Grant van Riessen

Hole-lifetime width: a comparison between theory and experiment

An extensive comparison of theoretical hole-lifetime width with experiment available for the L, M, and N hole levels of atomic elements 14≤Z≤92 is made. The independent-particle approach cannot provide reliable estimates of the hole-lifetime widths, except for the L23, M45 and N67 levels. The many-body theory can provide much more reliable estimates. The cause of discrepancies between theory and experiment is discussed in detail, in light of many-body effects of the hole decay. The present analysis of the hole-lifetime widths of several atomic levels of a large number of atomic elements elucidates not only the opening or closing of certain decay channels, but also the effect on emitted Auger electrons by intrinsic physical mechanisms, such as localization or delocalization of two-holes in a final state of the hole decay.

Effects of Plasma Treatment of Wool on the Uptake of Sulfonated Dyes with Different Hydrophobic Properties

A wool fabric has been subjected to an atmospheric-pressure treatment with a helium plasma for 30 seconds. X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry confirmed removal of the covalently-bound fatty acid layer (F-layer) from the surface of the wool fibers, resulting in exposure of the underlying, hydrophilic protein material. Dye uptake experiments were carried out at 50°C to evaluate the effects of plasma on the rate of dye uptake by the fiber surface, as well as give an indication of the adsorption characteristics in the early stages of a typical dyeing cycle. The dyes used were typical, sulfonated wool dyes with a range of hydrophobic characteristics, as determined by their partitioning behavior between water and n-butanol. No significant effects of plasma on the rate of dye adsorption were observed with relatively hydrophobic dyes. In contrast, the relatively hydrophilic dyes were adsorbed more rapidly (and uniformly) by the plasma-treated fabric. It was concluded that adsorption of hydrophobic dyes on plasma-treated wool was influenced by hydrophobic interactions, whereas electrostatic effects predominated for dyes of more hydrophilic character. On heating the dyebath to 90°C in order to achieve fiber penetration, no significant effect of the plasma treatment on the extent of uptake or levelness of a relatively hydrophilic dye was observed as equilibrium conditions were approached.

Whole-cell phase contrast imaging at the nanoscale using Fresnel Coherent Diffractive Imaging Tomography

X-ray tomography can provide structural information of whole cells in close to their native state. Radiation-induced damage, however, imposes a practical limit to image resolution, and as such, a choice between damage, image contrast, and image resolution must be made. New coherent diffractive imaging techniques, such Fresnel Coherent Diffractive Imaging (FCDI), allows quantitative phase information with exceptional dose efficiency, high contrast, and nano-scale resolution. Here we present three-dimensional quantitative images of a whole eukaryotic cell by FCDI at a spatial resolution below 70 nm with sufficient phase contrast to distinguish major cellular components. From our data, we estimate that the minimum dose required for a similar resolution is close to that predicted by the Rose criterion, considerably below accepted estimates of the maximum dose a frozen-hydrated cell can tolerate. Based on the dose efficiency, contrast, and resolution achieved, we expect this technique will find immediate applications in tomographic cellular characterisation.

X-ray photoelectron emission microscopy and time-of-flight secondary ion mass spectrometry analysis of ultrathin fluoropolymer coatings for stent applications.

Fluoropolymer plasma coatings have been investigated for application as stent coatings due to their chemical stability, conformability, and hydrophobic properties. The challenge resides in the capacity for these coatings to remain adherent, stable, and cohesive after the in vivo stent expansion, which can generate local plastic deformation of up to 25%. Plasma-coated samples have been prepared by a multistep process on 316L stainless steel substrates, and some coated samples were plastically deformed to mimic a stent expansion. Analyses were then performed by X-ray photoelectron spectroscopy (XPS), X-ray photoelectron emission microscopy (X-PEEM), and time-of-flight secondary ion mass spectrometry (TOF-SIMS) to determine the chemical and physical effects of such a deformation on both the coating and the interfacial region. While XPS analyses always showed a continuous coating with no significant effect of the deformation, TOF-SIMS and near-edge X-ray absorption fine structure (derived from X-PEEM) data indicated the presence of a certain density of porosity and pinholes in all coatings as well as sparse fissures and molecular fragmentation in the deformed ones. The smallness of the area fraction affected by the defects and the subtlety of the chemical changes could only be evidenced through the higher chemical sensitivity of these latter techniques.

Characterization of the Magnesium Alloy AZ31 Surface in the Ionic Liquid Trihexyl(tetradecyl)phosphonium Bis(trifluoromethanesulfonyl)amide

Commercially available magnesium alloy AZ31 is extensively used in structural engineering components although, like many magnesium-based materials, it suffers from poor corrosion resistance, particularly in marine environments, which limit wider application. Previously, the ionic liquid (IL) trihexyl(tetradecyl)phosphonium bis(trifluoromethanesulfonyl)amide ([P 66614 ][NTf 2 ]) was shown to improve the corrosion resistance of magnesium alloy AZ31 in humid environments and in the presence of chloride-containing aqueous environments. Here, we investigate the morphology and composition of the protective surface film that forms upon immersion of the Mg alloy in the IL, using grazing angle X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), time of flight-secondary-ion mass spectrometry (TOF-SIMS), solid-state NMR, and transmission electron microscopy (TEM). XRD indicates that an amorphous film is present on the surface subsequent to exposure to the [P 66614 ][NTf 2 ] IL, whereas XPS etching experiments indicate that the film is multilayered. The innermost layer is predominantly inorganic fluoride salts as well as native oxide/hydroxide surface species. TOF-SIMS spectra support these observations and indicate an outermost, thin, adherent layer of IL species. Multinuclear NMR spectroscopy confirms the presence of a multiphase composition as well as the presence of metal fluorides and complex organic species. The surface film appears to be of the order of 100 nm according to the TEM/energy-dispersive X-ray spectroscopy observations.

Rapid, low dose X-ray diffractive imaging of the malaria parasite Plasmodium falciparum.

Phase-diverse X-ray coherent diffractive imaging (CDI) provides a route to high sensitivity and spatial resolution with moderate radiation dose. It also provides a robust solution to the well-known phase-problem, making on-line image reconstruction feasible. Here we apply phase-diverse CDI to a cellular sample, obtaining images of an erythrocyte infected by the sexual stage of the malaria parasite, Plasmodium falciparum, with a radiation dose significantly lower than the lowest dose previously reported for cellular imaging using CDI. The high sensitivity and resolution allow key biological features to be identified within intact cells, providing complementary information to optical and electron microscopy. This high throughput method could be used for fast tomographic imaging, or to generate multiple replicates in two-dimensions of hydrated biological systems without freezing or fixing. This work demonstrates that phase-diverse CDI is a valuable complementary imaging method for the biological sciences and ready for immediate application.

Direct and core-resonant double photoemission from Cu(001)

We have measured the correlated electron pair emission from a Cu(001) surface by both direct and core-resonant channels upon excitation with linearly polarized photons of energy far above the 3p threshold. As expected for a single-step process mediated by electron correlation in the initial and final states, the two electrons emitted by the direct channel continuously share the sum of the energy available to them. The core-resonant channel is often considered in terms of successive and independent steps of photoexcitation and Auger decay. However, electron pairs emitted by the core-resonant channel also share their energy continuously to jointly conserve the energy of the complete process. By detecting the electron pairs in parallel over a wide range of energy, evidence of the core-resonant double photoemission proceeding by a coherent single-step process is most strikingly manifested by a continuum of correlated electron pairs with a sum energy characteristic of the process but for which the individual electrons have arbitrary energies and cannot meaningfully be distinguished as a photoelectron or Auger electron.

A soft X-ray beamline for quantitative nanotomography using ptychography

Soft X-ray nanotomography using ptychography allows quantitative imaging of the internal structure of biological and materials samples with high sensitivity. In this work, we describe progress toward the implementation of an interferometer-controlled microscope located at a beamline that provides coherent ux over the photon energy range of 200 to 2000 eV. Recent experimental results are presented to illustrate the potential for two- and three-dimensional imaging at the nanoscale.

The Ag M5N45N45 Auger photoelectron coincidence spectra of disordered Ag0.5Pd0.5 alloy

An effect of disorder broadening (DB) on the Ag M5N45N45 Auger spectra in the random substituted Ag0.5Pd0.5 has been investigated by Auger photoelectron coincidence spectroscopy (APECS). Data were collected for the Ag M5N45N45 Auger line coincident with the Ag 3d5/2 photoelectron line (and its higher and lower binding energy sides). It is shown that the broadening of the Ag M5N45N45 line is directly associated with the presence of disorder broadening of the Ag 3d5/2 photoelectron line. The APECS experiment is used to demonstrate the broadening in a novel way.

Simultaneous X-ray fluorescence and scanning X-ray diffraction microscopy at the Australian Synchrotron XFM beamline.

Owing to its extreme sensitivity, quantitative mapping of elemental distributions via X-ray fluorescence microscopy (XFM) has become a key microanalytical technique. The recent realisation of scanning X-ray diffraction microscopy (SXDM) meanwhile provides an avenue for quantitative super-resolved ultra-structural visualization. The similarity of their experimental geometries indicates excellent prospects for simultaneous acquisition. Here, in both step- and fly-scanning modes, robust, simultaneous XFM-SXDM is demonstrated.