Spectroscopy

X-ray absorption spectroscopy

When x-rays pass through any sort of material a proportion of them will be absorbed. measuring the amount of absorption with increasing x-ray energy reveals so-called edge structures where the level of absorption suddenly increases. This happens when an x-ray has sufficient energy to free or excite a bound electron within the material to a higher energy level.

The physical processes governing x-ray absorption involves the absorption of a photon of energy hv (h is the Planck constant and v is the frequency) resulting in the ejection of a photoelectron. The photoelectron is regarded as an outgoing spherical wave that may be scattered by the neighbouring atoms. At the absorbing atom, out-going and scattered waves interfere, modulating the absorption coefficient. Accordingly, an absorption spectrum exhibits oscillations of fine structure extending beyond the absorption edge. These oscillations gradually gradually die away as the x-ray energy increases. The oscillations, with occur relatively close to the edge (within about 40 eV) are known as NEXAFS (Near-Edge X-ray Absorption Fine Structure) or XANES (X-ray Absorption Near Edge Structure).

X-ray Absorption Spectroscopy (XAS) is a well-established quantitative analytical technique used by both academia and industry to garner atomic scale structural information for a wide range of systems in both liquid and solid form. XAS probes both the short and medium range order of a sample and as such is complementary to x-ray diffraction.

Analysis of the extended x-ray absorption fine structure (EXAFS) yields structural information such as bond lengths and coordination numbers. In closer proximity to the absorption edge, analysis of the multiple-scattering processes that dominate the x-ray absorption near-edge structure (XANES) yields chemical information such as the local coordination geometry and oxidation state of the absorbing atom.

Though the fine structure of an absorption spectrum was first observed experimentally approximately 100 years ago, it was not until the 1970s and the availability of the intense, tunable source of photons from synchrotron light that the underlying physics was correctly established and XAS changed forever from a qualitative observation to a quantitative analytical technique. Despite a 30 year history and current widespread usage, the XAS technique continues to evolve both experimentally in the development of time-resolved measurements, and theoretically, where analysis has been greatly improved by ab initio calculations.

Soft x-ray absorption

X-ray absorption occurs for both hard and soft x-rays. Soft x-rays are generally understood to be x-rays with insufficient energy to penetrate the beryllium window of a hard x-ray beamline but with energies higher than that of extreme ultraviolet light. They cover an energy range of importance for spectroscopic studies of the low atomic number elements. It is not possible to obtain x-ray absorption spectra of elements up to potassium on a typical hard x-ray beamline because they have no electrons with a binding energy above 4 keV. Clearly there are many elements of major importance in the twenty of lowest atomic number, and for this reason alone it is important to have access to both hard and soft x-rays. Moreover, even for the elements of atomic number greater than 20, additional chemical information can be obtained from different absorption edge fine structure because of the different unfilled orbitals involved in the absorption process. For example, in the case of chromium, K-edge (~6 kev) XAS is carried out on a hard x-ray beamline, but L-edge (~0.6 keV) XAS, which provides valuable additional information, can only be carried out on a soft x-ray beamline.

Soft x-rays have limited ability to penetrate some materials. They are well-suited to characterizing surfaces, near surface interfacial layers and thin films, rather than bulk properties. In these cases all elements of atomic number greater than 3 are potentially of interest, and absorption is usually measured indirectly by determining the total electron yield, partial electron yield, Auger electron yield, or fluorescence yield from the surface. Each of these yields is associated with a different analysis depth, ranging from the outermost few nanometres to more than 100 nanometres. It is possible to measure these yields simultaneously, and thereby obtain information towards the assembly of a non-destructive chemical depth profile.

XAS for surface and thin film studies may be carried out at a fixed angle of incidence of the x-ray beam on the specimen, or the angle of incidence may be varied from perpendicular to glancing. Angle-dependent XAS is a very important and widely used technique for the study of orientation effects in thin films, and of the alignment of adsorbed monolayers or multilayers.

X-ray photoelectron spectroscopy

Conventional x-ray photoelectron spectroscopy (XPS) is the most important and versatile technique for the chemical characterization of the surface of a material. In conventional XPS, soft x-rays of fixed energy are obtained from an aluminium anode, and because this photon energy is near 1.5 keV depending on the binding energies of the core electrons of interest, the photoelectron kinetic energies are such that the typical analysis depth is 2-5 nm. Clearly, only a small proportion of this analysis depth can be considered to be the true surface layer.

In synchrotron XPS, the photon energy can be tuned to vary the kinetic energy of the ejected photoelectrons, thereby varying the analysis depth. In particular, a photon energy can be selected to result in the photoelectrons of interest having a kinetic energy near 45 eV, the energy for which the inelastic mean free path is a minimum. In this way, the surface sensitivity of XPS can be maximized and an analysis depth of the order of two atomic layers can be achieved. By increasing the photon energy, the analysis depth is increased and information towards a non-destructive chemical depth profile can be obtained.

In synchrotron XPS, the photon energy can also be tuned to alter the photo-ionization cross-section for the electrons pf particular interest. In practice, it is often desirable to optimize the cross-section to enhance the sensitivity for a particular element, or to change the relative cross-section for a sub-shell in two elements, but of course in both cases a concomitant change in the depth analyzed would occur. This in synchrotron XPS, it is the tunability of the photon energy that is of the greatest importance, but of almost equal importance is the high photon flux, because this allows high-energy resolution to be selected in the trade-off between monochromator resolution and transmission trade-off. The high flux also allows photo-electron diffraction measurements to be made.

Fluorescence spectroscopy

In addition to absorption and scattering, some materials fluoresce when excited by UV light. In this case outer shell electrons in the atoms are raised to a state of higher energy by the light, and when they return to a state of lower energy the emit light of a specific colour or wavelength, or heat. Identification of the colour of the light will identify the particular atomic or molecular species, and its concentration can be measured from the intensity of the fluorescence. Because some energy is also lost as heat, the emitted light contains less energy and therefore is of a longer wavelength than the absorbed (or excitation light. The illumination required to fluoresce most materials ranges from visible light, through ultraviolet light to soft x-rays.

Fluorescence spectroscopy is a highly versatile tool. It is used for studying molecular interactions in analytical chemistry, biochemistry, cell biology, physiology, nephrology, cardiology, photochemistry and environmental science. It is widely used for rapid characterization of minerals, and can be used to fingerprint oil samples.

Fluorescence microscopy

In biological applications, where visible light is used, samples are often stained with fluorescent tags that bind to certain molecules. Then fluorescence can be used to identify the presence of these molecules in the sample.

However, it is possible that the fluorescence tags may interfere with the processes that they are meant to reveal. Their size alone may affect the cellular activity since some may be up to 10% of the size of the protein to which they are attached, and many molecules of interest are too small to be used with fluorescent labels at all. Many naturally-occuring substances exhibit a natural auto-fluorescence, but to exploit this the sample must be excited with deep UV light, which can be obtained with high intensity from a synchrotron. At the Daresbury Laboratory in the UK, this technique has been used with great success to provide new insights into the mechanisms of blood clotting and the effect of new hair treatments; hair also exhibits auto-fluorescence.

Vibrational spectroscopy

Infrared illumination causes atoms in a molecule to vibrate. The frequency of vibration is specific to the type of interatomic bond. For example, in a biological or polymer molecule, the bonds are mostly between carbon, hydrogen and oxygen. Each of these bonds vibrates at a different, characteristic frequency. The types of bond present and their intensity provides a unique signature for each molecule.

By analyzing the spectra it is possible to identify the structures and types of molecule with a high degree of discrimination. If a characteristic peak is selected and then imaged it is possible to build a picture of the distribution of specific molecules in the sample.

Infrared and Raman spectroscopy are very wisely used in Australia. Every reputable research or analytical laboratory in departments of chemistry, physics, materials science, biochemistry and microbiology would possess an infrared (IR) system, and many production facilities use the techniques for quality control.

Laboratory-based instruments are usually driven by globar light sources, which limit the achievable spatial resolution to 20 to 30 micron. Synchrotron light, which is highly collimated, polarized and at least 100 times more intense than these sources will vastly increase
the potential of the techniques:

• For the first time it will be possible to obtain spatial resolution down to the diffraction limit of 5 to 10 micron. When combined with confocal microscopy, it will be possible to locate and analyze individual components in a sample with dimensions typical of biological cells.
• recent advances in focal plane detectors, when coupled with high intensity illumination from a synchrotron and scanning confocal microscopy, will enable high resolution imaging in three dimensions in a realistic time frame, leading to wide application in anatomy and forensic science.
• The high brightness will enable high resolution infrared spectroscopy, which is important in atmospheric monitoring, gas analysis and molecular structure determination.
• Time-dependent studies of fast reactions will be possible