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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.

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At least 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Model-free estimation of the effective correlation time for C–H bond reorientation in amphiphilic bilayers: {sup 1}H–{sup 13}C solid-state NMR and MD simulations

Molecular dynamics (MD) simulations give atomically detailed information on structure and dynamics in amphiphilic bilayer systems on timescales up to about 1 μs. The reorientational dynamics of the C–H bonds is conventionally verified by measurements of {sup 13}C or {sup 2}H nuclear magnetic resonance (NMR) longitudinal relaxation rates R{sub 1}, which are more sensitive to motional processes with correlation times close to the inverse Larmor frequency, typically around 1-10 ns on standard NMR instrumentation, and are thus less sensitive to the 10-1000 ns timescale motion that can be observed in the MD simulations. We propose an experimental procedure for atomically resolved model-free estimation of the C–H bond effective reorientational correlation time τ{sub e}, which includes contributions from the entire range of all-atom MD timescales and that can be calculated directly from the MD trajectories. The approach is based on measurements of {sup 13}C R{sub 1} and R{sub 1ρ} relaxation rates, as well as {sup 1}H−{sup 13}C dipolar couplings, and is applicable to anisotropic liquid crystalline lipid or surfactant systems using a conventional solid-state NMR spectrometer and samples with natural isotopic composition. The procedure is demonstrated on a fully hydrated lamellar phase of 1-palmitoyl-2-oleoyl-phosphatidylcholine, yielding values of τ{sub e} from 0.1 ns for the methyl groups in the choline moiety and at the end of the acyl chains to 3 ns for the g{sub 1} methylene group of the glycerol backbone. MD simulations performed with a widely used united-atom force-field reproduce the τ{sub e}-profile of the major part of the acyl chains but underestimate the dynamics of the glycerol backbone and adjacent molecular segments. The measurement of experimental τ{sub e}-profiles can be used to study subtle effects on C–H bond reorientational motions in anisotropic liquid crystals, as well as to validate the C–H bond reorientation dynamics predicted in MD simulations of amphiphilic bilayers such as lipid membranes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Ultrafast vibrational spectroscopy (2D-IR) of CO{sub 2} in ionic liquids: Carbon capture from carbon dioxide’s point of view

The CO{sub 2}ν{sub 3} asymmetric stretching mode is established as a vibrational chromophore for ultrafast two-dimensional infrared (2D-IR) spectroscopic studies of local structure and dynamics in ionic liquids, which are of interest for carbon capture applications. CO{sub 2} is dissolved in a series of 1-butyl-3-methylimidazolium-based ionic liquids ([C{sub 4}C{sub 1}im][X], where [X]{sup −} is the anion from the series hexafluorophosphate (PF{sub 6}{sup −}), tetrafluoroborate (BF{sub 4}{sup −}), bis-(trifluoromethyl)sulfonylimide (Tf{sub 2}N{sup −}), triflate (TfO{sup −}), trifluoroacetate (TFA{sup −}), dicyanamide (DCA{sup −}), and thiocyanate (SCN{sup −})). In the ionic liquids studied, the ν{sub 3} center frequency is sensitive to the local solvation environment and reports on the timescales for local structural relaxation. Density functional theory calculations predict charge transfer from the anion to the CO{sub 2} and from CO{sub 2} to the cation. The charge transfer drives geometrical distortion of CO{sub 2}, which in turn changes the ν{sub 3} frequency. The observed structural relaxation timescales vary by up to an order of magnitude between ionic liquids. Shoulders in the 2D-IR spectra arise from anharmonic coupling of the ν{sub 2} and ν{sub 3} normal modes of CO{sub 2}. Thermal fluctuations in the ν{sub 2} population stochastically modulate the ν{sub 3} frequency and generate dynamic cross-peaks. These timescales are attributed to the breakup of ion cages that create a well-defined local environment for CO{sub 2}. The results suggest that the picosecond dynamics of CO{sub 2} are gated by local diffusion of anions and cations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Investigating vibrational relaxation in cyanide-bridged transition metal mixed-valence complexes using two-dimensional infrared and infrared pump-probe spectroscopies

Using polarization-selective two-dimensional infrared (2D IR) and infrared pumpprobe spectroscopies, we study vibrational relaxation of the four cyanide stretching (CN) vibrations found in [(NH3)5RuIIINCFeII(CN)5] (FeRu) dissolved in D2O or formamide and [(NC)5FeIICNPtIV(NH3)4NCFeII(CN)5] 4 (FePtFe) dissolved in D2O. These cyanide-bridged transition metal complexes serve as models for understanding the role high frequency vibrational modes play in metal-to-metal charge transfers over a bridging ligand. However, there is currently little information about vibrational relaxation and dephasing dynamics of the anharmonically coupled CN modes in the electronic ground state of these complexes. IR pump-probe experiments reveal that the vibrational lifetimes of the CN modes are 2 times faster when FeRu is dissolved in D2O versus formamide. They also reveal that the vibrational lifetimes of the CN modes of FePtFe in D2O are almost four times as long as for FeRu in D2O. Combined with mode-specific relaxation dynamics measured from the 2D IR experiments, the IR pump-probe experiments also reveal that intramolecular vibrational relaxation is occurring in all three systems on 1 ps timescale. Center line slope dynamics, which have been shown to be a measure of the frequency-frequency correlation function, reveal that the radial, axial, and trans CN modes exhibit a 3 ps timescale for frequency fluctuations. This timescale is attributed to the forming and breaking of hydrogen bonds between each mode and the solvent. The results presented here along with our previous work on FeRu and FePtFe reveal a picture of coupled anharmonic CN modes where the spectral diffusion and vibrational relaxation dynamics depend on the spatial localization of the mode on the molecular complex and its specific interaction with the solvent.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Study of the time-resolved, 3-dimensional current density distribution in solid metallic liners at 1 MA

We present a study of the time varying current density distribution in solid metallic liner experiments at the 1MA level. Measurements are taken using an array of magnetic field probes which provide 2D triangulation of the average centroid of the drive current in the load at 3 discrete axial positions. These data are correlated with gated optical self-emission imaging which directly images the breakdown and plasma formation region. Results show that the current density is azimuthally non-uniform, and changes significantly throughout the 100ns experimental timescale. Magnetic field probes show clearly motion of the current density around the liner azimuth over 10ns timescales. If breakdown is initiated at one azimuthal location, the current density remains non-uniform even over large spatial extents throughout the current drive. The evolution timescales are suggestive of a resistive diffusion process or uneven current distributions among simultaneously formed but discrete plasma conduction paths.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Picosecond sulfur K-edge X-ray absorption spectroscopy with applications to excited state proton transfer

Picosecond X-ray absorption (XA) spectroscopy at the S K-edge (∼2.4 keV) is demonstrated and used to monitor excited state dynamics in a small organosulfur molecule (2-Thiopyridone, 2TP) following optical excitation. Multiple studies have reported that the thione (2TP) is converted into the thiol (2-Mercaptopyridine, 2MP) following photoexcitation. However, the timescale and photochemical pathway of this reaction remain uncertain. In this work, time-resolved XA spectroscopy at the S K-edge is used to monitor the formation and decay of two transient species following 400 nm excitation of 2TP dissolved in acetonitrile. The first transient species forms within the instrument response time (70 ps) and decays within 6 ns. The second transient species forms on a timescale of ∼400 ps and decays on a 15 ns timescale. Time-dependent density functional theory is used to identify the first and second transient species as the lowest-lying triplet states of 2TP and 2MP, respectively. This study demonstrates transient S K-edge XA spectroscopy as a sensitive and viable probe of time-evolving charge dynamics near sulfur sites in small molecules with future applications towards studying complex biological and material systems.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Efficiently sampling conformations and pathways using the concurrent adaptive sampling (CAS) algorithm

Molecular dynamics (MD) simulations are useful in obtaining thermodynamic and kinetic properties of bio-molecules but are limited by the timescale barrier, i.e., we may be unable to efficiently obtain properties because we need to run microseconds or longer simulations using femtoseconds time steps. While there are several existing methods to overcome this timescale barrier and efficiently sample thermodynamic and/or kinetic properties, problems remain in regard to being able to sample un- known systems, deal with high-dimensional space of collective variables, and focus the computational effort on slow timescales. Hence, in this work, a new sampling method, called the “Concurrent Adaptive Sampling (CAS) algorithm,” has been developed to tackle these three issues and efficiently obtain conformations and pathways. The method is not constrained to use only one or two collective variables, unlike most reaction coordinate-dependent methods. Instead, it can use a large number of collective vari- ables and uses macrostates (a partition of the collective variable space) to enhance the sampling. The exploration is done by running a large number of short simula- tions, and a clustering technique is used to accelerate the sampling. In this paper, we introduce the new methodology and show results from two-dimensional models and bio-molecules, such as penta-alanine and triazine polymer

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2018

Theory of critical balance in plasma turbulence

Critical balance, a statement of the approximate equality of a timescale of motion parallel to the magnetic field with a timescale of turbulent motion perpendicular to the field, is analyzed from a statistical description of the relative motion of two particles consistent with two-point correlation in a nonlinear gyrokinetic system with collisions. Critical balance is understood from a premise that the fastest time to any spatial scale that encounters decorrelated motions sets an overall correlation time for both perpendicular and parallel scales. For a regime of weak collisionality, this leads to the approximate equality of the parallel streaming time and the perpendicular turbulent correlation time, which is a standard statement of critical balance. These two timescales are only approximately equal because of effects in parallel streaming dynamics and collisions, the former producing deviations tied to variations of perpendicular and parallel wavenumbers. Finally, analysis of a collisional regime shows that critical balance also applies in that limit with a modified relation between parallel and perpendicular spatial scales.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Partial-ionization deconfinement effect in magnetized plasma

In partially ionized plasma, where ions can be in different ionization states, each charge state can be described as a different fluid for the purpose of multi-ion collisional transport. In the case of two charge states, transport pushes plasma toward equilibrium, which is found to be a combination of local charge-state equilibrium and generalized pinch relations between ion fluids representing different charge states. Combined, these conditions lead to a dramatic deconfinement of ions. This deconfinement happens on the timescale similar but not identical to the multi-ion cross-field transport timescale, as opposed to electron–ion transport timescale in fully ionized plasma. Deconfinement occurs because local charge-state equilibration enforces the disparity in diamagnetic drift velocities of ion fluid components, which in turn leads to the cross-field transport due to ion–ion friction.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Curious cross-field transport effects in multi-ion, magnetized plasma

In contrast to single-ion plasma, multiple-ion-species plasma exhibits new, curious, and large transport effects. On short timescales, where ions exchange momentum, magnetized multi-ion plasma behaves as a most unusual substance, compressible across field lines in number density but incompressible in charge density. It takes 40 times longer for electrons to participate. In this ion–ion cross-field transport regime, we identified the charge-incompressibility heat pump effect, transferring heat both spatially and between species. Curiously, the direction of impurity transport strongly depends on plasma magnetization, characterized by the ratio of light ion gyrofrequency to the collision frequency between light and heavy ion species. The expulsion of heavy ion impurities from a hotspot occurs sufficiently quickly to be observable on MagLIF, so long as plasma becomes sufficiently collisionally magnetized under implosion. Even more curious, multi-ion transport changes its nature in partially ionized plasma, where ions occupy different charge states. In this regime, we identify a partial-ionization deconfinement effect. The combination of cross-field transport, ionization, and recombination leads to a net ion charge moving across magnetic field lines on the ion–ion transport timescale as opposed to the electron–ion transport timescale. Cross-field transport effects in multi-ion plasma are important in a number of applications, including nuclear fusion and plasma mass filters.

Mlodik, M. E. (ORCID:0000000343003941)↗
DOE OSTI2024

Emergent facilitation and glassy dynamics in supercooled liquids

In supercooled liquids, dynamical facilitation refers to a phenomenon where microscopic motion begets further motion nearby, resulting in spatially heterogeneous dynamics. This is central to the glassy relaxation dynamics of such liquids, which show super-Arrhenius growth of relaxation timescales with decreasing temperature. Despite the importance of dynamical facilitation, there is no theoretical understanding of how facilitation emerges and impacts relaxation dynamics. Here, we present a theory that explains the microscopic origins of dynamical facilitation. We show that dynamics proceeds by localized bond-exchange events, also known as excitations, resulting in the accumulation of elastic stresses with which new excitations can interact. At low temperatures, these elastic interactions dominate and facilitate the creation of new excitations near prior excitations. Using the theory of linear elasticity and Markov processes, we simulate a model, which reproduces multiple aspects of glassy dynamics observed in experiments and molecular simulations, including the stretched exponential decay of relaxation functions, the super-Arrhenius behavior of relaxation timescales as well as their two-dimensional finite-size effects. The model also predicts the subdiffusive behavior of the mean squared displacement (MSD) on short, intermediate timescales. Furthermore, we derive the phonon contributions to diffusion and relaxation, which when combined with the excitation contributions produce the two-step relaxation processes, and the ballistic–subdiffusive–diffusive crossover MSD behaviors commonly found in supercooled liquids.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

An Acrobatic Substrate Metamorphosis Reveals a Requirement for Substrate Conformational Dynamics in Trypsin Proteolysis

The molecular basis of enzyme catalytic power and specificity derives from dynamic interactions between enzyme and substrate during catalysis. While considerable effort has been devoted to understanding how conformational dynamics within enzymes affect catalysis, the role of conformational dynamics within protein substrates has not been addressed. Here in this paper, we examine the importance of substrate dynamics in the cleavage of Kunitz-BPTI protease inhibitors by mesotrypsin, finding that the varied conformational dynamics of structurally similar substrates can profoundly impact the rate of catalysis. A 1.4 Å crystal structure of a mesotrypsin-product complex formed with a rapidly cleaved substrate reveals a dramatic conformational change in the substrate upon proteolysis. Using long all-atom molecular dynamics simulations of acyl-enzyme intermediates with proteolysis rates spanning three orders of magnitude, we identify global and local dynamic features of substrates on the ns-μs timescale that correlate with enzymatic rates and explain differential susceptibility to proteolysis. By integrating multiple enhanced sampling methods for molecular dynamics, we model a viable conformational pathway between substratelike and product-like states, linking substrate dynamics on the ns-μs timescale with large collective substrate motions on the much slower timescale of catalysis. Our findings implicate substrate flexibility as a critical determinant of catalysis.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2017

Fission Chain Restart Theory

We present that fast nanosecond timescale neutron and gamma-ray counting can be performed with a (liquid) scintillator array. Fission chains in metal evolve over a timescale of tens of nanoseconds. If the metal is surrounded by moderator, neutrons leaking from the metal can thermalize and diffuse in the moderator. With finite probability, the diffusing neutrons can return to the metal and restart the fast fission chain. The timescale for this restart process is microseconds. A theory describing time evolving fission chains for metal surrounded by moderator, including this restart process, is presented. Finally, this theory is sufficiently simple for it to be implemented for real-time analysis.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

A coarse-grained computational model of the nuclear pore complex predicts Phe-Gly nucleoporin dynamics

The phenylalanine-glycine–repeat nucleoporins (FG-Nups), which occupy the lumen of the nuclear pore complex (NPC), are critical for transport between the nucleus and cytosol. Although NPCs differ in composition across species, they are largely conserved in organization and function. Transport through the pore is on the millisecond timescale. Here, to explore the dynamics of nucleoporins on this timescale, we use coarse-grained computational simulations. These simulations generate predictions that can be experimentally tested to distinguish between proposed mechanisms of transport. Our model reflects the conserved structure of the NPC, in which FG-Nup filaments extend into the lumen and anchor along the interior of the channel. The lengths of the filaments in our model are based on the known characteristics of yeast FG-Nups. The FG-repeat sites also bind to each other, and we vary this association over several orders of magnitude and run 100-ms simulations for each value. The autocorrelation functions of the orientation of the simulated FG-Nups are compared with in vivo anisotropy data. We observe that FG-Nups reptate back and forth through the NPC at timescales commensurate with experimental measurements of the speed of cargo transport through the NPC. Our results are consistent with models of transport where FG-Nup filaments are free to move across the central channel of the NPC, possibly informing how cargo might transverse the NPC.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2015

WHAT CAUSES THE INTER-SOLAR-CYCLE VARIATION OF TOTAL SOLAR IRRADIANCE?

The Physikalisch Meteorologisches Observatorium Davos total solar irradiance (TSI), Active Cavity Radiometer Irradiance Monitoring TSI, and Royal Meteorological Institute of Belgium TSI are three typical TSI composites. Magnetic Plage Strength Index (MPSI) and Mount Wilson Sunspot Index (MWSI) should indicate the weak and strong magnetic field activity on the solar full disk, respectively. Cross-correlation (CC) analysis of MWSI with three TSI composites shows that TSI should be weakly correlated with MWSI, and not be in phase with MWSI at timescales of solar cycles. The wavelet coherence (WTC) and partial wavelet coherence (PWC) of TSI with MWSI indicate that the inter-solar-cycle variation of TSI is also not related to solar strong magnetic field activity, which is represented by MWSI. However, CC analysis of MPSI with three TSI composites indicates that TSI should be moderately correlated and accurately in phase with MPSI at timescales of solar cycles, and that the statistical significance test indicates that the correlation coefficient of three TSI composites with MPSI is statistically significantly higher than that of three TSI composites with MWSI. Furthermore, the cross wavelet transform (XWT) and WTC of TSI with MPSI show that the TSI is highly related and actually in phase with MPSI at a timescale of a solar cycle as well. Consequently, the CC analysis, XWT, and WTC indicate that the solar weak magnetic activity on the full disk, which is represented by MPSI, dominates the inter-solar-cycle variation of TSI.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

EXPLICIT-IMPLICIT SCHEME FOR RELATIVISTIC RADIATION HYDRODYNAMICS

We propose an explicit-implicit scheme for numerically solving special relativistic radiation hydrodynamic equations, which ensures a conservation of total energy and momentum (matter and radiation). In our scheme, zeroth and first moment equations of the radiation transfer equation are numerically solved without employing a flux-limited diffusion approximation. For an hyperbolic term, of which the timescale is the light crossing time when the flow velocity is comparable to the speed of light, is explicitly solved using an approximate Riemann solver. Source terms describing an exchange of energy and momentum between the matter and the radiation via the gas-radiation interaction are implicitly integrated using an iteration method. The implicit scheme allows us to relax the Courant-Friedrichs-Lewy condition in optically thick media, where heating/cooling and scattering timescales could be much shorter than the dynamical timescale. We show that our numerical code can pass test problems of one- and two-dimensional radiation energy transport, and one-dimensional radiation hydrodynamics. Our newly developed scheme could be useful for a number of relativistic astrophysical problems. We also discuss how to extend our explicit-implicit scheme to the relativistic radiation magnetohydrodynamics.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

VARIABLE GAMMA-RAY EMISSION FROM THE CRAB NEBULA: SHORT FLARES AND LONG 'WAVES'

Gamma-ray emission from the Crab Nebula has been recently shown to be unsteady. In this paper, we study the flux and spectral variability of the Crab above 100 MeV on different timescales ranging from days to weeks. In addition to the four main intense and day-long flares detected by AGILE and Fermi-LAT between 2007 September and 2012 September, we find evidence for week-long and less intense episodes of enhanced gamma-ray emission that we call 'waves'. Statistically significant 'waves' show timescales of 1-2 weeks, and can occur by themselves or in association with shorter flares. We present a refined flux and spectral analysis of the 2007 September-October gamma-ray enhancement episode detected by AGILE that shows both 'wave' and flaring behavior. We extend our analysis to the publicly available Fermi-LAT data set and show that several additional 'wave' episodes can be identified. We discuss the spectral properties of the 2007 September 'wave'/flare event and show that the physical properties of the 'waves' are intermediate between steady and flaring states. Plasma instabilities inducing 'waves' appear to involve spatial distances l {approx} 10{sup 16} cm and enhanced magnetic fields B {approx} (0.5-1) mG. Day-long flares are characterized by smaller distances and larger local magnetic fields. Typically, the deduced total energy associated with the 'wave' phenomenon (E{sub w} {approx} 10{sup 42} erg, where E{sub w} is the kinetic energy of the emitting particles) is comparable with that associated to the flares, and can reach a few percent of the total available pulsar spin-down energy. Most likely, flares and waves are the product of the same class of plasma instabilities that we show acting on different timescales and radiation intensities.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

CONTINUUM VARIABILITY OF DEEPLY EMBEDDED PROTOSTARS AS A PROBE OF ENVELOPE STRUCTURE

Stars may be assembled in large growth spurts; however the evidence for this hypothesis is circumstantial. Directly studying the accretion at the earliest phases of stellar growth is challenging because young stars are deeply embedded in optically thick envelopes, which have spectral energy distributions that peak in the far-IR, where observations are difficult. In this paper, we consider the feasibility of detecting accretion outbursts from these younger stars by investigating the timescales for how the protostellar envelope responds to changes in the emission properties of the central source. The envelope heats up in response to an outburst, brightening at all wavelengths and with the emission peak moving to shorter wavelengths. The timescale for this change depends on the time for dust grains to heat and re-emit photons and the time required for the energy to escape the inner, optically thick portion of the envelope. We find that the dust response time is much shorter than the photon propagation time and thus the timescale over which the emission varies is set by time delays imposed by geometry. These times are hours to days near the peak of the spectral energy distribution and weeks to months in the sub-mm. The ideal location to quickly detect continuum variability is therefore in the mid- to far-IR, near the peak of the spectral energy distribution, where the change in emission amplitude is largest. Searching for variability in sub-mm continuum emission is also feasible, though with a longer time separation and a weaker relationship between the amount of detected emission amplitude and change in central source luminosity. Such observations would constrain accretion histories of protostars and would help to trace the disk/envelope instabilities that lead to stellar growth.

79 ASTRONOMY AND ASTROPHYSICS↗