Search NASA⌕ Search

SEARCH · Search NASA

Results for “dynamic sampling”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 505 records · Page 28
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Multiscale simulation of the ground and photo-induced charge-separated states of a molecular triad in polar organic solvent: Exploring the conformations, fluctuations, and free energy landscapes

Here, the approach of a multiscale simulation that combines quantum chemical calculations, classical molecular dynamics simulations, and statistical physics to integrate the information of the electronic states of a conformationally complex molecule into its structural distribution over an ensemble was performed to understand the influence of a polar organic solvent on the structural stability of carotene-porphyrin-fullerene molecular triad in both the ground and the photoinduced charge-separated excited states. The excited states of the triad were computed with the ab initio quantum chemical calculations using the algebraic diagrammatic construction through the second order correction (ADC(2)) method and the time-dependent density functional theory (TDDFT). The replica exchange molecular dynamics was used for the enhanced sampling of the ensemble in order to explore the phase space of the ground state and the photoinduced charge-separated excited state of triad in explicit tetrahydrofuran (THF) solvent. An importance sampling was strategically employed to bridge the gap between the two computational methods that aim to explore distinct realms of dynamics. We analyzed the free energy landscape, the structural fluctuations, the solvent arrangements, the static dielectric constant, and the interactions between the triad and the solvent molecules. The analysis of the free energy landscape of the triad indicates that the charge-separated excited state of the triad is thermodynamically stable in a linearly extended geometry, while the ground-state triad explores several extended and bent conformations that are populated in the local free energy minima separated by low free energy barriers at an order of thermal fluctuation (kBT). The stabilization of a linearly extended structure of the charge-separated excited state triad is predominantly due to the solvation interactions (van der Waals and electrostatic interactions) between the triad and the THF molecules as well as the interactions within the THF molecules. Differences in the charge distribution on a molecular triad induce slight changes in the dielectric property of THF near the triad. Our study suggests that by measuring the differences in a dielectric response of solvent near the triad, it is possible to provide insight into the population of the charge-separated electronic state of the triad relative to that of the ground state.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2025

A Flat Analyzer Polycapillary Spectrometer for X-ray Absorption Spectroscopy of Dilute Transition Metals at X-ray-Free Electron Lasers

X-ray absorption spectroscopy (XAS) coupled with highly intense pulses from an X-ray-free electron laser (XFEL) can be used to track ultrafast chemical dynamics. Nonetheless, measurements for dilute samples (≤1 mM) have been exceptionally challenging, as scattering background signals dominate over the sample’s X-ray fluorescence. We show that femtosecond time-resolved XAS measurements of sub-millimolar transition metal solutions are now possible at the Linac Coherent Light Source (LCLS) using a high-throughput polycapillary XAS spectrometer, designed, developed, and commissioned at LCLS and the Stanford Synchrotron Radiation Lightsource (SSRL). The instrument integrates three polycapillary optics that collect and collimate X-ray fluorescence emitted from the interaction point with a high solid angle. X-ray collimated fluorescence is then selectively diffracted by coupled graphite analyzer crystals. As a result, the contribution of scattered photons is suppressed. Experiments at the Pt L 3 -edge on 0.1 mM aqueous K 2 PtCl 6 (delivered via a 100 μm liquid jet) were successfully performed in the laser pump X-ray probe configuration at the LCLS XCS and XPP instruments. We report the transient spectra of hexachloroplatinate within the first 10 ps after a 266 nm photoexcitation. We observed a short-lived reduced intermediate (≈2 ps). The polycapillary X-ray spectrometer at LCLS now enables the efficient study of the dynamics of ultradilute transition metals in solution. This capability opens the door to investigating plasmonic systems, photocatalysts, enzymes, and other scarce and dilute samples.

LCLS↗
DOE OSTI2024

Modeling bicarbonate formation in an alkaline solution with multi-level quantum mechanics/molecular dynamics simulations

Understanding carbonate speciation and how it may be modulated is essential for the advancement of carbon dioxide (CO 2 ) capture and storage technologies, which often rely on the transformation of CO 2 into carbonate, e.g. via the formation of carbonate minerals. To date, few atomic-level, quantum-mechanics-based simulations have been carried out to characterize how carbonic acid (H 2 CO 3 ) and bicarbonate ($HCO^{-}_{3}$) form in aqueous solution, and how pH affects this process. Recently, Martirez and Carter utilized rare-event sampling density functional theory molecular dynamics simulations in combination with multi-level embedded correlated wavefunction theory, thus accounting for both solvent dynamics and electron correlation accurately, to elucidate the mechanism of H 2 CO 3 formation in neutral solution (J. Am. Chem. Soc., 145, 12561, 2023). Here, we perform a complementary simulation using the same method to map out the energetics of $HCO^{-}_{3}$ formation from dissolved CO 2 in basic solution. We find that, as in H 2 CO 3 formation, including water dynamics is important to obtain an accurate prediction of the energetics for the aforementioned reaction. Furthermore, only with MD did we identify the correct pathway for the reaction, in which water – not hydroxide – acts as the initial nucleophile and only at the transition state does it lose a proton.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2024

Advanced manufacturing provides tailor-made solutions for crystallography with x-ray free-electron lasers

Serial crystallography at large facilities, such as x-ray free-electron lasers and synchrotrons, evolved as a powerful method for the high-resolution structural investigation of proteins that are critical for human health, thus advancing drug discovery and novel therapies. However, a critical barrier to successful serial crystallography experiments lies in the efficient handling of the protein microcrystals and solutions at microscales. Microfluidics are the obvious approach for any high-throughput, nano-to-microliter sample handling, that also requires design flexibility and rapid prototyping to deal with the variable shapes, sizes, and density of crystals. Here, we discuss recent advances in polymer 3D printing for microfluidics-based serial crystallography research and present a demonstration of emerging, large-scale, nano-3D printing approaches leading into the future of 3D sample environment and delivery device fabrication from liquid jet gas-dynamic virtual nozzles devices to fixed-target sample environment technology.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Imaging the Thickness of Passivation Layers for Crystalline Silicon with Micron‐Scale Spatial Resolution Using Spectral Photoluminescence

Thin films deposited on crystalline silicon surfaces are critical components of many silicon‐based devices, including high‐efficiency photovoltaic cells. Knowledge of their optical and electronic properties is important in designing and fabricating efficient solar cells, especially as the device complexity increases and includes micron‐scale features. Here, a camera‐based photoluminescence (PL) technique is developed to image the thickness of specific passivating films on silicon wafers. The technique is entirely contactless and nondestructive, requires minimal sample preparation, and provides a large dynamic range in terms of field of view, with micron‐scale spatial resolution possible. It retains the advantages of PL imaging, a mainstream technique for photovoltaic material and device characterization, and it has some extra capabilities similar to those of ellipsometry. This newly developed technique is demonstrated on hydrogenated amorphous silicon and silicon nitride films, two important passivation technologies for high‐efficiency silicon solar cells.

Nguyen, Hieu T.↗
DOE OSTI2011

High temperature millimeter wave radiometric and interferometric measurements of slag-refractory interaction for application to coal gasifiers

Millimeter wave (MMW) radiometry can be used for simultaneous measurement of emissivity and temperature of materials under extreme environments (high temperature, pressure, and corrosive environments) such as in slagging coal gasifiers, where sensors have been identified as a key enabling technology need for process optimization. We present a state-of-the-art dual-channel MMW heterodyne radiometer with active interferometric capability that allows simultaneous radiometric measurements of sample temperature, emissivity, and flow dynamics to over 1873 K. Interferometric capability is supplied via a probe signal originating from the 137 GHz radiometer local oscillator (LO). The interferometric 'video' channels allow measurement of additional parameters simultaneously, such as volume expansion, thickness change, and slag viscosity along with temperature or emissivity. This capability has been used to demonstrate measurement of temperature and simulated coal slag infiltration into a chromia refractory brick sample as well as slag flow down a vertically placed refractory brick. Observed phenomena include slag melting and slumping, slag reboil and foam with oxygen evolution, and eventual failure of the alumina crucible through corrosion by the molten slag. These results show the promise of the MMW system for extracting quantitative and qualitative process parameters from operating slagging coal gasifiers, providing valuable information for process efficiency, control, and increased productivity.

01 COAL, LIGNITE, AND PEAT↗
DOE OSTI2020

Design and implementation of a J -coupled spectrometer for multidimensional structure and relaxation detection at low magnetic fields

In recent years, it has been realized that low and ultra-low field (mT-nT magnetic field range) nuclear magnetic resonance (NMR) spectroscopy can be used for molecular structural analysis. However, spectra are often hindered by lengthy acquisition times or require large sample volumes and high concentrations. Here, we report a low field (50 µT) instrument that employs a linear actuator to shuttle samples between a 1 T prepolarization field and a solenoid detector in a laboratory setting. The current experimental setup is benchmarked using water and 13 C-methanol with a single scan detection limit of 2x10 20 spins (3 μL, 55 M H 2 O) and detection of 2.9x10 19 (200 μL, 617 mM 13 C-methanol) spins with signal averaging. The system has a dynamic range of >3 orders of magnitude. Investigations of room-temperature relaxation dynamics of 13Cmethanol show that sample dilution can be used in lieu of sample heating to acquire spectra with linewidths comparable to high-temperature spectra. These results indicate that the T 1 and T 2 mechanisms are governed by both the proton exchange rate and the dissolved oxygen in the sample. Finally, a 2D correlation spectroscopy (COSY) experiment is reported, performed in the strong coupling regime that resolves the multiple resonances associated with the heteronuclear Jcoupling. The spectrum was collected using 10 times less sample and in less than half the time from previous reports in the strong coupling limit.

47 OTHER INSTRUMENTATION↗
DOE OSTI2024

Studying the Collective Functional Response of a Receptor in Alchemical Ligand Binding Free Energy Simulations with Accelerated Solvation Layer Dynamics

Ligand binding free energy simulations (LB-FES) that involve sampling of protein functional conformations have been longstanding challenges in research on molecular recognition. Particularly, modeling of the conformational transition pathway and design of the heuristic biasing mechanism are severe bottlenecks for the existing enhanced configurational sampling (ECS) methods. Inspired by the key role of hydration in regulating conformational dynamics of macromolecules, this report proposes a novel ECS approach that facilitates binding-associated structural dynamics by accelerated hydration transitions in combination with the lambda-exchange of free energy perturbation (FEP). Further, two challenging protein-ligand binding processes involving large configurational transitions of the receptor are studied, with hydration transitions at binding sites accelerated by Hamiltonian-simulated annealing of the hydration layer. Without the need for pathway analysis or ad hoc barrier flattening potential, LB-FES were performed with FEP/lambda-exchange molecular dynamics simulation at a minor overhead for annealing of the hydration layer. The LB-FES studies showed that the accelerated rehydration significantly enhances the collective conformational transitions of the receptor, and convergence of binding affinity calculations is obtained at a sweet-spot simulation time scale. Alchemical LB-FES with the proposed ECS strategy is free from the effort of trial and error for the setup and realizes efficient on-the-fly sampling for the collective functional response of the receptor and bound water and therefore presents a practical approach to high-throughput screening in drug discovery.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Quantum mechanical closure of partial differential equations with symmetries

We develop a statistical framework for the dynamical closure of spatiotemporal dynamics governed by partial differential equations. Employing the mathematical framework of quantum mechanics to embed the original classical dynamics into a quantum mechanical representation, we use the space of quantum density operators to model the unresolved degrees of freedom of the original dynamics in a statistical sense, and the framework of quantum measurement to predict their contributions to the resolved dynamics. The embedded dynamics is discretized by a positivity preserving process, leading to a compressed representation that is invariant under the dynamical symmetries of the resolved dynamics. We present a data based formulation of the closure scheme and apply it to a closure problem for the shallow water equations. The numerical results demonstrate that our closure model can accurately predict the main features of the true dynamics, including for out of sample initial conditions.

Delay embedding↗
DOE OSTI2015

Measuring consistent masses for 25 Milky Way globular clusters

We present central velocity dispersions, masses, mass-to-light ratios (M/Ls ), and rotation strengths for 25 Galactic globular clusters (GCs). We derive radial velocities of 1951 stars in 12 GCs from single order spectra taken with Hectochelle on the MMT telescope. To this sample we add an analysis of available archival data of individual stars. For the full set of data we fit King models to derive consistent dynamical parameters for the clusters. We find good agreement between single-mass King models and the observed radial dispersion profiles. The large, uniform sample of dynamical masses we derive enables us to examine trends of M/L with cluster mass and metallicity. The overall values of M/L and the trends with mass and metallicity are consistent with existing measurements from a large sample of M31 clusters. This includes a clear trend of increasing M/L with cluster mass and lower than expected M/Ls for the metal-rich clusters. We find no clear trend of increasing rotation with increasing cluster metallicity suggested in previous work.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Role of atomistic structure in the stochastic nature of conductivity in substoichiometric tantalum pentoxide

First-principles calculations of electrical conductivity (σ{sub o}) are revisited to determine the atomistic origin of its stochasticity in a distribution generated from sampling 14 ab-initio molecular dynamics configurations from 10 independently quenched models (n = 140) of substoichiometric amorphous Ta{sub 2}O{sub 5}, where each structure contains a neutral O monovacancy (V{sub O}{sup 0}). Structural analysis revealed a distinct minimum Ta-Ta separation (dimer/trimer) corresponding to each V{sub O}{sup 0} location. Bader charge decomposition using a commonality analysis approach based on the σ{sub o} distribution extremes revealed nanostructural signatures indicating that both the magnitude and distribution of cationic charge on the Ta subnetwork have a profound influence on σ{sub o}. Furthermore, visualization of local defect structures and their electron densities reinforces these conclusions and suggests σ{sub o} in the amorphous oxide is best suppressed by a highly charged, compact Ta cation shell that effectively screens and minimizes localized V{sub O}{sup 0} interaction with the a-Ta{sub 2}O{sub 5} network; conversely, delocalization of V{sub O}{sup 0} corresponds to metallic character and high σ{sub o}. The random network of a-Ta{sub 2}O{sub 5} provides countless variations of an ionic configuration scaffold in which small perturbations affect the electronic charge distribution and result in a fixed-stoichiometry distribution of σ{sub o}; consequently, precisely controlled and highly repeatable oxide fabrication processes are likely paramount for advancement of resistive memory technologies.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Characterization and flux assessment of airborne phthalates released from polyvinyl chloride consumer goods

Highlights: • Enhanced levels of semi-volatile organic compounds (SVOCs) can affect human health. • The major SVOCs like phthalates are released into indoor by many sources like plasticizers. • This study investigated the emission characteristics of diverse phthalates from plastic/rubber products. • Emission behavior of phthalates is described in relation to several key variables, e.g., types of PVC. The concentrations and fluxes of airborne phthalates were measured from five types of polyvinyl chloride (PVC) consumer products (vinyl flooring, wallcovering, child's toy, yoga mat, and edge protector) using a small chamber (impinger) system. Airborne phthalates released from each of those PVC samples were collected using sorbent (Tenax TA) tubes at three temperature control intervals (0, 3, and 6 h) under varying temperature conditions (25, 40, and 90 °C). A total of 11 phthalate compounds were quantified in the five PVC products examined in this study. To facilitate the comparison of phthalate emissions among PVC samples, their flux values were defined for total phthalates by summing the average fluxes of all 11 phthalates generated during the control period of 6 h. The highest flux values were seen in the edge protector sample at all temperatures (0.40 (25 °C), 9.65 (40 °C), and 75.7 μg m{sup −2} h{sup −1} (90 °C)) of which emission was dominated by dibutyl isophthalate. In contrast, the lowest fluxes were found in wallcovering (0.01 (25 °C) and 0.05 μg m{sup −2} h{sup −1} (40 °C)) and child's toy (0.23 μg m{sup −2} h{sup −1} (90 °C)) at each temperature level. The information regarding phthalate composition and emission patterns varied dynamically with type of PVC sample, controlled temperature, and duration of control.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2023

Benchmarking Quantum Simulators Using Ergodic Quantum Dynamics

Here we propose and analyze a sample-efficient protocol to estimate the fidelity between an experimentally prepared state and an ideal target state, applicable to a wide class of analog quantum simulators without advanced spatiotemporal control. Our protocol relies on universal fluctuations emerging from generic Hamiltonian dynamics, which we discover in the present work. It does not require fine-tuned control over state preparation, quantum evolution, or readout capability, while achieving near optimal sample complexity: a percent-level precision is obtained with ~103 measurements, independent of system size. Furthermore, the accuracy of our fidelity estimation improves exponentially with increasing system size. We numerically demonstrate our protocol in a variety of quantum simulator platforms, including quantum gas microscopes, trapped ions, and Rydberg atom arrays. We discuss applications of our method for tasks such as multiparameter estimation of quantum states and processes.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Probing ultrafast C–Br bond fission in the UV photochemistry of bromoform with core-to-valence transient absorption spectroscopy

UV pump–extreme UV (XUV) probe femtosecond transient absorption spectroscopy is used to study the 268 nm induced photodissociation dynamics of bromoform (CHBr3). Core-to-valence transitions at the Br(3d) absorption edge (∼70 eV) provide an atomic scale perspective of the reaction, sensitive to changes in the local valence electronic structure, with ultrafast time resolution. The XUV spectra track how the singly occupied molecular orbitals of transient electronic states develop throughout the C–Br bond fission, eventually forming radical Br and CHBr2 products. Complementary ab initio calculations of XUV spectral fingerprints are performed for transient atomic arrangements obtained from sampling excited-state molecular dynamics simulations. C–Br fission along an approximately CS symmetrical reaction pathway leads to a continuous change of electronic orbital characters and atomic arrangements. Two timescales dominate changes in the transient absorption spectra, reflecting the different characteristic motions of the light C and H atoms and the heavy Br atoms. Within the first 40 fs, distortion from C3v symmetry to form a quasiplanar CHBr2 by the displacement of the (light) CH moiety causes significant changes to the valence electronic structure. Displacement of the (heavy) Br atoms is delayed and requires up to ∼300 fs to form separate Br + CHBr2 products. We demonstrate that transitions between the valence-excited (initial) and valence + core-excited (final) state electronic configurations produced by XUV absorption are sensitive to the localization of valence orbitals during bond fission. The change in valence electron-core hole interaction provides a physical explanation for spectral shifts during the process of bond cleavage.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2015

Ferroelectric Switching by the Grounded Scanning Probe Microscopy Tip

The process of polarization reversal by the tip of scanning probe microscope was intensively studied for last two decades. Number of the abnormal switching phenomena was reported by the scientific groups worldwide. In particularly it was experimentally and theoretically shown that slow dynamics of the surface screening controls kinetics of the ferroelectric switching, backswitching and relaxation and presence of the charges carriers on the sample surface and in the sample bulk significantly change polarization reversal dynamics. Here we experimentally demonstrated practical possibility of the history dependent polarization reversal by the grounded SPM tip. This phenomenon was attributed to induction of the slowly dissipating charges into the surface of the grounded tip that enables polarization reversal under the action of the produced electric field. Analytical and numerical electrostatic calculations allow additional insight into nontrivial abnormal switching phenomena reported earlier.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Carbon Nanoparticles and Graphene Nanosheets as MALDI Matrices in Glycomics: a New Approach to Improve Glycan Profiling in Biological Samples

Glycomics continues to be a highly dynamic and interesting research area due to the need to comprehensively understand the biological attributes of glycosylation in many important biological functions such as the immune response, cell development, cell differentiation/adhesion, and host-pathogen interactions. Although matrix-assisted laser desorption ionization (MALDI) mass spectrometry (MS) has proven to be suitable for glycomic profiling studies, there is a need for improved sensitivity in the detection of native glycans, which ionize inefficiently. In this study, we investigated the efficiencies of graphene nanosheets (GNs) and carbon nanoparticles (CNPs) as MALDI matrices and co-matrices in glycan profiling. Our results indicated an enhancement of signal intensity by several orders of magnitude upon using GNs and CNPs in MALDI analysis of N-glycans derived from a variety of biological samples. Interestingly, increasing the amounts of CNPs and GNs improved not only the signal intensities but also prompted in-source decay (ISD) fragmentations, which produced extensive glycosidic and cross-ring cleavages. Our results indicated that the extent of ISD fragmentation could be modulated by CNP and GN concentrations, to obtain MS{sup 2} and pseudo-MS{sup 3} spectra. The results for glycan profiling in high salt solutions confirmed high salt-tolerance capacities for both CNPs and GNs. Finally, the results showed that by using CNPs and GNs as co-matrices, DHB crystal formation was more homogeneous which improved shot-to-shot reproducibility and sensitivity. .

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2017

A Novel Approach for Plate Impact Experiments to Determine the Dynamic Behavior of Materials Under Extreme Conditions

Here we present a unique approach to plate impact experiments for characterization of dynamic material behavior under extreme conditions, i.e., ultra-high strain-rates (~10 6 /s) and test temperatures up to 1000 °C. Strategic modifications are made to the existing singlestage gas-gun facility at CWRU to enable this approach. These include an extension of the gun barrel at the breech end of the gas-gun, which now incorporates a precision machined steel housing that carries a vertical heater well to accommodate an 800 W resistance coil heater with axial and rotational degrees of freedom. This custom designed heater assembly enables specimens placed on the flyer plate carried by the sabot to be heated uniformly across the diameter in a 100 mTorr vacuum prior to impact at the breech end of the gun barrel. A new sabot design is utilized to minimize heat transfer from the heated flyer plate to the sabot body. Using this modified gas-gun facility, the dynamic strength of pre-heated commercial purity polycrystalline aluminum samples is investigated at the onset of plastic flow in response to weak normal shock compression at test temperatures ranging from room to near melt point of aluminum. Lastly, the dynamic strength of aluminum samples, as inferred from the measured normal particle velocity history at the free (rear) surface of the target plate, show progressive weakening with increasing specimen temperatures in the temperature range 23–643 °C; at higher test temperatures, however, the rate of softening in dynamic strength is observed to weaken and even reverse as the sample temperatures approach the melt point of aluminum (test temperature ~643 °C).

36 MATERIALS SCIENCE↗
DOE OSTI2015

Preserving correlations between trajectories for efficient path sampling

Importance sampling of trajectories has proved a uniquely successful strategy for exploring rare dynamical behaviors of complex systems in an unbiased way. Carrying out this sampling, however, requires an ability to propose changes to dynamical pathways that are substantial, yet sufficiently modest to obtain reasonable acceptance rates. Satisfying this requirement becomes very challenging in the case of long trajectories, due to the characteristic divergences of chaotic dynamics. Here, we examine schemes for addressing this problem, which engineer correlation between a trial trajectory and its reference path, for instance using artificial forces. Our analysis is facilitated by a modern perspective on Markov chain Monte Carlo sampling, inspired by non-equilibrium statistical mechanics, which clarifies the types of sampling strategies that can scale to long trajectories. Viewed in this light, the most promising such strategy guides a trial trajectory by manipulating the sequence of random numbers that advance its stochastic time evolution, as done in a handful of existing methods. In cases where this “noise guidance” synchronizes trajectories effectively, as the Glauber dynamics of a two-dimensional Ising model, we show that efficient path sampling can be achieved for even very long trajectories.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗