69 Matching Results

Search Results

Advanced search parameters have been applied.

Dimensionality and its effects upon the valence electronic structure of ordered metallic systems

Description: The system c(10x2)Ag/Cu(001) was investigated with Angle-Resolved Photoemission (ARP), Low Energy Electron Diffraction (LEED) and Auger Electron Spectroscopy (AES). LEED and AES provided the calibration of a quartz microbalance used to measure the amount of silver evaporated onto the copper single crystal and also established the monolayer geometrical structure at one monolayer exposure. An off-normal ARP bandmapping study performed with polarized HeI and NeI radiation demonstrated the electronically two-dimensional nature of the silver d-bands at coverages of near one monolayer. The states at the surface Brillouin Zone center were assigned upon the basis of their polarization dependences and a structural model of hexagonal symmetry. A normal emission ARP experiment was performed at the Stanford Synchrotron Radiation Laboratory (SSRL) over the photon energy range of 6 to 32 eV. Data from it documented the evolution of the valence electronic structure of the silver overlayer from a two-dimensional hexagonal valence to a three-dimensional behavior converging towards that of bulk Ag(111). A structural study was attempted using the ARP technique of Normal Emission Photoelectron Diffraction over the photon energy range of 3.4 to 3.7 keV at SSRL, the results of which are inconclusive.
Date: July 1, 1983
Creator: Tobin, J.G.
Partner: UNT Libraries Government Documents Department

MCSNA: Experimental Benchmarking of Pu Electronic Structure

Description: The objective of this work is to develop and/or apply advanced diagnostics to the understanding of aging of Pu. Advanced characterization techniques such as photoelectron and x-ray absorption spectroscopy will provide fundamental data on the electronic structure of Pu phases. These data are crucial for the validation of the electronic structure methods. The fundamental goal of this project is to narrow the parameter space for the theoretical modeling of Pu aging. The short-term goal is to perform experiments to validate electronic structure calculations of Pu. The long-term goal is to determine the effects of aging upon the electronic structure of Pu. Many of the input parameters for aging models are not directly measurable. These parameters will need to be calculated or estimated. Thus a First Principles-Approach Theory is needed, but it is unclear what terms are important in the Hamiltonian. (H{Psi} = E{Psi}) Therefore, experimental data concerning the 5f electronic structure are needed, to determine which terms in the Hamiltonian are important. The data obtained in this task are crucial for reducing the uncertainty of Task LL-01-developed models and predictions. The data impact the validation of electronic structure methods, the calculation of defect properties, the evaluation of helium diffusion, and the validation of void nucleation models. The importance of these activities increases if difficulties develop with the accelerating aging alloy approach.
Date: January 29, 2007
Creator: Tobin, J G
Partner: UNT Libraries Government Documents Department

Actinides and Rare Earths Topical Conference (Code AC)

Description: Actinide and the Rare Earth materials exhibit many unique and diverse physical, chemical and magnetic properties, in large part because of the complexity of their f electronic structure. This Topical Conference will focus upon the chemistry, physics and materials science in Lanthanide and Actinide materials, driven by 4f and 5f electronic structure. Particular emphasis will be placed upon 4f/5f magnetic structure, surface science and thin film properties. For the actinides, fundamental actinide science and its role in resolving technical challenges posed by actinide materials will be stressed. Both basic and applied experimental approaches, including synchrotron-radiation-based investigations, as well as theoretical modeling and computational simulations, are planned to be part of the Topical Conference. Of particular importance are the issues related to the potential renaissance in Nuclear Fuels, including synthesis, oxidation, corrosion, intermixing, stability in extreme environments, prediction of properties via benchmarked simulations, separation science, environmental impact and disposal of waste products.
Date: November 24, 2009
Creator: Tobin, J G
Partner: UNT Libraries Government Documents Department

Russian Contract Procurement Document

Description: This contract supports the enhancement of physical protection or nuclear material control and accounting systems at institutes or enterprises of the newly independent states under the material protection control and accounting (MPC&A) program. The contract is entered into pursuant to the MPC&A Program, a gratuitous technical assistance program, in accordance with the bilateral Agreements between the Russian Federation and the United States of America concerning the Safe and Secure Transportation, Storage and Destruction of Weapons and the Prevention of Weapons Proliferation of June 1992, as extended and amended by Protocol signed of June 1999, Agreement between the Government of the Russian Federation regarding Cooperation in the Area of Nuclear Materials Physical Protection, Control and Accounting of October 1999 and the Russian Federation law of May 1999 on the taxation exemption of gratuitous technical assistance with Russian Federation under registration No.DOE001000.
Date: March 29, 2010
Creator: Tobin, J G
Partner: UNT Libraries Government Documents Department

Soft X-Ray and Vacuum Ultraviolet Based Spectroscopy of the Actinides

Description: The subjects of discussion included: VUV photoelectron spectroscopy, X-ray photoelectron spectroscopy, Synchrotron-radiation-based photoelectron spectroscopy, Soft x-ray absorption spectroscopy, Soft x-ray emission spectroscopy, Inverse photoelectron spectroscopy, Bremstrahlung Isochromat Spectroscopy, Low energy IPES, Resonant inverse photoelectron spectroscopy.
Date: March 17, 2011
Creator: Tobin, J. G.
Partner: UNT Libraries Government Documents Department

Letter to Russia

Description: Below is the referee report. It is not as bad as it seems at first. The manuscript has not been rejected. Instead, the referee is 'not recommending publication.' On the APS website, the status is 'with authors,' instead of 'not under consideration.' Thus, this manuscript is still alive, but we will need to work on it. Please take a look at what the referee says below and let me know how you would respond. I will do the same. Hopefully, we will be able to respond well and find a way for this manuscript to get into PRB. According to the introduction of their manuscript, the authors intend to study the electronic structure of clusters of Pu atoms and, among other things, to illustrate how the properties of the cluster's central region approach those of the bulk Pu metal as the cluster size increases. It is then somewhat surprising to find out that all the 'cluster' calculations discussed in the paper are in fact set up in such a way that they model the bulk properties - the clusters are embedded in a kind of mean field that is designed to approximate the rest of an infinite lattice (the authors call it the extended cluster scheme). Consequently, all the observed finite-size effects are essentially artificial since they represent the inaccuracies of the embedding procedure. The results for the finite clusters themselves do not carry a direct physical meaning (which contradicts authors statements from the introduction), only the extrapolation to the infinite cluster would, if done properly. The authors propose that the number of 5f electrons n{_}5f is a linear function of the cubic root of N, where N is the number of atoms in the cluster. This function fits the calculated data well (Fig. 8), but, as the authors indeed ...
Date: March 15, 2012
Creator: Tobin, J G
Partner: UNT Libraries Government Documents Department

X-Ray Absorption Spectroscopy of Uranium Dioxide

Description: After the CMMD Seminar by Sung Woo Yu on the subject of the x-ray spectroscopy of UO2, there arose some questions concerning the XAS of UO2. These questions can be distilled down to these three issues: (1) The validity of the data; (2) The monchromator energy calibration; and (3) The validity of XAS component of the figure shown. The following will be shown: (1) The data is valid; (2) It is possible to calibrate the monchromator; and (3) The XAS component of the above picture is correct. The remainder of this document is in three sections, corresponding to these three issues.
Date: December 10, 2010
Creator: Tobin, J G
Partner: UNT Libraries Government Documents Department

Final Version: Orbital Specificity in the Unoccupied States of UO2 from Resonant Inverse Photoelectron Spectroscopy

Description: One of the crucial questions of all actinide electronic structure determinations is the issue of 5f versus 6d character and the distribution of these components across the density of states. Here, a break-though experiment is discussed, which has allowed the direct determination of the U5f and U6d contributions to the unoccupied density of states (UDOS) in Uranium Dioxide. A novel Resonant Inverse Photoelectron (RIPES) and X-ray Emission Spectroscopy (XES) investigation of UO{sub 2} is presented. It is shown that the U5f and U6d components are isolated and identified unambiguously.
Date: March 12, 2012
Creator: Tobin, J G & Yu, S W
Partner: UNT Libraries Government Documents Department

Soft X-ray Studies of Pu Electronic Structure: Past Lessons and Future Directions

Description: Photoelectron Spectroscopy (PES) and X-ray Absorption Spectroscopy (XAS, Figure 1) have contributed greatly to our improved understanding of Pu electronic structure. From these and related measurements, the following has been determined: (1) The Pu 5f spin-orbit splitting is large; (2) The number of Pu5f electrons is near 5; and (3) The Pu 5f spin-orbit splitting effect dominates 5f itineracy. Significant questions remain concerning the nature of Pu electronic structure. Perhaps the missing piece of the puzzle is the direct experimental determination of the unoccupied electronic structure using high energy inverse photoelectron spectroscopy or Bremstrahlung Isochromat Spectroscopy (BIS). Past BIS studies of Th and U indicate the feasibility and utility of Pu studies.
Date: February 7, 2008
Creator: Tobin, J. G. & Yu, S. W.
Partner: UNT Libraries Government Documents Department

Investigations of Magnetic Overlayers at the Advanced Photon Source

Description: Magnetic overlayers of Fe and Co have been investigated with X-ray Magnetic Circular Dichroism in X-ray Absorption Spectroscopy (XMCD-ABS) and Photoelectron Spectroscopy (PES), including Spin-Resolved Photoelectron Spectroscopy (SRPES), at Beamline 4 at the Advanced Photon Source (APS). Particular emphasis was placed upon the interrogation of the 2p levels of the Fe.
Date: June 26, 2009
Creator: Tobin, J G; Yu, S & Butterfield, M T
Partner: UNT Libraries Government Documents Department

THE TWO-DIMENSIONAL VALENCE ELECTRONIC STRUCTURE OF A MONOLYAER OF Ag ON Cu(00l)

Description: The metal overlayer system c(10x2)Ag/Cu(001) was studied at coverages near one monolayer with angle-resolved photoemission. The observed spectroscopic features indicate a two-dimensional d-band electronic structure that can be interpreted using a model with planar, hexagonal symmetry in which crystal field effects dominate over spin-orbit effects.
Date: May 1, 1985
Creator: Tobin, J. G.; Robey, S. W. & Shirley, D. A.
Partner: UNT Libraries Government Documents Department

Pu Workshop Letter

Description: In preparation for the upcoming Pu Workshop in Livermore, CA, USA, during July 14 and 15, 2006, we have begun to give some thought as to how the meeting will be structured and what will be discussed. Below, you will find our first proposal as to the agenda and contents of the meeting. From you, we need your feedback and suggestions concerning the desirability of each aspect of our proposal. Hopefully, we will be able to converge to a format that is acceptable to all parties. First, it now appears that we will be limited to three main sessions, Friday morning (July 14), Friday afternoon (July 14) and Saturday morning (July 15). The Pu Futures Meeting will conclude on Thursday, July 13. Following a social excursion, the Russian participants will be transported from Monterey Bay to their hotel in Livermore. We anticipate that the hotel will be the Residence Inn at 1000 Airway Blvd in Livermore. However, the hotel arrangements still need to be confirmed. We expect that many of our participants will begin their travels homeward in the afternoon of Saturday, July 15 and the morning of Sunday, July 16. Associated with the three main sessions, we propose that there be three main topics. Each session will have an individual focus. Because of the limited time available, we will need to make some judicious choices concerning the focus and the speakers for each session. We will also have a poster session associated with each session, to facilitate discussions, and a rotating set of Lab Tours, to maximize participation in the tour and minimize the disruption of the speaking schedule. Presently, we are planning a tour of the Dynamical Transmission Electron Microscope (DTEM) facilities, but this is still in a preliminary stage. We estimate that for each session and topic, ...
Date: March 6, 2006
Creator: Tobin, J G; Schwartz, A J & Fluss, M
Partner: UNT Libraries Government Documents Department

Photoelectron Spectroscopy of U Oxide at LLNL

Description: In our laboratory at LLNL, an effort is underway to investigate the underlying complexity of 5f electronic structure with spin-resolved photoelectron spectroscopy using chiral photonic excitation, i.e. Fano Spectroscopy. Our previous Fano measurements with Ce indicate the efficacy of this approach and theoretical calculations and spectral simulations suggest that Fano Spectroscopy may resolve the controversy concerning Pu electronic structure and electron correlation. To this end, we have constructed and commissioned a new Fano Spectrometer, testing it with the relativistic 5d system Pt. Here, our preliminary photoelectron spectra of the UO{sub 2} system are presented. X-ray photoelectron spectroscopy has been used to characterize a sample of UO{sub 2} grown on an underlying substrate of Uranium. Both AlK{alpha} (1487 eV) and MgK{alpha} (1254 eV) emission were utilized as the excitation. Using XPS and comparing to reference spectra, it has been shown that our sample is clearly UO{sub 2}.
Date: March 2, 2010
Creator: Tobin, J. G.; Yu, S.; Chung, B. W. & Waddill, G. D.
Partner: UNT Libraries Government Documents Department

FY04 LDRD Final Report:Properties of Actinide Nanostructures

Description: Two papers completely describe the objectives and work performed in this laboratory directed research and development (LDRD) project. The first paper published in Review of Scientific Instruments (UCRL-JC-152913) describes the purpose, construction, and operation of a novel instrument to produce and characterize actinide nanostructures by pulsed laser deposition. The second paper submitted to Physical Review B (UCRL-JRNL-209427) describes our work quantifying the oxidation of pulsed laser deposited depleted uranium nanostructures by following the evolution of the electronic structure.
Date: February 22, 2005
Creator: Hamza, A. V.; Trelenberg, T. W. & Tobin, J. G.
Partner: UNT Libraries Government Documents Department

Oxidation of uranium nanoparticles produced via pulsed laser ablation

Description: An experimental apparatus designed for the synthesis, via pulsed laser deposition, and analysis of metallic nanoparticles and thin films of plutonium and other actinides was tested on depleted uranium samples. Five nanosecond pulses from a Nd:YAG laser produced films of {approx}1600 {angstrom} thickness that were deposited showing an angular distribution typical thermal ablation. The films remained contiguous for many months in vacuum but blistered due to induced tensile stresses several days after exposure to air. The films were allowed to oxidize from the residual water vapor within the chamber (2 x 10{sup -10} Torr base pressure). The oxidation was monitored by in-situ analysis techniques including x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and scanning tunneling microscopy (STM) and followed Langmuir kinetics.
Date: December 7, 2005
Creator: Trelenberg, T W; Glade, S C; Tobin, J G; Felter, T E & Hamza, A V
Partner: UNT Libraries Government Documents Department

Spin Polarized Electron Energy Loss Spectroscopy on Fe(100) Thin Films Grown on Ag(100)

Description: We report sharp spin-dependent energy loss features in electron scattering from bcc Fe(100) thin films grown on Ag(100). Majority spin features are observed at {approx}1.8 and 2.5 eV energy loss, and a minority spin feature is observed at {approx}2.0 eV energy loss. The majority spin peaks are attributed to spin-flip exchange scattering from the Fe films, with the lowest energy feature corresponding to the exchange splitting for the Fe. The minority spin peak is attributed to non-flip exchange scattering with an energy corresponding to the separation between occupied and unoccupied minority spin bands.
Date: December 1, 2004
Creator: Komesu, T; Waddill, G D & Tobin, J G
Partner: UNT Libraries Government Documents Department

Contract B590089: Technical Evaluation of the Pu Cluster Calculations

Description: Using Synchrotron-Radiation-based Photoelectron Spectroscopy and X-ray Absorption Spectroscopy, the theoretical results within recent progress reports supplied under Contract B590089 have been evaluated. Three appendices are included: A is from Progress Report I; B is from Progress Report II; and C is from an earlier calculation by M. Ryzhkov. The comparisons between the LLNL experimental data and the Russian calculations are quite favorable. The Cluster calculations may represent a new and useful avenue to address unresolved questions within the field of actinide electron structure, particularly that of Pu.
Date: November 18, 2011
Creator: Tobin, J G; Ryzhkov, M & Mirmelstein, A
Partner: UNT Libraries Government Documents Department

Spin-Orbit Effects in Spin-Resolved L2,3 Core Level Photoemission of 3d Ferromagnetic Thin Films

Description: We present spin-resolved 2p core level photoemission for the 3d transition metal films of Fe and Co grown on Cu(100). We observe clear spin asymmetry in the main 2p core level photoemission peaks of Fe and Co films consistent with trends in the bulk magnetic moments. The spin polarization can be strongly enhanced, by variation of the experimental geometry, when the photoemission is undertaken with circularly polarized light, indicating that spin-orbit interaction can have a profound in spin polarized photoemission. Further spin polarized photoemission studies using variable circularly polarized light at high photon energies, high flux are indicated, underscoring the value of synchrotron measurements at facilities with increased beam stability.
Date: October 2, 2007
Creator: Komesu, T; Waddill, G D; Yu, S W; Butterfield, M & Tobin, J G
Partner: UNT Libraries Government Documents Department

Element-specific magnetometry with linear dichroism in photoemission

Description: In this paper, we investigate the magnetic linear dichroism in the core-level photoemission spectra of the binary alloys Co<sub>x</sub>Ni<sub>l-x</sub> and Fe<sub>x</sub>Ni<sub>1-x</sub>/Cu(100). These epitaxial films have fee structures, but very different magnetic behavior.We show that the magnetic linear dichroism in x-ray photoemission (XMLD) signal tracks the magnetization in these alloys. Comparison with recent SQUID data provides a quantitative check and endorses the view that XMLD monitors the element-specific magnetometry.
Date: July 1, 1998
Creator: Tobin, J G
Partner: UNT Libraries Government Documents Department

Application of MXCD to magnetic thin-film sensors

Description: While Magnetic X-ray Circular Dichroism (MXCD) has been applied extensively to the extraction of elemental magnetic moments in various magnetic multilayers, the configuration of actual devices imposes certain constraints on the application of MXCD to devices. Using a set of real, thin-film spin valve devices with varying Cu spacer layer thicknesses, we demonstrate the correlation between MXCD and R-H measurements on those devices as well as the restrictions on the interpretation of MXCD data imposed by both the device topology and the formulation of realistic error estimates.
Date: May 17, 1996
Creator: Bedrossian, P.J.; Tobin, J.G. & Jankowski, A.F.
Partner: UNT Libraries Government Documents Department