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At least 505 records · Page 28
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Medium resolution studies of extreme ultraviolet emission from N2 by electron impact - The effect of predissociation on the emission cross section of the b 1Pi(u) state

The electron impact induced fluorescence spectrum of N2 is measured in the 102-134 nm range at a 0.05 nm spectral resolution. The spectral measurements provide the emission cross sections of the transitions of the b 1Pi(u)-X 1Sigma(+)g Birge-Hopfield I-band system. The structure and vibrational population distribution of this system are strongly affected by a configuration interaction of the valence b 1Pi(u) and Rydberg c 1Pi(u) and o 1Pi(u) states. The excitation function (0-400 eV) for the b-X (1,2) transition is measured and a modified Born approximation analytic model is applied to calculate the oscillator strength for the b 1Pi(u)-X band system. With the exception of the v-prime = 1 level, vibrational levels of the b 1Pi(u) state predissociate with a branching ratio of between 0.95 and 1.00. Predissociation of the b 1Pi(u) state contributes approximately six percent of the total dissociation cross section of N2 by electron impact at 100 eV.

James, Geoffrey K.↗
DOE OSTI2020

Neutron-proton pairing correction in the extended isovector and isoscalar pairing model

An extended O ST (8) model with multi j-orbits is constructed based on the angular momentum decomposition with “pseudo”-spin S for valence nucleons in a j-orbit. It is shown that the isovector S = 0 and T=1 pairs are exactly the J=0 and T=1 pairs in a given j-orbit, while the isoscalar S = 1 pairs are linear combinations of J=odd pairs, with which the pairing Hamiltonian can be used to estimate isovector and isoscalar pairing interactions. As an example of the model application, some low-lying J = 0 + level energies of even-even and odd-odd A=18 – 28 nuclei up to the half-filling in the ds-shell above the 16 O core are fit by the model and compared with the fitting results of the same Hamiltonian in the O LST (8) form. It has been verified from the fitting of both the models that the isoscalar pairing interaction can be neglected in the lower energy part of the spectra of these ds-shell nuclei as far as binding energies and a few J=0 + excited levels of these nuclei are concerned. With the mean-field plus isovector pairing interaction only, neutron-neutron, proton-proton, and neutron-proton pairing contributions at the ground or the lowest J=0 + state of these nuclei are estimated. It is shown that the isovector np-pairing contribution to the binding in the odd-odd N=Z nuclei is systematically larger than that in the even-even nuclei. Furthermore, the isoscalar np-pair content at the lowest J=0+ state of these nuclei is also estimated. In both the O ST (8) and O LST (8) models, it is clearly shown that the isoscalar-pair content in the lowest J=0 + state of the N=Z and N=Z±2 nuclei increases with increasing of the valence nucleons, especially in those even-even nuclei, which indicates the isoscalar pairing correlation to be of importance at low-lying states of N=Z and N=Z±2 nuclei, especially in those even-even nuclei with more valence nucleons up to the half-filling, even though the isoscalar pairing interaction is negligible.

20 ≤ A ≤ 38↗
DOE OSTI2021

New Compounds and Phase Selection of Nickel Sulfides via Oxidation State Control in Molten Hydroxides

Molten salts are promising reaction media candidates for the discovery of novel materials; however, they offer little control over oxidation state compared to aqueous solutions. Here, we demonstrated that when two hydroxides are mixed, their melts become fluxes with tunable solubility, which are surprisingly powerful solvents for ternary chalcogenides and offer effective paths for crystal growth to new compounds. We report that precise control of the oxidation state of Ni is achievable in mixed molten LiOH/KOH to grow single crystals of all known ternary K-Ni-S compounds. It is also possible to access several new phases, including a new polytope of β-K 2 Ni 3 S 4 , as well as low-valence KNi 4 S 2 and K 4 Ni 9 S 11 . KNi 4 S 2 is a two-dimensional low-valence nickel-rich sulfide, and β-K 2 Ni 3 S 4 has a hexagonal lattice. Moreover, using KNi 4 S 2 as a template, we obtained a new layered binary Ni 2 S by topotactic deintercalation of K. The new binary Ni 2 S has a van der Waals gap and can function as a new host layer for intercalation chemistry, as demonstrated by the intercalation of LiOH between its layers. The oxidation states of low-valence KNi 4 S 2 and Ni 2 S were studied using X-ray absorption spectroscopy and X-ray photoelectron spectroscopy. Density functional theory calculations showed large antibonding interactions at the Fermi level for both KNi 4 S 2 and Ni 2 S 2 corresponding to the flat-bands with large Ni-d$_{x^2}$$_{y^2}$ character.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Watching Excited State Dynamics with Optical and X-ray Probes: The Excited State Dynamics of Aquocobalamin and Hydroxocobalamin

Femtosecond time-resolved X-ray absorption (XANES) at the Co K-edge, X-ray emission (XES) in the Co Kβ and valence-to-core regions, and broadband UV–vis transient absorption are combined to probe the femtosecond to picosecond sequential atomic and electronic dynamics following photoexcitation of two vitamin B12 compounds, hydroxocobalamin and aquocobalamin. Polarized XANES difference spectra allow identification of sequential structural evolution involving first the equatorial and then the axial ligands, with the latter showing rapid coherent bond elongation to the outer turning point of the excited state potential followed by recoil to a relaxed excited state structure. Time-resolved XES, especially in the valence-to-core region, along with polarized optical transient absorption suggests that the recoil results in the formation of a metal-centered excited state with a lifetime of 2–5 ps. In conclusion, this combination of methods provides a uniquely powerful tool to probe the electronic and structural dynamics of photoactive transition-metal complexes and will be applicable to a wide variety of systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Quadrupolar excitons and hybridized interlayer Mott insulator in a trilayer moiré superlattice

Transition metal dichalcogenide (TMDC) moiré superlattices, owing to the moiré flatbands and strong correlation, can host periodic electron crystals and fascinating correlated physics. The TMDC heterojunctions in the type-II alignment also enable long-lived interlayer excitons that are promising for correlated bosonic states, while the interaction is dictated by the asymmetry of the heterojunction. Here we demonstrate a new excitonic state, quadrupolar exciton, in a symmetric WSe 2 -WS 2 -WSe 2 trilayer moiré superlattice. The quadrupolar excitons exhibit a quadratic dependence on the electric field, distinctively different from the linear Stark shift of the dipolar excitons in heterobilayers. This quadrupolar exciton stems from the hybridization of WSe 2 valence moiré flatbands. The same mechanism also gives rise to an interlayer Mott insulator state, in which the two WSe 2 layers share one hole laterally confined in one moiré unit cell. In contrast, the hole occupation probability in each layer can be continuously tuned via an out-of-plane electric field, reaching 100% in the top or bottom WSe 2 under a large electric field, accompanying the transition from quadrupolar excitons to dipolar excitons. Our work demonstrates a trilayer moiré system as a new exciting playground for realizing novel correlated states and engineering quantum phase transitions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1997

Electronic Structure of TlBa2CaCu2O(7-Delta)

The core levels of TlBa2CaCu2O(7-delta) (Tl-1212) epitaxial films have been measured with X-ray photoelectron spectroscopy (XPS). The valence electronic structure has been determined using the full-potential linear muffin-tin-orbital band-structure method and measured with XPS. The calculations show that a van Hove singularity (VHS) lies above the Fermi level (E(sub F)) for the stoichiometric compound (delta = 0.5), while for 50% oxygen vacancies in the Tl-O layer (delta = 0.5) E(sub F) is in close proximity to the VHS. Samples annealed in nitrogen (to reduce the hole overdoping by the removal of oxygen) exhibit higher core-level binding energies and a higher T(sub c), consistent with a shift of E(sub F) closer to the VHS. Comparisons are made to the core levels and valence bands of Tl2Ba2CaCu2O(8 + delta)(Tl-2212) and HgBa2CaCu2O)6 + delta) (Hg- 1212). The similarity of the Cu 2p(sub 3/2) spectra for Tl-1212 and Tl-2212 indicates that the number of Tl-O layers has little effect on the Cu-O bonding. However, the Tl-1212 and Hg-1212 Cu 2p(sub 3/2) signals exhibit differences which suggest that the replacement of T(sup 3+) with Hg(sup 2+) results in a decrease in the O 2p right arrow Cu 3d charge-transfer energy and differences in the probabilities of planar vs apical oxygen charge transfer and/or Zhang-Rice singlet-state formation. Differences between the Tl-1212 and the Tl-2212 and Hg-1212 measured valence bands are consistent with the calculated Cu 3d and (Tl,Hg) 6s/5d partial densities of states.

Vasquez, R. P.↗
NASA NTRS1988

Theoretical confirmation of a 4-Delta ground state for FeH

Large CAS SCF/multireference configuration interaction (MRCI) and modified coupled pair functional (MCPF) calculations are carried out for the 4-Delta and 6-Delta states of FeH using very large Gaussian basis sets. At the MCPF level, 3s and 3p correlation preferentially lowers the 4-Delta state by 0.10 eV. Adding this inner-shell correlation effect to our best CAS SCF/MRCI+Q valence treatment results in a 4-Delta-6 Delta separation of 0.16 eV. This supports the interpretation of the photodetachment spectra of Stevens et al. (1983) that places the 6-Delta state about 0.25 eV above the 4-Delta ground state.

Bauschlicher, Charles W., Jr.↗
DOE OSTI2020

Influence of Polymorphism on the Electronic Structure of Ga 2 O 3

The search for new wide-band-gap materials is intensifying to satisfy the need for more advanced and energy-efficient power electronic devices. Ga 2 O 3 has emerged as an alternative to SiC and GaN, sparking a renewed interest in its fundamental properties beyond the main β-phase. Here, three polymorphs of Ga 2 O 3 , α, β, and ε, are investigated using X-ray diffraction, X-ray photoelectron and absorption spectroscopy, and ab initio theoretical approaches to gain insights into their structure–electronic structure relationships. Valence and conduction electronic structure as well as semicore and core states are probed, providing a complete picture of the influence of local coordination environments on the electronic structure. State-of-the-art electronic structure theory, including all-electron density functional theory and many-body perturbation theory, provides detailed understanding of the spectroscopic results. The calculated spectra provide very accurate descriptions of all experimental spectra and additionally illuminate the origin of observed spectral features. This paper provides a strong basis for the exploration of the Ga 2 O 3 polymorphs as materials at the heart of future electronic device generations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Interface-modulated kinetic differentials in electron and hole transfer rates as a key design principle for redox photocatalysis by Sb2VO5/QD heterostructures

The efficient conversion of solar energy to chemical energy represents a critical bottleneck to the energy transition. Photocatalytic splitting of water to generate solar fuels is a promising solution. Semiconductor quantum dots (QDs) are prime candidates for light-harvesting components of photocatalytic heterostructures, given their size-dependent photophysical properties and band-edge energies. A promising series of heterostructured photocatalysts interface QDs with transition-metal oxides which embed midgap electronic states derived from the stereochemically active electron lone pairs of p-block cations. Here, we examine the thermodynamic driving forces and dynamics of charge separation in Sb2VO5/CdSe QD heterostructures, wherein a high density of Sb 5s2-derived midgap states are prospective acceptors for photogenerated holes. Hard-x-ray valence band photoemission spectroscopy measurements of Sb2VO5/CdSe QD heterostructures were used to deduce thermodynamic driving forces for charge separation. Interfacial charge transfer dynamics in the heterostructures were examined as a function of the mode of interfacial connectivity, contrasting heterostructures with direct interfaces assembled by successive ion layer adsorption and reaction (SILAR) and interfaces comprising molecular bridges assembled by linker-assisted assembly (LAA). Transient absorption spectroscopy measurements indicate ultrafast (<2 ps) electron and hole transfer in SILAR-derived heterostructures, whereas LAA-derived heterostructures show orders of magnitude differentials in the kinetics of hole (<100 ps) and electron (∼1 ns) transfer. The interface-modulated kinetic differentials in electron and hole transfer rates underpin the more effective charge separation, reduced charge recombination, and greater photocatalytic efficiency observed for the LAA-derived Sb2VO5/CdSe QD heterostructures.

Chemistry↗
DOE OSTI2021

Strategies for solving the excited-state self-consistent-field problem for highly excited and multiply ionized states

The dynamics of molecules exposed to intense x-ray radiation involve a large number of multiply ionized and highly excited electronic configurations. To model these dynamics a reliable and efficient electronic structure model is imperative. Employing the Hartree-Fock-Slater electronic structure model in combination with the maximum overlap method, we quantify the associated convergence failures when calculating electronic states of carbon monoxide with multiple vacancies in the core and valence levels. Here, we characterize these cases and describe strategies to overcome the convergence problems. The described techniques not only eliminate all convergence issues for CO but also result in a significant reduction of convergence failures for simulations of the x-ray-induced multiple ionization dynamics of the phenol molecule.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2023

Lattice vibrations and energy landscape of the isoelectronic semiconductor series CuBr, ZnSe, GaAs, and Ge: The special case of CuBr and its $\mathcal{d}$-level chemistry

Here we have examined the lattice vibrations and the energy landscape of the isoelectronic diamond and zincblende semiconductor series CuBr, ZnSe, GaAs, and Ge. Vibrations are found to be an increasing function of ionicity, with the cation sublattice always vibrating more strongly than the anion sublattice. These findings are consistent with density functional theory (DFT) calculations of the energy landscape and temperature-dependent molecular dynamics simulations of the atomic-position fluctuations. For CuBr, inclusion of the Cu 3d Hubbard U term is found necessary to stabilize the zincblende structure and to bring its vibrational amplitudes into agreement with experiment. In addition, vibrations are found to strongly affect the CuBr near-edge x-ray absorption fine structure that we have successfully modeled by including displacements in our theoretical Bethe-Salpeter equation calculations. Reverse Monte Carlo structural refinements using large atomic configurations to simultaneously fit x-ray absorption and x-ray total-scattering data support these conclusions, and they reveal strong Cu-Br first-neighbor correlations and asymmetric distributions of interatomic distances in the temperature ranges of both negative and positive thermal expansion. Delineation of the CuBr valence band photoelectron spectrum into its Cu 3d and Br 4p states uniquely reveals their covalent mixing and further supports the DFT results.

36 MATERIALS SCIENCE↗
NASA NTRS1977

Molecular cluster theory of CO chemisorption on a nickel /100/ surface

Self-consistent Hartree-Fock-Slater molecular cluster models for the chemisorption of carbon monoxide on a (100) transition metal surface are presented. Energy levels and charge distribution for the CO:Ni5 cluster in C sub 4v symmetry are obtained, and the variation of binding energies with height of the CO molecule above the surface of nickel is studied in detail. Comparison is made with experimental binding energy spectra and with the multiple-scattering results of Batra and Bagus. The redistribution in energy of free-atom valence levels is studied by means of local-densities-of-states diagrams.

Ellis, D. E.↗
NASA NTRS1981

Electron spectroscopy of the diamond surface

The diamond surface is studied by ionization loss spectroscopy and Auger electron spectroscopy. For surfaces heated to temperatures not exceeding 900 C, the band gap was found to be devoid of empty states in the absence of electron beam effects. The incident electron beam generates empty states in the band gap and loss of structure in the valence band for these surfaces. A cross section of 1.4 x 10 to the -19th sq cm was obtained for this effect. For surfaces heated to temperatures exceeding 900 C the spectra were identical to those from surfaces modified by the electron beam. The diamond surface undergoes a thermal conversion in its electronic structure at about 900 C.

Pepper, S. V.↗
NASA NTRS1986

Delta excitations and shell-model information in heavy-ion, charge-exchange reactions

We calculate total cross sections for coherent pion production using localized plane-wave approximations for the shell-structure of valence nucleons that are excited to delta particles in the intermediate state in the (12C, 12B) and (12C, 12N) charge-exchange, heavy-ion reactions. We find comparable agreement to projectile downshift data for 12C(12C, 12B)12N. Then we improve the formalism by replacing the localized plane wave bound states with harmonic oscillator states which are imbedded in a multipole expansion approach and calculate pion differential cross sections to test for the sensitivity of the spectra to the single-particle mass parameter.

NASA Program Space Medicine↗
DOE OSTI2023

Absence of mixed valency for Pr in pristine and hole-doped PrNiO 2

Infinite-layer nickelates (𝑅⁢NiO 2 ) exhibit some distinct differences as compared to cuprate superconductors, leading to a debate concerning the role of rare-earth ions (𝑅=La,Pr,Nd) in the low-energy many-body physics. Although rare-earth 4⁢𝑓 orbitals are typically treated as inert “core” electrons in studies, this approximation has been questioned. An active participation of 4⁢𝑓 states is most likely for PrNiO 2 based on an analogy to cuprates where Pr cuprates differ significantly from other cuprates. Here, we adopt density functional plus dynamical mean-field theory to investigate the role of Pr 4⁢𝑓 orbitals and more generally the correlated electronic structure of PrNiO 2 and its hole-doped variant. We find that the Pr 4⁢𝑓 states are insulating and show no evidence for either a Kondo resonance or Zhang-Rice singlet formation as they do not have any hybridization channels near the Fermi energy. The biggest effects of hole doping are to shift the Pr 5⁢𝑑 and 4⁢𝑓 states further away from the Fermi energy whereas enhancing the Ni 3⁢𝑑−O 2⁢𝑝 hybridization, thus, reducing correlation effects as the O 2⁢𝑝 states get closer to the Fermi energy. We again find no evidence for either Kondo or Zhang-Rice physics for the 4⁢𝑓 states upon hole doping. Finally, we conclude by commenting on implications for other reduced valence nickelates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Configuration–Interaction Perturbation Theory Calculations of Pu II

Configuration–interaction perturbation theory (CI–PT) is applied to calculations of low-energy states of Pu II. This ion is quite challenging due to a large number of possible determinants arising from seven valence electrons and strong relativistic effects. The CI–PT calculations agree with experiments for the energies and g-factors for many low-energy states that allowed positive identification of the theoretical levels. Isotope shifts were also used to aid in identification, and, in case of the odd states, fitting with three independent parameters was used to match theoretical isotope shifts to the experimental values with good accuracy. The CI–PT approach tested here on the Pu II ion can be generally used to calculate properties of many complex atoms, including U I that can find application in fundamental and applied science.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2023

Orbital polarization, charge transfer, and fluorescence in reduced-valence nickelates

Herein this paper presents a simple formalism for calculating X-ray absorption (XAS) and resonant inelastic x-ray scattering (RIXS) that has as input orbital-resolved density of states from a single particle or many-body ab initio calculation and is designed to capture itinerant-like features. We use this formalism to calculate both the XAS and RIXS with input from DFT and DFT + DMFT for the recently studied reduced valence nickelates R 4 Ni 3 O 8 and RNiO 2 (R = rare earth), and these results are then contrasted with those for the cuprate CaCuO 2 and the unreduced nickelate R 4 Ni 3 O 10 . In contrast to the unreduced R 4 Ni 3 O 10 , the reduced valence nickelates as well as the cuprate show strong orbital polarization due to the dominance of x 2 – y 2 orbitals for the unoccupied 3d states. We also reproduce two key aspects of a recent RIXS experiment for R 4 Ni 3 O 8 : (i) a charge transfer feature between 3d and oxygen 2p states whose energy we find to decrease as one goes from RNiO 2 to R 4 Ni 3 O 8 to the cuprate, and (ii) an energy-dependent polarization reversal of the fluorescence line that arises from hybridization of the unoccupied 3z 2 –r 2 states with R 5d states. We end with some implications of our results for the nature of the 3d electrons in reduced valence nickelates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗