figured mirrors with nm accuracy to preserve a coherent wavefront, they have obtained a focused small beam of 0.95 × 1.20 μm² at 10 keV. The estimated achievable power density at the sample position is 6 × 10¹⁷ W/cm². For more information, see the paper, "Focusing of X-ray free-electron laser pulses with reflective optics", H. Yumoto et al., Nature Photonics, 7, 43 (2013).

At Linac Coherent Light Source (LCLS), Stanford, USA, a series of experimental works has been carried out based on the core-level excitation and relaxation process. One recently published paper from Stanford reports the resonant generation of Kα emission from aluminum foil (1μm thick) in a solid-plasma state created by irradiating very strong X-ray free-electron laser pulses (less than 80 fs time width, 1.6×10¹² photons/pulse). In the experiment, quasimonochromatic (0.5% bandwidth) X-ray pulses in the energy range of 1480-1580 eV (below and slightly above the K edge of ground state Al) were focused onto a 3μm diameter spot on the sample, with a corresponding peak intensity in excess of 10¹⁷ W/cm². To analyze the X-ray spectra, the research group employed a wavelength-dispersive X-ray spectrometer with a flat ADP (101) crystal and an X-ray CCD camera. Since the same atom can absorb multiple photons contained in the single pulse time width, with L-shell holes being created and leading to the excitation of a K-shell electron into one of these L-holes, the Kα X-rays are produced. The research group studied many such emission spectra produced by tuning the XFEL energy to the K-L transitions of those highly charged ions that have transition energies below the K edge of the cold material. It was also found that resonance emission peaks broaden significantly, and this was explained as opacity effects. Because of the intensity-dependent optical depth, the transparent sample at low intensity thickens optically with an intense XFEL pulse. For more information, see the paper, "Resonant Kα Spectroscopy of Solid-Density Aluminum Plasmas", B. I. Cho et al., Phys. Rev. Lett., 109, 245003 (2012).

A Chinese group recently published a paper proposing a new interpretation of neon's absorption of extremely strong X-ray photons from an X-ray free electron laser, which was experimentally studied at Stanford in 2010 (see, L. Young et al., Nature, 466, 56 (2010)). Although the ordinary absorption edge of neon is around 867 eV, the energy becomes higher than usual because of multiple ionization. Therefore, detailed studies were done between 800 eV and 2000 eV at Stanford at that time. The main discussion here is the large discrepancies between theory and experiment found at 1050 eV, where the rates of K-shell absorption 1s → 4p of Ne⁶⁺ and 1s → 3p of Ne⁷⁺ are larger than the direct single-photon ionization rates by more than one order of magnitude. The authors of this paper propose that the inner-shell resonant absorption (IRA) effects be considered as the mechanism. They showed that resonant photopumping of K-shell electrons to the L, M, or even higher bound orbitals can provide an interaction strength that is two or three orders of magnitude larger than that in the continuum level. Only when the IRA effects were taken into account were the observed charge state distributions explained well. For more information, see the paper, "Inner-shell resonant absorption effects on evolution dynamics of the charge state distribution in a neon atom interacting with ultraintense x-ray pulses", W. Xiang et al., Phys. Rev. A86, 061401(R) (2012).

The recent advent of the X-ray free-electron laser (XFEL) based on self-amplified spontaneous emission (SASE) has brought new opportunities in X-ray physics and many scientific applications. On the other hand, the shot-noise start-up in the SASE mechanism lends an inherent stochastic character to X-ray pulses, leading to rather large variations both in wavelength and intensity. One strategy to solve the problem is to use an XFEL pulse to create a population inversion in a medium which then lases in the X-ray region (See, N. Rohringer et al., Nature, 481, 488 (2012)). Alternatively, resonant core excitation can be used as well. Recently, a theoretical chemistry group led by Professor F. Gel'mukhanov (Royal Institute of Technology, Sweden) has published a prediction of X-ray lasing based on resonant core excitation of a molecule to a state which is subject to ultrafast dissociation, i.e., a state in which dissociation precedes the femtosecond core hole decay. As an example, Cl 2p_1/2 → 6σ excitation of the HCl molecule by an XFEL pulse and the subsequent ultrafast dissociation were studied. For more information, see the paper, "Dissociative X-ray Lasing", Q. Miao et al., Phys. Rev. Lett., 109, 233905 (2012).

Dr. Okamoto (Canon, Japan) and his colleagues have reported X-ray waveguiding based on electromagnetism in photonic crystals, using a waveguide consisting of a pair of claddings sandwiching a core with a periodic structure. For more information, see the paper, "X-ray Waveguide Mode in Resonance with a Periodic Structure", K. Okamoto et al., 109, 233907 (2012).

Coherent X-ray diffraction imaging is a promising new technique to observe samples in material science and biology with a spatial resolution of around 10 nm. However, the range of applications is still not very wide, because the method requires that the X-ray source be highly coherent both laterally and longitudinally. Thus, one of the most important questions for users is the feasibility of the technique when only a partially coherent source is available. A research group led by Professor K. Nugent (University of Melbourne, Australia) has recently reported some quite good news on this issue. So far, it has been often said that the lateral coherence length should be at least twice the greatest spatial extent of the object. The longitudinal coherence length is determined by the bandwidth of the monochromatic X-ray beam. According to the present study, one could relax the minimal criteria by a factor of 2 for both lateral coherence length and longitudinal coherence length, if the coherence properties are known either a priori or through experiment. In other words, more flux could be made available at the sample position for the coherent X-ray diffraction imaging experiments with the use of a partially coherent X-ray source. For more information, see the paper, "Diffraction imaging: The limits of partial coherence", B. Chen et al., Phys. Rev. B86, 235401 (2012).