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1st quarter 1999 TheLos Actinide Alamos National Research Laboratory N u c l e a r M aQuarterly t e r i a l s R e s e a r c h a n d T e c h n o l o g y a U.S. Department of Energy Laboratory Photoemission Studies at the Advanced Light Source Shed Light on Plutonium Phase Characteristics In This Issue The physical characteristics of any given material are largely derived from the behavior of its valence electrons. Valence 1 electrons are the lowest-energy electrons in a material and Photoemission are responsible for the formation of chemical bonds. Typi- Studies at the cally, in a metal, electrons are either localized around a particu- Advanced Light lar atom or are delocalized (i.e., shared by all the atoms in the crystal) Source Shed Light on throughout the entire metal. The actinide series is interesting because as Plutonium Phase the atomic number increases across the series, the electrons in the Characteristics actinide metals transit from delocalized 5f electrons 4 (Ac–Pu) to localized 5f electrons (Pu–No). Plutonium (element 94) is located right at this transition. O2F in FOOF Holds Promise for Pu This placement in the series leads to pluto- Decontamination nium metal being one of the most com- plex materials known. Metallic 6 plutonium displays six allotropic Nuclear Materials phases (α, β, γ, δ, δ′, and ε) at stan- Including “Orphans” dard pressure. A 20% volume Require a Comprehen- expansion occurs during the sive Management change from the α phase to the Strategy δ phase. These physical properties 8 have been attributed to the 5f valence Wing 9 Hot Cells electrons changing from delocalized states to Support Work localized states as the crystal structure changes from Involving Highly the α to the δ phase. XBD9811-03028.tif photo courtesy Radioactive Materials Soft x-ray techniques (photonof University energyof California, in Berkeley the range of 10–1000 eV) such as photoelectron; x-ray emission; and near-edge resonant photoemission spectrum 11 x-ray absorption spectroscopies have been used to determine the Publications and electronic structure of many (in fact most) materials. However, these Invited Talks techniques have not been fully utilized on the actinides. The safety is- sues involved in handling the actinides make it necessary to minimize 12 NewsMakers the amount of radioactive materials used in the measurements. To our knowledge, the only synchrotron radiation source in the world where soft x-ray measurements have been performed on plutonium is the Spectromicroscopy Facility at Beam Line 7.0.1 at the Advanced Light Source (ALS). The ALS is the premier synchrotron radiation facil- ity in the world for the study of the electronic structure of materi- als using soft x-ray photons. The Spectromicroscopy Facility was first used for resonant photoemission measurements on plutonium oxide in 1993. Our first measurements on plutonium were conducted in the fall of 1998. continued on page 2 Nuclear Materials Research and Technology/Los Alamos National Laboratory 1 Actinide Research Quarterly Photoemission Studies at the Advanced Light Source Shed Light on Plutonium Phase Characteristics (continued) The Spectromicroscopy Facility is de- Figure 2 shows the Pu 4f core-level photo- This article was signed so that measurements can be made on emission spectra from the α- and δ-plutonium contributed by small quantities of hazardous material. This samples. The spectrum arising from core-level Jeff Terry and facility has a photon flux of 1013 photon/sec at photoemission is sensitive to energy differ- Roland Schulze a photon energy of 100 eV with 0.01 eV resolu- ences in the initial state (no core hole) and the (both NMT-11). tion. The high photon flux allows one to focus final state (core hole and free photoelectron). Others contributing the beam down to a size of 50 microns and still The two core-level spectra in Figure 2 are simi- to the research were Jason have enough light intensity at the sample for lar in that they both show two features, a sharp Lashley (MST measurements to be conducted in a reasonable feature at low binding energy and a broad Division), Tom time frame, 1–10 minutes per spectrum. There- feature at higher binding energy. The low- Zocco (NMT-11), fore, the sample size can be on the order of 100 binding-energy feature can be ascribed to a J. Doug Farr microns in diameter. This greatly minimizes metallic initial state with a delocalized final (NMT-11), Eli the amount of plutonium on site during the state. As the 5f electrons are more delocalized Rotenberg, Keith experiment. The microfocussing capability in the α- than in the δ-plutonium, the greater Heinzelman, and also allowed us to focus onto a single δ- number of delocalized electrons in the α- David Shuh plutonium microcrystallite grown by Jason plutonium leads to greater intensity in this (Lawrence Lashley (MST Division). peak than the δ-plutonium. Berkeley We performed core-level photoemission, Presently, the higher-binding-energy fea- Laboratory), and Mike Blau and valence band photoemission, and near-edge ture is not completely understood. It is likely Jim Tobin x-ray absorption spectroscopy on both poly- that this feature has two states contributing to α δ (Lawrence crystalline -plutonium and -plutonium it. We believe that, in simplistic forms, one Livermore National microcrystals. Only the photoemission possible component contributing to the feature Laboratory). experiments will be described here. Photo- is made up of emission to an electronic final electron spectroscopy is predicated upon the state with localized 5f electrons. If this is the photoelectric effect first described by Einstein case, it suggests that both localized and delo- in 1905. The experimental photoemission calized 5f electrons are present in the α- and setup at the δ-plutonium phases. The second possible com- Spectromicroscopy ponent of this feature is from an initial state Facility is shown in that has been oxidized. Figure 1. An incident Valence band spectra show that a small photon is absorbed amount of oxygen remains on the surface after by an atom in the the sample preparation. The binding energy of solid, and an electron PuOx is higher than that of the metal and is ejected. An electron would be expected to be seen around the posi- energy analyzer is tion of the unknown high-binding-energy fea- used to measure the ture. A final determination of the exact nature direction and kinetic of this high-binding-energy feature will shed energy of the emitted light on the poorly understood valence elec- electron. The kinetic tronic structure of plutonium, as well as on the energy of the photo- presence of localized and delocalized 5f elec- electron is directly trons in the different plutonium phases. related to its binding Our initial goal of the valence electronic energy in the solid. structure measurements was to determine the density-of-state of the 5f valence electrons in Figure 1. The photoemission setup at the α- and δ-plutonium. This experiment can be Spectromicroscopy Facility at ALS. The position of the directly compared with electronic structure analyzer is fixed with respect to the incoming photons. calculations performed by theoreticians. Fig- Angle-resolved photoemission is performed by rotating the ures 3(a and b) show the resonant photoemis- sample. sion spectra of the valence band from δ- and 2 Nuclear Materials Research and Technology/Los Alamos National Laboratory 1st quarter 1999 α 2.0 -plutonium, respec- Pu 4f Core Level Sharp Figure 2. Core-level photoemission Broad Manifold hν = 850 eV Metallic tively. In a resonant of States spectra from a large crystallite δ- Pass Energy 6 eV State photoemission experi- Alpha plutonium sample and a polycrystalline 1.5 ment, valence band Delta α-plutonium sample are shown. These photoemission spectra spectra were collected with a photon are collected as the pho- 1.0 energy of 850 eV and an analyzer pass energy of 6 eV. Note that two large ton energy is scanned Intensity through a core-level components are visible in each spec- absorption edge. This 0.5 trum, a sharp feature at low binding can result in an en- energy and a broad feature at higher hancement of the emis- binding energy. sion from specific 0 -450 -440 -430 -420 -410 valence levels. In the Binding Energy (eV) case of plutonium, scanning the photon energy through the 5d A large theoretical Figure 3. The 5d–5f resonant photo- absorption edge results in a resonant enhance- effort has been under- emission spectrum with analyzer ment of the 5f valence emission. The 5d–5f taken to understand pass energy of 1 eV showing the 5f resonant photoemission measurements in plu- the plutonium data density of states in the valence band tonium are a measurement of the 5f contribu- that we have collected. from (a) a large crystallite δ-plutonium tion to the valence density of states. The next phase of the sample, and (b) a polycrystalline α- Two types of information are obtained in valence-band, elec- plutonium sample. the resonant photoemission measurements tronic structure mea- that can greatly enhance the understanding of surements will involve measuring the electron plutonium metal. Two-dimensional data slices dispersion relation (band structure) of the va- can be taken in either the constant-photon- lence electrons in δ-plutonium, and the energy direction or the constant-binding- data will be compared with theoreti- energy direction. Slices taken with a constant cal calculations. The band struc- photon energy are the equivalent to standard ture can only be measured on photoemission spectra with the exception that a single crystal of a material, specific valence states are emphasized.

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