Actinide Science with Soft X-ray Synchrotron Radiation

Several workshops, some dating back more than fifteen years, recognised both the potential scientific impact and opportunities that would be made available by the capability to investigate actinide materials in the vacuum ultraviolet (VUV)/soft X-ray region of the synchrotron radiation (SR) spectrum. This spectral region revolutionised the approach to surface materials chemistry and physics nearly two decades ago. The actinide science community was unable to capitalise on these SR methodologies for the study of actinide materials until recently because of radiological safety concerns. The Advanced Light Source (ALS) at LBNL is a third-generation light source providing state-of-the-art performance in the VUV/soft X-ray region. Along with corresponding improvements in detector and vacuum technology, the ALS has rendered experiments with small amounts of actinide materials possible. In particular, it has been the emergence and development of microspectroscopic techniques that have enabled investigations of actinide materials at the ALS. The primary methods for the experimental investigation of actinide materials in the VUV/soft X-ray region are the complementary photoelectron spectroscopies, near-edge X-ray absorption fine structure (NEXAFS) and X-ray emission spectroscopy (XES) techniques. Resonant photo-emission is capable of resolving the 5f electron contributions to actinide bonding and can be used to characterise the electronic structure of actinide materials. This technique is clearly a most important methodology afforded by the tunable SR source. Core level and valence band photoelectron spectroscopies are valuable for the characterisation of the electronic properties of actinide materials, as well as for general analytical purposes. High-resolution core-level photo-emission and resonant photo-emission measurements from the α (monoclinic) and δ (FCC) allotropic phases of plutonium metal have been collected on beam line 7.0 at the ALS and the spectra show pronounced differences between the phases. The Pu 4f7/2 core level spectra obtained from both phases have sharp metallic features indicative of core hole shielding by de-localised electrons. However, the screening is enhanced in the α-Pu spectrum and suggests a larger degree of de-localisation than in δ-Pu. The results from the resonant valence band photo-emission measurements show greater spectral enhancement from the δ-Pu than the α-Pu, which is also consistent with a greater degree of localisation in δ-Pu. One specific consequence of third-generation VUV/soft X-ray SR source development is the renaissance of XES techniques. XES is an atom specific probe, complementary to both photo-emission and absorption, that is especially amenable to studies of buried, disordered and bulk materials systems. Thus, XES is well suited for characterising the electronic properties of actinide materials since complications with surface contamination and sample preparation can be avoided. Results from XES