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At least 523 records · Page 29
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Interplay of Crystal Structure and Magnetic Properties of the Eu 5.08-x Sr x Al 3 Sb 6 Solid Solution

Zintl phases containing rare-earth metals have gained attention due to their magnetic, electronic, and thermoelectric properties. Eu 5.08 Al 3 Sb 6 is a new structure type (monoclinic space group C2/m) that can be described as a pseudorock-salt EuSb motif with the Eu-centered Sb octahedra at the origin of the unit cell, and on the C-face center, containing either Eu (8%) or an Al 4 tetrahedron modeled as a dual tetrahedron (37.5%). The complete solid solution of Eu 5.08–x Sr x Al 3 Sb 6 can be prepared; however, the cation totals vacillate from 5 to 5.24 depending on the Al content. Al K-edge XANES shows a shift to higher energy relative to the Al metal but at slightly lower energy relative to AlSb, indicating an intermediate oxidation state closer to +3 than 0. The lack of an Al K-edge shift with the incorporation of Sr suggests that changes in Sr content do not have a meaningful impact on the electronics of the Al tetrahedra. Investigation of the solid solution structures provides evidence for classifying this structure type as a polar intermetallic phase with variable composition. Magnetization measurements were collected for the solid solution and show complex magnetic ordering with competing ferromagnetic and antiferromagnetic interactions as the Sr content increases.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Visible-Light-Enhanced Cobalt-Catalyzed Hydrogenation: Switchable Catalysis Enabled by Divergence between Thermal and Photochemical Pathways

The catalytic hydrogenation activity of the readily-prepared, coordinatively saturated cobalt(I) precatalyst, (R,R)-( iPr DuPhos)Co(CO) 2 H ((R,R)- iPr DuPhos = (+)-1,2-bis[(2R,5R)-2,5-diisopropylphospholano]benzene) is described. While efficient turnover was observed with a range of alkenes upon heating to 100 ºC, the catalytic performance of the cobalt catalyst was markedly enhanced upon irradiation with blue light at 35 °C. Here, this improved reactivity enabled hydrogenation of terminal, di- and trisubstituted alkenes, alkynes, and carbonyl compounds. A combination of deuterium labeling studies, hydrogenation of alkenes containing radical clocks and experiments probing relative rates support a hydrogen atom transfer pathway under thermal conditions that is enabled by a relatively weak cobalt–hydrogen bond of 56 kcal/mol. In contrast, data for the photocatalytic reactions support light-induced dissociation of a carbonyl ligand followed by a coordination-insertion sequence where the product is released by combination of a cobalt alkyl intermediate with the starting hy-dride, (R,R)-( iPr DuPhos)Co(CO) 2 H. These results demonstrate the versatility with catalysis with Earth-abundant metals as pathways involving open- versus closed-shell intermediates can be switched by the energy source.

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

Effect of Co/SiO 2 Single-Site Heterogeneity on Small Alkane Dehydrogenation Kinetics

Atomically dispersed, high-spin Co(II) atoms in distorted tetrahedral coordination to an amorphous silica (am-SiO 2 ) support, and more recently in zeolite frameworks, are active and selective for light alkane dehydrogenation. This paper investigates how variations in the geometry of the active sites affect the ethane dehydrogenation activity of atomically dispersed Co(II) on an am-SiO 2 support. We generate a distribution of sites and determine the geometric parameters that exhibit the strongest correlation with the coordination geometry and activity of the metal atom by means of linear dimensionality reduction techniques. We perform electronic structure calculations and microkinetic modeling and deduce the mechanism and kinetics for a representative sample of sites. Irrespective of the active site geometry, the rate of ethane dehydrogenation is governed by the β-hydride elimination, which involves quartet-doublet spin-crossing and proceeds adiabatically due to strong spin-orbit coupling. Informed by the complete microkinetic analysis of the sites, we derive the reduced rate expression as a function of three site-dependent quantities. We show that these site-dependent quantities correlate with the energy of formation of the ethyl intermediate that forms via C-H bond activation. This correlation allows us to derive the site-averaged rate for the entire distribution of sites. Among various sites, the tri-coordinate and planar tetra-coordinate Co sites exhibit higher Lewis acidity than the tetrahedral sites, and consequently, higher initial rates. Finally, we discuss the implications for more active catalysts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Silica-copper catalyst interfaces enable carbon-carbon coupling towards ethylene electrosynthesis

Membrane electrode assembly (MEA) electrolyzers offer a means to scale up CO 2 -to-ethylene electroconversion using renewable electricity and close the anthropogenic carbon cycle. To date, excessive CO 2 coverage at the catalyst surface with limited active sites in MEA systems interferes with the carbon-carbon coupling reaction, diminishing ethylene production. With the aid of density functional theory calculations and spectroscopic analysis, here we report an oxide modulation strategy in which we introduce silica on Cu to create active Cu-SiO x interface sites, decreasing the formation energies of OCOH* and OCCOH*—key intermediates along the pathway to ethylene formation. We then synthesize the Cu-SiO x catalysts using one-pot coprecipitation and integrate the catalyst in a MEA electrolyzer. By tuning the CO 2 concentration, the Cu-SiO x catalyst based MEA electrolyzer shows high ethylene Faradaic efficiencies of up to 65% at high ethylene current densities of up to 215 mA cm -2 ; and features sustained operation over 50 h.

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

Tracking the reaction networks of acetaldehyde oxide and glyoxal oxide Criegee intermediates in the ozone-assisted oxidation reaction of crotonaldehyde

The reaction of unsaturated compounds with ozone (O 3 ) is recognized to lead to the formation of Criegee intermediates (CIs), which play a key role in controlling the atmospheric budget of hydroxyl radicals and secondary organic aerosols. The reaction network of two CIs with different functionality, i.e. acetaldehyde oxide (CH 3 CHOO) and glyoxal oxide (CHOCHOO) formed in the ozone-assisted oxidation reaction of crotanaldehyde (CA), is investigated over a temperature range between 390 K and 840 K in an atmospheric pressure jet-stirred reactor (JSR) at a residence time of 1.3 s, stoichiometry of 0.5 with a mixture of 1% crotonaldehyde, 10% O 2 , at an fixed ozone concentration of 1000 ppm and 89% Ar dilution. Molecular-beam mass spectrometry in conjunction with single photon tunable synchrotron vacuum-ultraviolet (VUV) radiation is used to identify elusive intermediates by means of experimental photoionization energy scans and ab initio threshold energy calculations for isomer identification. Addition of ozone (1000 ppm) is observed to trigger the oxidation of CA already at 390 K, which is below the temperature where the oxidation reaction of CA was observed in the absence of ozone. The observed CA + O 3 product, C 4 H 6 O 4 , is found to be linked to a ketohydroperoxide (2-hydroperoxy-3-oxobutanal) resulting from the isomerization of the primary ozonide. Products corresponding to the CIs uni- and bi-molecular reactions were observed and identified. A network of CI reactions is identified in the temperature region below 600 K, characterized by CIs bimolecular reactions with species like aldehydes, i.e., formaldehyde, acetaldehyde, and crotonaldehyde and alkenes, i.e., ethene and propene. The region below 600 K is also characterized by the formation of important amounts of typical low-temperature oxidation products, such as hydrogen peroxide (H 2 O 2 ), methyl hydroperoxide (CH 3 OOH), and ethyl hydroperoxide (C 2 H 5 OOH). Detection of additional oxygenated species such as alcohols, ketene, and aldehydes are indicative of multiple active oxidation routes. This study provides important information about the initial step involved in the CIs assisted oligomerization reactions in complex reactive environments where CIs with different functionalities are reacting simultaneously. It provides new mechanistic insights into ozone-assisted oxidation reactions of unsaturated aldehydes, which is critical for the development of improved atmospheric and combustion kinetics models.

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

On the statistical theory of self-gravitating collisionless dark matter flow: Scale and redshift variation of velocity and density distributions

The statistics of velocity and density fields are crucial for cosmic structure formation and evolution. Here, this paper extends our previous work on the two-point second-order statistics for the velocity field [Phys. Fluids 35, 077105 (2023)] to one-point probability distributions for both density and velocity fields. The scale and redshift variation of density and velocity distributions are studied by a halo-based non-projection approach. First, all particles are divided into halo and out-of-halo particles so that the redshift variation can be studied via generalized kurtosis of distributions for halo and out-of-halo particles, respectively. Second, without projecting particle fields onto a structured grid, the scale variation is analyzed by identifying all particle pairs on different scales $r$. We demonstrate that: (i) Delaunay tessellation can be used to reconstruct the density field. The density correlation, spectrum, and dispersion functions were obtained, modeled, and compared with the N-body simulation; (ii) the velocity distributions are symmetric on both small and large scales and are non-symmetric with a negative skewness on intermediate scales due to the inverse energy cascade on small scales with a constant rate $\varepsilon_u$; (iii) On small scales, the even order moments of pairwise velocity $\Delta u_L$ follow a two-thirds law $\propto{(-\varepsilon_ur)}^{2/3}$, while the odd order moments follow a linear scaling $\langle(\Delta u_L)^{2n+1}\rangle=(2n+1)\langle(\Delta u_L)^{2n}\rangle\langle\Delta u_L\rangle\propto{r}$; (iv) The scale variation of the velocity distributions was studied for longitudinal velocities $u_L$ or $u_L^{'}$, pairwise velocity (velocity difference) $\Delta u_L$=$u_L^{'}$-$u_L$ and velocity sum $\Sigma u_L$=$u^{'}_L$+$u_L$. Fully developed velocity fields are never Gaussian on any scale, despite that they can initially be Gaussian; (v) On small scales, $u_L$ and $\Sigma u_L$ can be modeled by a $X$ distribution to maximize the entropy of the system. The distribution of $\Delta u_L$ can be different; (vi) On large scales, $\Delta u_L$ and $\Sigma u_L$ can be modeled by a logistic or a $X$ distribution, while $u_L$ has a different distribution; (vii) the redshift variation of the velocity distributions follows the evolution of the $X$ distribution involving a shape parameter $\alpha(z)$ decreasing with time.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

Streaming Large-Scale Microscopy Data to a Supercomputing Facility

Data management is a critical component of modern experimental workflows. As data generation rates increase, transferring data from acquisition servers to processing servers via conventional file-based methods is becoming increasingly impractical. The 4D Camera at the National Center for Electron Microscopy generates data at a nominal rate of 480 Gbit s -1 (87,000 frames s -1 ⁠), producing a 700 GB dataset in 15 s. To address the challenges associated with storing and processing such quantities of data, we developed a streaming workflow that utilizes a high-speed network to connect the 4D Camera’s data acquisition system to supercomputing nodes at the National Energy Research Scientific Computing Center, bypassing intermediate file storage entirely. In this work, we demonstrate the effectiveness of our streaming pipeline in a production setting through an hour-long experiment that generated over 10 TB of raw data, yielding high-quality datasets suitable for advanced analyses. Additionally, we compare the efficacy of this streaming workflow against the conventional file-transfer workflow by conducting a postmortem analysis on historical data from experiments performed by real users. Our findings show that the streaming workflow significantly improves data turnaround time, enables real-time decision-making, and minimizes the potential for human error by eliminating manual user interactions.

4D-STEM↗
DOE OSTI2021

Nuclear Data and Cross Section Testing Using ENDF/B-VIII.0

With the release of the Evaluated Nuclear Data File (ENDF)/B-VIII.0 library, nuclear criticality safety practitioners and engineers have access to the latest cross section sets available for their analyses. However, these cross sections must be rigorously tested and validated to ensure that the nuclear data are responsive to the needs of the individuals responsible for developing, implementing, and maintaining computational tools for criticality safety applications. Thus, the ENDF/B-VIII.0 library is tested and validated with a large collection of experiments that were vetted by the International Criticality Safety Benchmark Evaluation Project and made available in the International Handbook of Evaluated Criticality Safety Benchmark Experiments. A selection of benchmark experiments for use within the criticality safety community were prepared and reviewed within the Verified, Archived Library of Inputs and Data (VALID), which is maintained by the Nuclear Energy and Fuel Cycle Division at Oak Ridge National Laboratory. The performance of the ENDF/B-VIII.0 library is assessed by using VALID models of benchmark experiments with the beta 12 version of SCALE 6.3 KENO V.a and KENO-VI Monte Carlo codes. The performance is compared with the results obtained from with the ENDF/B-VII.1 library. This report considers multigroup (MG) and continuous energy (CE) formats of the ENDF/B-VIII.0 and -VII.1 libraries. The benchmark experiments within VALID that validate the ENDF/B-VIII.0 library cover 15 broad system categories by using a range of fissile materials, uranium enrichments, plutonium isotopic vectors, and mixed uranium/plutonium systems. These forms are represented as metals, solutions, or various arrays of rods or plates that cover a variety of neutron energy spectra: thermal, fast, mixed, and intermediate. Over 600 cases were considered for use with the KENO V.a and KENO-VI codes with the ENDF/B-VIII.0 library. The results of the Monte Carlo comparison of ENDF/B-VIII.0 to ENDF/B-VII.1 with both KENO V.a and KENO-VI indicate that there is a less than 0.53% Δk difference between the bias of calculated k eff from the expected values. The CE ENDF/B-VIII.0 library results in smaller magnitude biases than the ENDF/B-VII.1 data for HEU-MET-FAST, HEU-SOL-THERM, IEU-MET-FAST, LEU-SOL-THERM, PU-SOL-THERM, and U233-MET-FAST systems, while the MG results yielded smaller magnitude biases for HEU-MET-FAST, HEU-SOL-THERM, IEU-MET-FAST, LEU-COMP-THERM, LEU-SOL THERM, MIX-COMP-FAST, and U233-MET-FAST systems. Most notable are the adjustments to the plutonium and 233 U cross section data, which has resulted in noticeably lower biases in the ENDF/B VIII.0 results for the mixed, plutonium, and 233 U systems. Results of the sensitivity data file comparison generated from TSUNAMI-3D for selected VALID cases for the ENDF/B-VIII.0 library indicate a very high level of agreement with correlation coefficients of the effect of nuclear data uncertainty on k eff (the c k integral parameter) all above 0.99. This indicates that cases with the ENDF/B-VIII.0 library would see very similar responses to any nuclear data errors or change as those with the ENDF/B-VII.1 library.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2021

Ultracool Dwarfs Observed with the Spitzer Infrared Spectrograph. I. An Accurate Look at the L-to-T Transition at ~300 Myr from Optical Through Mid-infrared Spectrophotometry

Here we present Spitzer IRS 5–14 μm spectra and 16 μm and 22 μm photometry of the T2.5 companion to the ~300 Myr old G0V star HN Peg. We incorporate previous 0.8–5 μm observations to obtain the most comprehensive spectral energy distribution (SED) of an intermediate-gravity L/T-transition dwarf that, together with an accurate Gaia EDR3 parallax of the primary, enables us to derive precise fundamental parameters. We find that young (≈0.1–0.3 Gyr) early-T dwarfs on average have ≈140 K lower effective temperatures, ≈20% larger radii, and similar bolometric luminosities compared to ≳1 Gyr old field dwarfs with similar spectral types. Our accurate infrared spectrophotometry offers new detail at wavelengths where the dominant carbon-bearing molecules have their strongest transitions: at 3.4 μm for methane and at 4.6 μm for carbon monoxide. We assess the performance of various widely available photospheric models and find that models with condensates and/or clouds better reproduce the full SED of this moderately young early-T dwarf. However, cloud-free models incorporating a more general convective instability treatment reproduce at least the low-resolution near-infrared spectrum similarly well. Our analysis of R ≈ 2300 J-band spectra shows that the near-infrared potassium absorption lines in HN Peg B have similar strengths to those seen in both younger and older T2–T3 dwarfs. We conclude that while alkali lines are well established as surface gravity indicators for L-type or warmer stars, they are insensitive to surface gravity in early-T dwarfs.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1976

On build-up of magnetic energy in the solar atmosphere

The dynamic response of the solar atmosphere is examined with the use of self-consistent numerical solutions to the complete set of nonlinear two-dimensional hydromagnetic equations. Of particular interest are the magnetic-energy buildup and the velocity field established by emerging flux at the base of an existing magnetic loop structure in a stationary atmosphere. For a plasma with a relatively low beta (0.03), the magnetic-energy buildup is approximately twice that of the kinetic energy, while the buildup in magnetic energy first exceeds but is eventually overtaken by the kinetic energy for a plasma with an intermediate beta (3). The increased magnetic flux causes the plasma to flow upward near the loop center and downward near the loop edges for the low-beta plasma. The plasma eventually flows downward throughout the lower portion of the loop carrying the magnetic field with it for the intermediate beta plasma. It is hypothesized that this latter case, and possibly the other case as well, may provide a reasonable simulation of the disappearance of prominences by flowing down into the chromosphere (a form of disparition brusque).

Nakagawa, Y.↗
NASA NTRS1988

Accretion shock geometries in the magnetic variables

The first self consistent shock models for the AM Herculis-type systems successfully identified the dominant physical processes and their signatures. These homogenous shock models predict unpolarized, Rayleigh-Jeans optical spectra with sharp cutoffs and rising polarizations as the shocks become optically thin in the ultraviolet. However, the observed energy distributions are generally flat with intermediate polarizations over a broad optical band. These and other observational evidence support a non-homogenous accretion profile which may extend over a considerable fraction of the stellar surface. Both the fundamental assumptions underlying the canonical 1-D shock model and the extension of this model to inhomogenous accretion shocks were identified, for both radial and linear structures. The observational evidence was also examined for tall shocks and little evidence was found for relative shock heights in excess of h/R(1) greater than or equal to 0.1. For several systems, upper limits to the shock height can be obtained from either x ray or optical data. These lie in the region h/R(1) is approximately 0.01 and are in general agreement with the current physical picture for these systems. The quasi-periodic optical variations observed in several magnetic variables may eventually prove to be a major aid in further understanding their accretion shock geometries.

Stockman, H. S.↗
NASA NTRS1991

Composite Struts Would Damp Vibrations

New design of composite-material (fiber/matrix laminate) struts increases damping of longitudinal vibrations without decreasing longitudinal stiffness or increasing weight significantly. Plies with opposing chevron patterns of fibers convert longitudinal vibrational stresses into shear stresses in intermediate viscoelastic layer, which dissipate vibrational energy. Composite strut stronger than aluminum strut of same weight and stiffness.

Dolgin, Benjamin P.↗
NASA NTRS1991

Development of an electrostatic propulsion engine using sub-micron powders as the reaction mass

Asteroid sample return missions would benefit from development of an improved rocket engine. Chemical rockets achieve their large thrust with high mass consumption rate (dm/dt) but low exhaust velocity; therefore, a large fraction of their total mass is fuel. Present day ion thrusters are characterized by high exhaust velocity, but low dm/dt; thus, they are inherently low thrust devices. However, their high exhausy velocity is poorly matched to typical mission requirements and therefore, wastes energy. A better match would be intermediate between the two forms of propulsion. This could be achieved by electrostatically accelerating solid powder grains, raising the possibility that interplanetary material could be processed to use as reaction mass. An experiment to study the charging properties of sub-micron sized powder grains is described. If a suitable material can be identified, then it could be used as the reaction mass in an electrostatic propulsion engine. The experiment employs a time of flight measurement to determine the exhaust velocity (v) of various negatively charged powder grains that were charged and accelerated in a simple device. The purpose is to determine the charge to mass ratio that can be sustained for various substances. In order to be competitive with present day ion thrusters, a specific impulse (v/g) of 3000 to 5000 seconds is required. Preliminary results are presented. More speculatively, there are some mission profiles that would benefit from collection of reaction mass at the remote asteroid site. Experiments that examine the generation of sub-micron clusters by electrostatic self-disruption of geologically derived material are planned.

Herbert, F.↗
NASA NTRS1991

Energy Models for One-Carrier Transport in Semiconductor Devices

Moment models of carrier transport, derived from the Boltzmann equation, made possible the simulation of certain key effects through such realistic assumptions as energy dependent mobility functions. This type of global dependence permits the observation of velocity overshoot in the vicinity of device junctions, not discerned via classical drift-diffusion models, which are primarily local in nature. It was found that a critical role is played in the hydrodynamic model by the heat conduction term. When ignored, the overshoot is inappropriately damped. When the standard choice of the Wiedemann-Franz law is made for the conductivity, spurious overshoot is observed. Agreement with Monte-Carlo simulation in this regime required empirical modification of this law, or nonstandard choices. Simulations of the hydrodynamic model in one and two dimensions, as well as simulations of a newly developed energy model, the RT model, are presented. The RT model, intermediate between the hydrodynamic and drift-diffusion model, was developed to eliminate the parabolic energy band and Maxwellian distribution assumptions, and to reduce the spurious overshoot with physically consistent assumptions. The algorithms employed for both models are the essentially non-oscillatory shock capturing algorithms. Some mathematical results are presented and contrasted with the highly developed state of the drift-diffusion model.

Jerome, Joseph W.↗
NASA NTRS1993

IRAS 21391 + 5802 - A study in intermediate mass star formation

We present infrared and millimeter wavelength observations of the cold IRAS source 21391 + 5802 and its associated molecular core. Infrared observations at lambda = 3.5 microns reveal a heavily obscured, central point source which is coincident with a compact lambda = 2.7 mm continuum and C18O emission region. The source radiates about 310 solar luminosities, primarily at FIR wavelengths, suggesting that it is a young stellar object of intermediate mass. The steeply rising spectral energy distribution and the large fraction of the system mass residing in circumstellar material imply that IRAS 21391 + 5802 is in an early stage of evolution. The inferred dust temperature indicates a temperature gradient in the core. A comprehensive model for the surrounding core of dust and gas is devised to match the observed dust continuum emission and multitransition CS emission from this and previous studies. We find a r exp -1.5 +/- 0.2 density gradient consistent with that of a gravitationally evolved core and a total core mass of 380 solar masses. The observed dust emission is most consistent with a lambda exp -1.5 - lambda exp -2 dust emissivity law; for a lambda exp -2 law, the data are best fit by a mass opacity coefficient of 3.6 x 10 exp -3 sq cm/g at lambda = 1.25 mm.

Wilking, Bruce↗
NASA NTRS1994

Proposed law of nature linking impacts, plume volcanism, and Milankovitch cycles to terrestrial vertebrate mass extinctions via greenhouse-embryo death coupling

A greenhouse-physiological coupling killing mechanism active among mammals, birds, and reptiles has been identified. Operating via environmental thermal effects upon the maternal core-skin blood flow critical to the survival and development of embryos, it reduces the flow of blood to the uterine tract. Today, during hot summers, this phenomena kills embryos on a vast, global scale. Because of sensitivity of many mammals to modern heat, a major modern greenhouse could reduce population numbers on a global scale, and potentially trigger population collapses in the more vulnerable parts of the world. In the geological past, the killing mechanism has likely been triggered into action by greenhouse warming via impact events, plume volcanism, and Earth orbital variations (Milankovitch cycles). Earth's biosphere is maintained and molded by the flow of energy from the solar energy source to Earth and on to the space energy sink (SES). This SES energy flow maintains Earth's biosphere and its living components, as open, intermediate, dissipative, nonequilibrium systems whose states are dependent upon the rate of energy flowing through them. Greenhouse gases such as CO2 in the atmosphere influence the SES energy flow rate. Steady-state flow is necessary for global ecological stability (autopoiesis). Natural fluctuations of the C cycle such as rapid releases of CO2 from the mantle, or oceans, disrupt steady-state SES flow. These fluctuations constantly challenge the biosphere; slowdown of SES energy flow drives it toward thermodynamical equilibrium and stagnation. Fluctuations induced by impact event, mantle plume volcanism, and Milankovitch cycles can grow into structure-breaking waves triggering major perturbations of Earth's C cycle and mass extinctions. A major C cycle perturbation involves readjustment of the outer physiochemical spheres of the Earth: the atmosphere, hydrosphere, and lithosphere; and by necessity, the biosphere. A greenhouse, one manifestation of a major C cycle perturbation, is the most dangerous natural phenomenon that life on Earth can experience. Greenhouse conditions existed during the KT mass extinctions of 65 m.y. ago, and the Pleistocene-Holocene (P-H) mammalian extinctions of 10,000-12,000 yr ago. Coupling climatology to reproductive physiology via effects of ambient air temperature upon uterine blood flow to developing embryos accounts for the extinctions via established physiological principles.

Mclean, D. M.↗
NASA NTRS2010

Implication of the Observable Spectral Cutoff Energy Evolution in XTE J1550-564

The physical mechanisms responsible for production of the non-thermal emission in accreting black holes should be imprinted in the observational appearances of the power law tails in the X-ray spectra from these objects. Variety of spectral states observed from galactic black hole binaries by it Rossi X-ray Timing Explorer (RXTE) allow examination of the photon upscattering under different accretion regimes. We revisit of RXTE data collected from the black hole X-ray binary XTE J1550-564 during two periods of X-ray activity in 1998 and 2000 focusing on the behavior of the high energy cutoff of the power law part of the spectrum. For the 1998 outburst the Iran- sition from the low-hard state to the intermediate state was accompanied by a gradual decrease in the cutoff energy which then showed a sharp reversal to a clear increasing trend during the further evolution towards the very high and high-soft states. However, the 2000 outburst showed only the decreasing part of this pattern. Notably, the photon indexes corresponding to the cutoff increase for the 1998 event are much higher than the index values reached during the 2000 rise transition. We attribute this difference in the cutoff' energy behav- for to the different partial contributions of the thermal and non-thermal (bulk motion) Comptonization in photon upscattering. Namely, during the 1998 event the higher accretion rate presumably provided more cooling to the Comptonizing media and thus reducing the effectiveness of the thermal upscattering process. Under these conditions the bulk motion takes a leading role in boosting the input soft photons. Monte Carlo simulations of the The physical mechanisms responsible for production of the non-thermal emission in accreting black holes should be imprinted in the observational apperances of the power law tails in the X-ray spectra from these objects. Variety of spectral states observed from galactic black hole binaries by it Rossi X-ray Timing Explorer (RXTE) allow examination of the photon upscattering under different accretion regimes. We revisit of RXTE data collected from the black hole X-ray binary XTE J1550-564 during two periods of X-ray activity in 1998 and 2000 focusing on the behavior of the high energy cutoff of the power law part of the spectrum. For the 1998 outburst the Iran- sition from the low-hard state to the intermediate state was accompanied by a gradual decrease in the cutoff energy which then showed a sharp reversal to a clear increasing trend during the further evolution towards the very high and high-soft states. However, the 2000 outburst showed only the decreasing part of this pattern. Notably, the photon indexes corresponding to the cutoff increase for the 1998 event are much higher than the index values reached during the 2000 rise transition. We attribute this difference in the cutoff' energy behav- for to the different partial contributions of the thermal and non-thermal (bulk motion) Comptonization in photon upscattering. Namely, during the 1998 event the higher accretion rate presumably provided more cooling to the Comptonizing media and thus reducing the effectiveness of the thermal upscattering process. Under these conditions the bulk motion takes a leading role in boosting the input soft photons. Monte Carlo simulations of the Comptonization in a bulk motion region near an accreting black hole by Laurent & Titarchuk (2010) strongly support this scenario. strongly support this scenario

Titarchuk, Lev↗
DOE OSTI2025

Resolution of Selectivity Steps of CO Reduction Reaction on Copper by Quantum Monte Carlo

Electrochemical reduction of carbon monoxide to valuable fuels and chemicals on copper surfaces remains a challenging area in catalysis due to a limited understanding of adsorption mechanisms and reaction pathways. Although density functional theory (DFT)-based studies have investigated these processes, their accuracy varies across different functionals. Here, in this study, we present the application of fixed-node diffusion Monte Carlo (FNDMC) to benchmark the adsorption energies of CO*, H*, and key CO reduction reaction (CORR) intermediates, COH* and CHO* on the Cu(111) surface. Our results for CO* and H* adsorption energies closely align with experimentally measured chemisorption reactions, highlighting the limitations of DFT and providing site-specific energy comparisons that are often not available experimentally. Additionally, we explore the effect of explicit solvation, demonstrating how water stabilizes the COH* over CHO*, thus suggesting a critical role of COH* in CORR. Finally, we release our high-accuracy FNDMC benchmarks for testing and developing new DFT functionals for electrocatalysis. Overall, this study underscores the potential of FNDMC for detailed surface chemistry studies and offers new insights into catalytic processes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗