February 2013 Archives

Dr. P. Korecki (Jagiellonian University, Poland) and his colleagues have recently published a fairly impressive, successful 3D analysis of Cu₃Au (001) single crystal by white-beam X-ray fluorescence holograms measured using a 50W tungsten X-ray tube (50 kV, 1 mA, with 0.8mm Al filter). Primary X-ray photons at the aperture, which is placed at 340 mm from the source, are around 2×10⁸ counts/sec. The sample was positioned 610 mm from the sample, and was rotated relative to the incident beam around two axes (θ, φ). The X-ray fluorescence intensity of Cu K and Au L lines was measured by a Si drift detector (SDD) with a 25 mm² effective area, placed at a distance of 12 mm from the sample. The typical counting rate was around 10⁵ counts/sec, and the total acquisition time was ~90 h, i.e., 4 days. It was demonstrated that a 3D image of the sample was reconstructed from the recorded holograms. Readers might be surprised to know that such a non-efficient experiment can be done even with a low power source. As the authors claim at the end of this paper, the measuring time can be reasonably shortened by the use of more powerful laboratory X-ray sources. For more information, see the paper, "Element sensitive holographic imaging of atomic structures using white x rays", K. M. Da.browski et al., Phys. Rev. B87, 064111 (2013).

A team led by Professor C. T. Chantler (University of Melbourne, Australia) has published vanadium Kβ spectra from metallic foil, measured with medium energy resolution but with high accuracy. For more information, see the paper, "Characterization of the Kβ spectral profile for vanadium", L. F. Smale et al., Phys. Rev. A87, 022512 (2013).

The extremely high peak power of an X-ray free electron laser pulse can be an attractive tool for clarifying the core-level excitation and relaxation process. Recently, Dr. B. Rudek and his colleagues have reported their time-of-flight ion spectroscopy studies on sequential inner-shell multiple ionization of krypton at photon energies at 2 keV and 1.5 keV, which are higher than the LI (~1.92 keV) and lower than the LIII (~1.67 keV) edges for ordinary neutral krypton, respectively. The experiments were done with two X-ray pulse widths (5 and 80 fs) and various pulse energies (from 0.07 to 2.6 mJ), at the Linac Coherent Light Source (LCLS), Stanford, USA. The highest charge state observed at 1.5 keV photon energy (below the LI edge) is Kr¹⁷⁺; at 2 keV photon energy (above the LIII edge), it is Kr²¹⁺. It was found that theoretical calculations based on a rate-equation model can explain the obtained experimental data for 1.5 keV, but fails to do so at 2 keV, where the experimental spectrum shows higher charge states. They discussed that this enhancement is due to a resonance-enhanced X-ray multiple ionization mechanism, i.e., resonant excitations followed by autoionization at charge states higher than Kr¹²⁺, where direct L-shell photoionization at 2 keV is energetically closed. For more information, see the paper, "Resonance-enhanced multiple ionization of krypton at an x-ray free-electron laser", B. I. Cho et al., Phys. Rev. A87, 023413 (2013).

Dr. B. Kanngießer (Technische Universität Berlin, Germany) and her colleagues have recently reported further advances in 3D chemical mapping using a confocal X-ray fluorescence setup. The research group has obtained nondestructive reconstruction of stratified systems with constant elemental composition but with varying chemical compounds. For more information, see the paper, "Three-Dimensional Chemical Mapping with a Confocal XRF Setup", L. Luhl et al., Anal. Chem., Article ASAP (DOI: 10.1021/ac303749b).

In spite of the recent advent of few fs pulse X-ray free-electron laser sources, so far, synchronization between optical lasers and X-ray pulses has been challenging, and the jitter, typically, 100~200 fs r.m.s., has limited the time-resolution of the measurement. At the Linac Coherent Light Source (LCLS), Stanford, scientists have recently solved this problem by introducing a "measure-and-sort" approach, which records all single-shot data with time information to ensure resorting of the data. In the beamline, the same optical laser beam is split into three beams: with the first, the relative delay between laser and X-ray is encoded into wavelength by using a broadband chirped supercontinuum; in the second, the temporal delay is spatially encoded; in the third, pump-probe experiments are performed with time-sorting tools. It was concluded that the error in the delay time between optical and X-ray pulses can be substantially improved to 6 fs r.m.s., leading to time-resolved measurement with only a few fs resolution. For more information, see the paper, "Achieving few-femtosecond time-sorting at hard X-ray free-electron lasers", M. Harmand et al., Nature Photonics, doi:10.1038/nphoton.2013.11; published online, February 17, 2013.

One promising application of laser-matter interactions is generating hot suprathermal electrons with keV-MeV energy, which enables excitation of the K shell of the target material. Recently, Dr. G. Cristoforetti (Intense Laser Irradiation Laboratory, Italy) and his colleagues have reported some interesting experiments on the laser pulse polarization effect on the Kα yield and line shape. The research group studied the interaction of an ultrashort laser pulse (λ = 800 nm, τ = 40 fs) with a Ti foil under intense irradiation. The K X-ray emission was analyzed by a quartz crystal and a CCD camera, and it was found that the energy of Kα lines shift a few eV up to around 15 eV, depending on the pulse polarization. Such dependence can be discussed by considering the efficiency of hot electron generation. For more information, see the paper, "Spatially resolved analysis of Kα x-ray emission from plasmas induced by a femtosecond weakly relativistic laser pulse at various polarizations", G. Cristoforetti et al., Phy. Rev. E87, 023103 (2013).

Coherent X-ray diffractive imaging has made remarkable progress over the past 15 years. The technique basically reconstructs real space microscopic images with the spatial resolution of nm without the use of lenses, mainly because of the ability to retrieve phases. However, it relies on the degree of high coherence of the available X-ray photon beam, and, until now, almost all experimental studies have been subject to some limits. It is not very easy to satisfy the ideal conditions, mainly because of the partial coherence of the beam itself and some decoherence caused by imperfect detection as well as the dynamic motions of the sample. Dr. P. Thibaut (Technische Universität München, Germany) and his colleague have recently reported their analytical studies into extending ptychography by formulating it as low-rank mixed states. The procedure is closely related to quantum state tomography and is equally applicable to high-resolution microscopy, wave sensing and fluctuation measurements. They concluded that some of the most stringent experimental conditions in ptychography can be relaxed, and susceptibility to imaging artifacts is reduced even when the coherence conditions are not ideal. For more information, see the paper, "Reconstructing state mixtures from diffraction measurements", P. Thibault et al., Nature, 494, 68 (2013).