Vacuum Ultraviolet Spectroscopy

User community

The number of users with existing synchrotron experience in the VUV area in Australia is significant and are members of a community comprised of more than eight groups; this community is of small to moderate size compared with other areas of sychrotron science. This restricted participation is probably due to a lack of access through ASRP to VUV beamlines. These active groups are, however, distinguished by their extensive experience and an advanced capability in terms of both synchrotron and end-station instrumentation.

Access duration: Due to the ultra-high vacuum nature of VUV beamlines, and the essential requirement to prepare atomically clean single-crystal samples in situ, a typical beamline visit is of 2-3 weeks duration for solid state experiments. For gas phase experimentation custom built end-stations are commonly required with possible consequences in terms of time lost changing instrumentation. Experiments involving coincidence techniques are particularly demanding of time.

The number of groups that can be accommodated at a given VUV beamline is consequently severely limited. Using Germany's BESSY facility as a model, eight user groups can be offered two 2-week periods per year. On this model, the beamline at the Australian Synchrotron will either be fully subscribed, or oversubscribed.

Research applications

Existing synchrotron facilities dedicated to the VUV region include comparable beamlines in operation at BESSY (Berlin), Elettra (Trieste) and ALS (Berkeley). A wide range of spectroscopic and microscopic techniques is in evidence at these facilities and many of these will be mirrored in Australia in the VUV facility at the Australian Synchrotron when it is operational.

Angle-resolved photoemission spectroscopy

Angle resolved photoemission spectroscopy is the pre-eminent technique for the elucidation of the electronic properties of atoms molecules and solids.

Solid state studies

Although the photoemission techniques has many applications at higher energies, it is in the VUV region of the spectrum that angle-resolved data is essential for providing a sufficiently detailed view of the electronic band structure of crystalline solids.

By 2007, state of the art instrumentation will require energy resolution of a few meV coupled with angular resolution significantly better than 1 degree. Instrumentation will be required to resolve fine structure in many materials of technological value and to investigate electronic structure beyond the one-electron approximation.

The unique availability at synchrotron sources of circularly polarized radiation that can be rapidly switched between helicities enables a suite of magnetic dichroism experiments to be undertaken, which can reveal spin-orbit interactions in the conduction band to derive the electronic structure of magnetic materials.

Applications include understanding the properties of shape memory alloys, of collosal magneto-resistance alloys and of strongly correlated materials such as the cobaltates and high-temperature superconductors.

Photoemission from the valence band is very attractive for studying chemisorption on superconductor surfaces.

The study of low binding energy core lines of vacuum fractured conducting (or small band gap) mineral single crystals, in conjuction with detailed valence band studies can illuminate inter-relationships between crystal and electronic structure, bulk vs surface states and, in the end, the reactivity of such materials.

Gas phase studies

Angle resolved photomemission measurements have traditionally been used to validate molecular orbital calculations of both stable and transient molecules. Synchrotron VUV studies allow investigation of electron correlation effects in bound and continuum states, electron exchange and spin-orbit effects, interference and coherence, orbital and magnetic dichroism effects and relativistic effects.

New techniques, such as the angle-resolved photoelectron-photoion coincidence spectroscopy now make possible, for the first time, the study of electron ion momentum vector correlations and reveal intermolecular scattering and associated interference. Exploration of the dynamics of the photoexitation of free molecules is another example of strong current interest, as is high resolution VUV spectroscopy of atmospheric molecules.

Double photoionization studied using conicidence angular distribution techniques, may be used to investigate the important question of coherence in atomic physics.

A recently developed technique with great promise involves the investigation of the intrinsic width of selected atomic states via fluorescent, rather than photo-ion emission. This technique also has particular merit for the optimization and the determination of the the resolution of VUV beamlines.

Pulsed multi-bunched operating modes of a synchrotron enable an extensive list of time-of-flight spectroscopies, including rotational-vibrational fragmentation modes and lifetimes of molecules as well as inner-shell effects.

Adsorbate studies

Bonds and nano-characterization: Synchrotron based angle-resolved VUV photoemission will enable studies of the identity and bonding of the successive surface intermediates in various CVD processes: the bonding of amino acides with metal surfaces; the characterization of carbon nanotubes, gallium nitride nanowires and silicon quantum wire arrays.

Biosystems

Surface science has recently neen increasing in prominence in the biomedical area, based on the fact that many biological reactions occur at surfaces. This any fundmental understanding of the incompatibility of a medical device must take into account the properties of proteins and cells at interfaces, and the characteristics of local biological reactions. Principles worked out in surface science laboratories are likely to become the basis for ways of improving the function and durability of materials featured in a wide range of medical products.

The electronic properties and interactions of matter at an atomic level in biological environments are largely unknown. Yet the detailed understanding of these systems is crucial to the successful development of many new technologies that have direct impact within our community. Applications include medical implants, delivery systems such as radiopharmaceuticals, as well as biosensors and chips for diagnostics, biomimetic materials such as artificial skin and organs, and novel artificial photosynthetic devices.

VUV light is able to probe the valence and low-lying core states of many elements in the periodic table. The interaction of such states ultimately controls the complex interactions and properties observed in biological systems. The high flux and small spot size produced by the VUV beamline will allow for many ground-breaking experiments and studies to be performed on biosystems from a sub-Angstrom to micron scale.

As most biosystems are made of several functioning parts, small spot microscopy on objects as tiny as only a few nanometres to as larges as several microns in size would be of tremendous importance in order to determine accurately the electronic state of each part.