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Soft x-ray spectroscopy
Australian Synchrotron beamlines

Soft x-ray spectroscopy

Soft x-rays are generally understood to be x-rays in the energy range 100-3,000 eV. They have insufficient energy to penetrate the beryllium window of a hard x-ray beamline but have energies higher than that of extreme ultraviolet light.

Introduction

Soft x-rays are generally understood to be x-rays in the energy range 100-3,000 eV. They have insufficient energy to penetrate the beryllium window of a hard x-ray beamline but have energies higher than that of extreme ultraviolet light. Soft x-rays cover an energy range of importance for spectroscopic studies of many elements in the periodic table. They are well suited to characterising surfaces and near-surface interfacial layers.

The soft x-ray beamline will be set up primarily for XAS of low atomic number elements and x-ray photoelectron spectroscopy (XPS), although there are other significant research applications such as photo-desorption and threshold x-ray excited Auger electron spectroscopy (XAES).

There will be two end stations to enable experimental flexibility, with the second end station capable of XPS and XAS mapping of samples in special atmospheres at pressures of up to 20 torr.

The photon range of the beamline encompasses the K edges of C, O and N at high resolution >10,000.  The collection of core level NEXAFS and high resolution photoelectron spectroscopy are key elements in the study of (bio)organic systems.

The beamline will also produce circularly polarised light between 100 to 1000 eV.  This is of particular interest for the L edges of the 1st row transition metals, Cr…Cu.  Magnetic X-ray Circular Dichroism (MXCD) can be used to separate the magnetic orbital and spin components of magnetic systems.

Synchrotron soft x-ray spectroscopy provides distinctive information for numerous research areas ranging from fundamental studies in solid state physics and nanotechnology to applied chemical problems in catalysis and coal combustion.

Techniques

Main Experimental Techniques

X-ray Photoelectron Spectroscopy (XPS)
Near Edge X-ray Absorption Spectroscopy (NEXAFS) in the following modes:

  • Total Electron Yield (TEY)
  • Partial Electron Yield (PEY)
  • Total Fluoresence Yield (TFY)
  • Auger Electron Yield (AEY)

Earth and environmental sciences

Synchrotron soft x-ray techniques are already opening up new ways to address the complex problems arising from earth resource utilisation, and this contribution is expected to increase in areas such as environmentally sustainable ore extraction, mineral processing, coal combustion and soil use.

The surface chemistry of metal sulfides is of major importance in the separation of the valuable and unwanted components in base metal ores, in the hydrometallurgical processing of a concentrate to produce the corresponding metal from the sulfide, and in the leaching of rejected material in waste heaps. Since the application of synchrotron XPS to mineral fracture surfaces, the importance of surface chemical states arising from relaxation of the outermost layer following fracture has become evident.

The enhanced surface sensitivity provided by synchrotron XPS, as well as the ability of angle-dependent x-ray absorption near edge structure (XANES) to reveal orientation, have also assisted elucidation of the mechanism by which flotation reagents interact with the surface of minerals.

Determination of the chemical forms of heteroatoms, such as nitrogen, in a large molecular weight or complex material such as coal is a case where soft x-ray absorption spectroscopy is used to complement conventional XPS for low atomic number non-surface chemical characterisation. This information is sought in research to minimise the generation of undesirable species such as NOx in coal combustion.

The ambient mapping facility on the second end station will enable measurements to be made at pressures of up to 20 torr with high spatial resolution. It will provide maps of surfaces when exposed to a variety of atmospheres including water, oxygen, helium and nitrogen. Thus, for instance, it will be possible to follow mineral oxidation, hydrolysis or collector adsorption in real time.

Physical and material sciences

A niche area in which Australia has been successful to date is the development of thin film materials for electronic and optoelectronic devices. Thin films of materials with particular chemical and/or physical properties such as piezoelectricity are typically deposited onto an appropriate substrate by one form of chemical vapour deposition, and during the development phase for both precursor and deposition conditions, the physical and chemical properties of the film must be determined. Variable-angle XAS from a synchrotron source can augment initial conventional XPS analysis to reveal the orientation of film crystallites.

Chemical and biochemical sciences

In many chemical, biochemical and earth science-related systems it is essential to obtain information about a species while it is in contact with an aqueous or other liquid environment and while it is at a particular electrochemical potential. A wet cell for non-microscopic XAS will be of considerable interest to researchers in a number of fields. In particular, with silicon nitride cell windows, the carbon K-edge near 0.3 keV can be studied, and the region between the carbon K-edges and the oxygen K-edge near 530 eV is often referred to as the 'water window'. It is expected that it would be possible to investigate solid electrode surfaces and even particulate slurries, with the electrochemical potential established by redox reagents in the liquid flowing through the cell.

For materials that can be investigated under ultra high vacuum, XAS is able to provide chemical information that is difficult to obtain by non-synchrotron techniques such as conventional XPS.

For biological samples that need to be kept in damp conditions, or where it is desired to study the action of catalysts in special environments, the ambient mapping facility will be used.

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