Debye-Waller effects in Bethe-Salpeter calculations: Bridging the gap between XANES and EXAFS.

We present a method to incorporate Debye-Waller effects on core-excitation spectra within frameworks other than real-space, multiple-scattering formulations. The method draws ideas used in multiple-scattering theory to realize effects of variations in interatomic distances, and the method accomplishes this without benefit of the atom-by-atom nature in which multiple-scattering calculations are formulated. We test the method in four metals and one semiconductor over a range of temperatures and compare all theoretical results to experiment.

Fast, efficient, and accurate dielectric screening using a local real-space approach.

Various many-body perturbation theory techniques for calculating electron behavior rely on , the screened Coulomb interaction. Computing requires complete knowledge of the dielectric response of the electronic system, and the fidelity of the calculated dielectric response limits the reliability of predicted electronic and structural properties. As a simplification, calculations often begin with the random-phase approximation (RPA).

Partial densities of states from x-ray absorption and Auger electron spectroscopy: Use of core-hole memory.

Ag edge x-ray absorption and - Auger electron spectra have been measured and simulated for a variety of x-ray electric-field polarization and Auger electron emission directions. The theory relies on density-functional theory, use of the Bethe-Salpeter equation, atomic multiplet theory, and a simplified model for the Auger line shape and its dependence on photon energy. We also demonstrate that partial densities of states for , , and symmetry partial-wave channels at the Ag site in the solid can be deduced from the experimental measurements with only atomic theoretical input, i.

Revisiting the K-edge X-ray absorption fine structure of Si, Ge-Si alloys, and the isoelectronic series: CuBr, ZnSe, GaAs, and Ge.

Extended X-ray absorption fine structure (EXAFS) has evolved into an unprecedented local-structure technique that is routinely used to study materials' problems in the biological, chemical, and physical sciences. Like many other experimental techniques, EXAFS also requires that several key atomic parameters must be known before structural information can be quantitatively determined. Utilizing current analytical methods, we revisit the isoelectronic series CuBr, ZnSe, GaAs, and Ge originally studied by Stern during the early development of EXAFS [E.

Design, calibration, and application of a cryogenic low-background infrared radiometer for spectral irradiance and radiance measurements from 4 μm to 20 μm wavelength.

We have constructed, calibrated, and tested a cryogenic low-background infrared radiometer for both spectral radiance and irradiance measurements over the 4 μm to 20 μm wavelength range. The primary purpose of the Missile Defense Transfer Radiometer (MDXR) is to measure absolute irradiance or radiance from cryogenic infrared test chamber sources using a photoconductive Si:As Blocked Impurity Band (BIB) detector and a set of spectral filters. The MDXR also includes an absolute cryogenic radiometer (ACR) and a Fourier transform spectrometer (FTS).

Core hole processes in x-ray absorption and photoemission by resonant Auger-electron spectroscopy and first-principles theory.

Electron-core hole interactions are critical for proper interpretation of core-level spectroscopies commonly used as analytical tools in materials science. Here we utilize resonant Auger-electron spectroscopy to uniquely identify exciton, shake, and charge-transfer processes that result from the sudden creation of the core hole in both x-ray-absorption and photoemission spectra. These effects are captured for the transition-metal compounds SrTiO and MoS by fully , combined real-time cumulant, and Bethe-Salpeter equation approaches to account for core hole dynamics and screening.

Charge-transfer satellites and chemical bonding in photoemission and x-ray absorption of SrTiO and rutile TiO: Experiment and first-principles theory with general application to spectroscopic analysis.

First-principles, real-time-cumulant, and Bethe-Salpeter-equation calculations fully capture the detailed satellite structure that occurs in response to the sudden creation of the core hole in both photoemission and x-ray absorption spectra of the transition-metal compounds SrTiO and rutile TiO. Analysis of the excited-state, real-space charge-density fluctuations betrays the physical nature of these many electron excitations that are shown to reflect the materials' solid-state electronic structure and chemical bonding. This first-principles development of the cumulant-based core hole spectral function is generally applicable to other systems and should become a standard tool for all similar spectroscopic analysis going beyond the quasiparticle physics of the photoelectric effect.

A new model dielectric function for loss functions and electron damping.

Trends in the zeroth frequency moment of the imaginary part of the dielectric function are studied for a wide range of metals, semiconductors and insulators. These results are combined with estimates for the inverse-first moment (related by Kramers-Kronig relations to the static dielectric function) and knowledge of the first moment from the f-sum rule. Matching all three moments allows for construction of a model dielectric function that reasonably predicts the loss function at different values of momentum and lifetime damping effects on occupied and unoccupied electron states.

Invited Article: Refined analysis of synchrotron radiation for NIST's SURF III facility.

We have developed a new method for the exact calculation of synchrotron radiation for the National Institute of Standards and Technology Synchrotron Ultraviolet Radiation Facility, SURF III. Instead of using the Schwinger formula, which is only an approximation, we develop formulae based on Graf's addition theorem for Bessel functions and accurate asymptotic expansions for Hankel functions and Bessel functions. By measuring the radiation intensity profile at two distances from the storage ring, we also confirm an apparent vertical emittance that is consistent with the vertical betatron oscillations that are intentionally introduced to extend beam lifetime by spreading the electron beam spatially.

Acceleration of diffraction calculations in cylindrically symmetrical optics.

We have significantly accelerated diffraction calculations using three independent acceleration devices. These innovations are restricted to cylindrically symmetrical systems. In the first case, we consider Wolf's formula for integrated flux in a circular region following diffraction of light from a point source by a circular aperture or a circular lens.

Imaging Catalytic Activation of CO on CuO (110): A First-Principles Study.

Balancing global energy needs against increasing greenhouse gas emissions requires new methods for efficient CO reduction. While photoreduction of CO is promising, the rational design of photocatalysts hinges on precise characterization of the surface catalytic reactions. CuO is a promising next-generation photocatalyst, but the atomic-scale description of the interaction between CO and the CuO surface is largely unknown, and detailed experimental measures are lacking.

Local atomic geometry and Ti 1s near-edge spectra in PbTiO and SrTiO.

We study Ti 1s near-edge spectroscopy in PbTiO at various temperatures above and below its tetragonal-to-cubic phase transition, and in SrTiO at room temperature. molecular dynamics (AIMD) runs on 80-atom supercells are used to determine the average internal coordinates and their fluctuations. We determine that one vector local order parameter is the dominant contributor to changes in spectral features: the displacement of the Ti ion with respect to its axial O neighbors in each Cartesian direction, as these displacements enhance the cross section for transitions to E-derived core-hole exciton levels.

Quantitative first-principles calculations of valence and core excitation spectra of solid C.

We present calculated valence and C 1s near-edge excitation spectra of solid C and experimental results measured with high-resolution electron energy-loss spectroscopy. The near-edge calculations are carried out using three different methods: solution of the Bethe-Salpeter equation (BSE) as implemented in the OCEAN suite (Obtaining Core Excitations with methods and the NIST BSE solver), the excited-electron core-hole approach (XCH), and the constrained-occupancy method using the Stockholm-Berlin core-excitation code, StoBe. The three methods give similar results and are in good agreement with experiment, though the BSE results are the most accurate.

Accurate X-Ray Spectral Predictions: An Advanced Self-Consistent-Field Approach Inspired by Many-Body Perturbation Theory.

Constrained-occupancy delta-self-consistent-field (ΔSCF) methods and many-body perturbation theories (MBPT) are two strategies for obtaining electronic excitations from first principles. Using the two distinct approaches, we study the O 1s core excitations that have become increasingly important for characterizing transition-metal oxides and understanding strong electronic correlation. The ΔSCF approach, in its current single-particle form, systematically underestimates the pre-edge intensity for chosen oxides, despite its success in weakly correlated systems.

Ultrafast modulation of electronic structure by coherent phonon excitations.

Femtosecond x-ray absorption spectroscopy with a laser-driven high-harmonic source is used to map ultrafast changes of x-ray absorption by femtometer-scale coherent phonon displacements. In LiBH, displacements along an phonon mode at 10 THz are induced by impulsive Raman excitation and give rise to oscillatory changes of x-ray absorption at the Li K-edge. Electron density maps from femtosecond x-ray diffraction data show that the electric field of the pump pulse induces a charge transfer from the to neighboring Li ions, resulting in a differential Coulomb force that drives lattice vibrations in this virtual transition state.

Refined treatment of single-edge diffraction effects in radiometry.

This work treats diffraction corrections in radiometry for cases of point and extended sources in cylindrically symmetrical three-element systems. It considers diffraction effects for spectral power and total power in cases of Planck sources. It improves upon an earlier work by the author by giving a simpler rendering of leading terms in asymptotic expansions for diffraction effects and reliable estimates for the remainders.

Results of aperture area comparisons for exo-atmospheric total solar irradiance measurements.

Exo-atmospheric solar irradiance measurements made by the solar irradiance community since 1978 have incorporated limiting apertures with diameters measured by a number of metrology laboratories using a variety of techniques. Knowledge of the aperture area is a critical component in the conversion of radiant flux measurements to solar irradiance. A National Aeronautics and Space Administration (NASA) Earth Observing System (EOS) sponsored international comparison of aperture area measurements of limiting apertures provided by solar irradiance researchers was performed, the effort being executed by the National Institute of Standards and Technology (NIST) in coordination with the EOS Project Science Office.

Numerical quantification of the vibronic broadening of the SrTiO3 Ti L-edge spectrum.

The 2p(5)3d(1) excited state of the Ti(4+) ion in SrTiO(3) couples to e(g) distortions of the local oxygen cage, leading to a Jahn-Teller vibronic broadening of the excited states. We quantify this contribution to the broadening of the spectral features of the Ti L edge of SrTiO(3) by solving a model Hamiltonian, taking parameters for the Hamiltonian from previous first-principles calculations. Evaluation of the model Hamiltonian indicates that vibronic coupling accounts for the majority of the broadening observed for the L(3) edge, but only a minority of the L(2)-edge spectral width.

Electronic structure of crystalline 4He at high pressures.

Using inelastic x-ray scattering techniques, we have succeeded in probing the high-pressure electronic structure of helium at 300 K. Helium has the widest known valence-conduction band gap of all materials a property whose high-pressure response has been inaccessible to direct measurements. We observed a rich electron excitation spectrum, including a cutoff edge above 23 eV, a sharp exciton peak showing linear volume dependence, and a series of excitations and continuum at 26 to 45 eV.

Bethe-Salpeter equation calculations of core excitation spectra.

We present a hybrid approach for Bethe-Salpeter equation (BSE) calculations of core excitation spectra, including x-ray absorption (XAS), electron energy loss spectra (EELS), and nonresonant inelastic x-ray scattering (NRIXS). The method is based on wave functions from the plane-wave pseudopotential code ABINIT; atomic core-level states and projector augmented wave (PAW) transition matrix elements; the NIST core-level BSE solver; and a many-pole self-energy model to account for final-state broadening and self-energy shifts. Multiplet effects are also approximately accounted for.

Many-Pole Model of Inelastic Losses Applied to Calculations of XANES.

Conventional Kohn-Sham band-structure methods for calculating deep-core x-ray spectra typically neglect photoelectron self-energy effects, which give rise to an energy-dependent shift and broadening of the spectra. Here an procedure is introduced to correct for these effects. The method is based on calculations of the GW self-energy using a many-pole model and a calculation of the dielectric function in the long wavelength limit using either the FEFF8 real-space Green's function code, or the AI2NBSE interface between the National Institute of Standards and Technology (NIST) Bethe-Salpeter equation solver (NBSE) and the ABINIT pseudopotential code.

The local electronic structure of alpha-Li3N.

New theoretical and experimental investigations of the occupied and unoccupied local electronic densities of states (DOS) are reported for alpha-Li(3)N. Band-structure and density-functional theory calculations confirm the absence of covalent bonding character. However, real-space full-multiple-scattering (RSFMS) calculations of the occupied local DOS find less extreme nominal valences than have previously been proposed.

Two innovations in diffraction calculations for cylindrically symmetrical systems.

Two mathematical innovations are presented that relate to calculating propagation of radiation through cylindrically symmetrical systems using Kirchhoff diffraction theory. The first innovation leads to an efficient means of computing Lommel functions of two arguments (u and nu), typically denoted by U(n)(u, nu) and V(n)(u, nu). This can accelerate computations involving Fresnel diffraction by circular apertures or lenses.

Revised formulas for diffraction effects with point and extended sources.

Revised formulas to estimate diffraction effects in radiometry for point and extended sources are derived. They are found to work as well as or better than previous formulas. In some instances the formulas can be written in closed form; otherwise their evaluation entails performing simple integrations as indicated.