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Results for “time resolved spectroscopy”

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 505 records · Page 28
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Spectroscopy and dynamics of the v = 1 levels derived from the OH(D) stretching modes in the I‾∙HDO complex using CW and time-resolved, resonant two-photon infrared excitation of the cryogenically cooled ions

The vibrational energy levels of the two isotopomers adopted by the I‾∙HDO ion-molecule complex occur such that the OH(D) stretching fundamentals span its dissociation energy, thus enabling a spectroscopic investigation of the dynamics displayed by a system prepared in the vicinity of the dissociation threshold. This regime is explored using infrared photoexcitation of the mass-selected complexes cooled in a cryogenic radiofrequency (Paul) ion trap. Survey spectra are obtained at modest resolution using two-color, IR-IR photodissociation with nanosecond lasers to establish the level structure and unimolecular decay dynamics of the v = 1 and 2 levels of the bound OH(D) oscillator. The v = 1 levels are prepared by fixed frequency excitation in the trap and the absorption spectra arising from this excited state are probed by photofragmentation of the complex with a second pulsed IR laser after a variable delay time (0 to 20 ms). The transitions to levels above the dissociation threshold for I‾ + HDO formation are observed to be sharp (~5 cm -1 FWHM). At low pressure, the bound OD (v = 1) population relaxes very slowly (~3 ms), consistent with resonant fluorescence in the IR. The collisional quenching rate constants of this level by the He buffer gas were estimated to be on the order 3 x 10 -12 cm 3 /s based on a crude Stern-Volmer analysis. Here, the rotational fine structure and linewidths (≲0.01 cm -1 FWHM) arising from transitions of the non-bonded OH stretch fundamental of the OD-bound isotopomer that lies just above the dissociation energy are determined using single photon photodissociation by excitation of the 10 K ions with a single-frequency, CW IR laser in the ion trap.

Cryogenic ion infrared spectroscopy↗
DOE OSTI2024

Transient absorption measurements of excitonic dynamics in 3 R -MoS 2

The excitonic dynamics in MoS 2 monolayer, bilayer, and bulk flakes with different stacking orders, namely 3R and 2H, are investigated through transient absorption spectroscopy at room temperature. Samples are obtained by the mechanical exfoliation of bulk MoS 2 crystals with 3R and 2H stacking orders. Photoluminescence spectroscopy is performed to characterize and compare the samples. Time-resolved differential reflection measurements reveal that monolayer flakes exfoliated from the 3R and 2H crystals exhibit similar exciton lifetimes of a few picoseconds. However, the exciton lifetime in the 3R bilayer flake is approximately 85 ps, about twice as long as that in the 2H bilayer. This prolonged exciton lifetime is attributed to the reduction of the electron-hole wavefunction overlap in the 3R bilayer due to the built-in polarization. Similarly, in bulk 3R MoS 2 , the exciton lifetime is around 220 ps, approximately twice as long as those in bulk 2H MoS 2 . The extension of the exciton lifetime in 3R transition metal dichalcogenides could be advantageous for their applications in optoelectronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

TDCOSMO. XII. Improved Hubble constant measurement from lensing time delays using spatially resolved stellar kinematics of the lens galaxy

Strong-lensing time delays enable measurement of the Hubble constant ($H_{0}$) independently of other traditional methods. The main limitation to the precision of time-delay cosmography is mass-sheet degeneracy (MSD). Some of the previous TDCOSMO analyses broke the MSD by making standard assumptions about the mass density profile of the lens galaxy, reaching 2% precision from seven lenses. However, this approach could potentially bias the $H_0$ measurement or underestimate the errors. For this work, we broke the MSD for the first time using spatially resolved kinematics of the lens galaxy in RXJ1131$-$1231 obtained from the Keck Cosmic Web Imager spectroscopy, in combination with previously published time delay and lens models derived from Hubble Space Telescope imaging. This approach allowed us to robustly estimate $H_0$, effectively implementing a maximally flexible mass model. Following a blind analysis, we estimated the angular diameter distance to the lens galaxy $D_{\rm d} = 865_{-81}^{+85}$ Mpc and the time-delay distance $D_{\Delta t} = 2180_{-271}^{+472}$ Mpc, giving $H_0 = 77.1_{-7.1}^{+7.3}$ km s$^{-1}$ Mpc$^{-1}$ - for a flat $\Lambda$ cold dark matter cosmology. The error budget accounts for all uncertainties, including the MSD inherent to the lens mass profile and the line-of-sight effects, and those related to the mass-anisotropy degeneracy and projection effects. Our new measurement is in excellent agreement with those obtained in the past using standard simply parametrized mass profiles for this single system ($H_0 = 78.3^{+3.4}_{-3.3}$ km s$^{-1}$ Mpc$^{-1}$) and for seven lenses ($H_0 = 74.2_{-1.6}^{+1.6}$ km s$^{-1}$ Mpc$^{-1}$), or for seven lenses using single-aperture kinematics and the same maximally flexible models used by us ($H_0 = 73.3^{+5.8}_{-5.8}$ km s$^{-1}$ Mpc$^{-1}$). This agreement corroborates the methodology of time-delay cosmography.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Synchrotron based transient x-ray absorption spectroscopy for emerging solid-state energy materials

The rational design of cutting-edge materials for an efficient solar energy conversion process is a challenging task, which demands a fundamental understanding of the mechanisms operative during the photoinduced physical and chemical reactions. In response to these issues, progress in the field has steered attention toward the use of time-resolved spectroscopic techniques to resolve the multiple intermediate species involved in these photoinduced reactions. Thanks to the advent of pump–probe technique, which leads to the development of various time-resolved spectroscopic methods, significant progress has been made in understanding the photophysical and photochemical properties (e.g., excited state dynamics, charge transfer mechanism, charge separation dynamics, etc.) of energy materials. Synchrotron-based x-ray transient absorption (XTA) spectroscopy is one of the most important time-resolved techniques to unravel the direct correlation of the material structure with their photophysical properties owing to its unique capability in directly observing electronic and structural evolution simultaneously. As a result, the aim of this work is to provide a systematic overview of the recent progress in using XTA for capturing the structural dynamics associated with excited state and charge separation dynamics in emerging solid-state energy materials.

14 SOLAR ENERGY↗
DOE OSTI2023

Femtosecond symmetry breaking and coherent relaxation of methane cations via x-ray spectroscopy

Understanding the relaxation pathways of photoexcited molecules is essential to gain atomistic-level insight into photochemistry. Here, we performed a time-resolved study of ultrafast molecular symmetry breaking through geometric relaxation (Jahn-Teller distortion) on the methane cation. Attosecond transient absorption spectroscopy with soft x-rays at the carbon K-edge revealed that the distortion occurred within 10 ± 2 femtoseconds after few-femtosecond strong-field ionization of methane. The distortion activated coherent oscillations in the asymmetric scissoring vibrational mode of the symmetry-broken cation, which were detected in the x-ray signal. These oscillations were damped within 58 ± 13 femtoseconds because vibrational coherence was lost with the energy redistributing into lower-frequency vibrational modes. This study completely reconstructs the molecular relaxation dynamics of this prototypical example and opens avenues for exploring complex systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Ultrafast Relaxations in Ruthenium Polypyridyl Chromophores Determined by Stochastic Kinetics Simulations

Maximizing the efficiency of solar energy conversion using dye assemblies rests on understanding where the energy goes following absorption. Transient spectroscopies in solution are useful for this purpose, and the time-resolved data are usually analyzed with a sum of exponentials. This treatment assumes that dynamic events are well separated in time, and that the resulting exponential prefactors and phenomenological lifetimes are related directly to primary physical values. Such assumptions break down for coincident absorption, emission, and excited state relaxation that occur in transient absorption and photoluminescence of tris(2,2'-bipyridine)ruthenium(2+) derivatives, confounding the physical meaning of the reported lifetimes. Here, we use inductive modeling and stochastic chemical kinetics to develop a detailed description of the primary ultrafast photophysics in transient spectroscopies of a series of Ru dyes, as an alternative to sums of exponential analysis. Commonly invoked three-level schemes involving absorption, intersystem crossing (ISC), and slow nonradiative relaxation and incoherent emission to the ground state cannot reproduce the experimentally measured spectra. The kinetics simulations reveal that ultrafast decay from the singlet excited state manifold to the ground state competes with ISC to the triplet excited state, whose efficiency was determined to be less than unity. The populations predicted by the simulations are used to estimate the magnitudes of transition dipoles for excited state excitations and evaluate the influence of specific ligands. The mechanistic framework and methodology presented here are entirely general, applicable to other dye classes, and can be extended to include charge injection by molecules bound to semiconductor surfaces.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Discovery of an Intrinsic Antiferromagnetic Semiconductor EuSc 2 Te 4 With Magnetism‐Driven Nonlinear Transport

Magnetic topological materials have recently emerged as a promising platform for studying quantum geometry by the nonlinear transport in thin film devices. In this work, an antiferromagnetic (AFM) semiconductor EuSc₂Te₄ as the first bulk crystal that exhibits quantum geometry-driven nonlinear transport is reported. This material crystallizes into an orthorhombic lattice with AFM order below 5.2 K and a bandgap of less than 50 meV. The calculated band structure aligns with the angle-resolved photoemission spectroscopy spectrum. The AFM order preserves combined space-time inversion symmetry but breaks both spatial inversion and time-reversal symmetry, leading to the nonlinear Hall effect (NLHE). Nonlinear Hall voltage measured in bulk crystals appears at zero field, peaks near the spin-flop transition as the field increases, and then diminishes as the spin moments align into a ferromagnetic order. This field dependence, along with the scaling analysis of the nonlinear Hall conductivity, suggests that the NLHE of EuSc₂Te₄ involves contributions from quantum metric, in addition to extrinsic contributions, such as spin scattering and junction effects. Furthermore, this NLHE is found to have the functionality of broadband frequency mixing, indicating its potential applications in electronics. This work reveals a new avenue for studying magnetism-induced nonlinear transport in magnetic materials.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Engineering Band Gap and Photoconduction in Semiconducting Metal Organic Frameworks: Metal Node Effect

We report a systematic study on the correlation of the metal nodes in M-THQ conducting MOFs (M = Fe, Ni, Cu and Zn; THQ = tetra-hydroxybenzoquinone) with their structure, photophysical property, and photoconductivity. We found that structural preference in these MOFs is controlled by metal node identity where Cu prefers a square planar coordination which leads to a 2D Kagome type structure. Fe, Ni and Zn prefer an octahedral sphere which leads to a 3D structure. Fe-THQ has the smallest band gap and highest photoconduction as well as a long-lived ligand to metal charge transfer state due to the mixed valence state revealed by time resolved optical and X-ray absorption and tera hertz spectroscopy. Furthermore, these results demonstrate the importance of the metal node in tuning the photophysical and photocatalytic properties of MOFs.

14 SOLAR ENERGY↗
DOE OSTI2022

Ion-Pair Reorganization Regulates Reactivity in Photoredox Catalysts

Cyclometalated and polypyridyl complexes of d6 metals are promising photoredox catalysts, using light to drive reactions with high kinetic or thermodynamic barriers via the generation of reactive radical intermediates. However, while tuning of their redox potentials, absorption energy, excited-state lifetime and quantum yield are well-known criteria for modifying activity, other factors could be important. Here we show that dynamic ion-pair reorganization controls the reactivity of a photoredox catalyst, [Ir[dF(CF3)ppy]2(dtbpy)]X. Time-resolved dielectric-loss experiments show how counter-ion identity influences excited-state charge distribution, evincing large differences in both the ground- and excited-state dipole moment depending on whether X is a small associating anion (PF6-) that forms a contact-ion pair versus a large one that either dissociates or forms a solvent-separated pair (BArF4-). These differences correlate with the reactivity of the photocatalyst toward both reductive and oxidative electron transfer, amounting to a 4-fold change in selectivity toward oxidation versus reduction. These results suggest that ion pairing could be an underappreciated factor that modulates reactivity in ionic photoredox catalysts.

catalysis↗
DOE OSTI2023

Charged particle diagnostics for inertial confinement fusion and high-energy-density physics experiments

MeV-range ions generated in inertial confinement fusion (ICF) and high-energy-density physics experiments carry a wealth of information, including fusion reaction yield, rate, and spatial emission profile; implosion areal density; electron temperature and mix; and electric and magnetic fields. Here, the principles of how this information is obtained from data and the charged particle diagnostic suite currently available at the major US ICF facilities for making the measurements are reviewed. Time-integrating instruments using image plate, radiochromic film, and/or CR-39 detectors in different configurations for ion counting, spectroscopy, or emission profile measurements are described, along with time-resolving detectors using chemical vapor deposited diamonds coupled to oscilloscopes or scintillators coupled to streak cameras for measuring the timing of ion emission. A brief description of charged-particle radiography setups for probing subject plasma experiments is also given. The goal of the paper is to provide the reader with a broad overview of available capabilities, with reference to resources where more detailed information can be found.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2022

Thioester synthesis by a designed nickel enzyme models prebiotic energy conversion

The formation of carbon–carbon bonds from prebiotic precursors such as carbon dioxide represents the foundation of all primordial life processes. In extant organisms, this reaction is carried out by the carbon monoxide dehydrogenase (CODH)/acetyl coenzyme A synthase (ACS) enzyme, which performs the cornerstone reaction in the ancient Wood–Ljungdahl metabolic pathway to synthesize the key biological metabolite, acetyl-CoA. Despite its significance, a fundamental understanding of this transformation is lacking, hampering efforts to harness analogous chemistry. To address these knowledge gaps, we have designed an artificial metalloenzyme within the azurin protein scaffold as a structural, functional, and mechanistic model of ACS. We demonstrate the intermediacy of the Ni I species and requirement for ordered substrate binding in the bioorganometallic carbon–carbon bond-forming reaction from the one-carbon ACS substrates. The electronic and geometric structures of the nickel-acetyl intermediate have been characterized using time-resolved optical, electron paramagnetic resonance, and X-ray absorption spectroscopy in conjunction with quantum chemical calculations. Moreover, we demonstrate that the nickel-acetyl species is chemically competent for selective acyl transfer upon thiol addition to biosynthesize an activated thioester. Drawing an analogy to the native enzyme, a mechanism for thioester generation by this ACS model has been proposed. The fundamental insight into the enzymatic process provided by this rudimentary ACS model has implications for the evolution of primitive ACS-like proteins. Ultimately, these findings offer strategies for development of highly active catalysts for sustainable generation of liquid fuels from one-carbon substrates, with potential for broad applications across diverse fields ranging from energy storage to environmental remediation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Time-resolved probing of laser-induced nanostructuring processes in liquids

Laser synthesis and processing of colloids (LSPC) in liquids has gained widespread applications in producing nanomaterials of different classes of solids. While the technical processes in different cases of ablation, fragmentation or colloidal fusion may look macroscopically different in each application, the underlying fundamental mechanisms are always the same cascade of laser interaction with matter, non-thermal or thermal energy deposition, phase transitions, and the subsequent structure formation processes. Disentangling these mechanisms represents a veritable challenge, as ultrafast and structurally sensitive experimental methods are required. This review presents a discussion of how state-of-the-art experimental protocols using ultrafast lasers and sensitive structural probes, such as electrons or X-rays are able to address this challenge. In particular, it is possible to investigate LSPC on single objects using single probe pulses and avoid accumulation effects in a heterogeneous sample. The presented results capture structure formation with femtosecond and atomic scale resolution. Ultrafast time-resolved probing approaches are key to revealing the transient states and pathways that govern material transformation in LSPC.

X-ray scattering↗
DOE OSTI2020

Towards full surface Brillouin zone mapping by coherent multi-photon photoemission

Abstract We report a novel approach for coherent multi-photon photoemission in the entire Brillouin zone with infrared light that is readily implemented in a laboratory setting. We excite a solid state material, Ag(110), with intense femtosecond laser pulses to excite higher-order multi-photon photoemission; angle-resolved electron spectroscopic acquisition records photoemission at large in-plane momenta involving optical transitions from the occupied to unoccupied bands of the sample that otherwise might remain hidden by the photoemission horizon. We propose this as a complementary ultrafast method to time- and angle-resolved two-color, e.g. infrared pump and extreme ultraviolet probe, photoemission spectroscopy, with the advantage of being able to measure and control the coherent electron dynamics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Hydrogenation of CO 2 to Methanol on a Au δ+ –In 2 O 3–x Catalyst

CO 2 hydrogenation to methanol has attracted increasing attention with the development of renewable hydrogen. A big challenge is to identify catalysts able to achieve high conversion and selectivity. In this work, we report an In 2 O 3 -supported Au catalyst that exhibits excellent performance for hydrogenation of CO 2 selectively to methanol. In situ characterizations using time-resolved X-ray diffraction, ambient-pressure X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy confirm that a strong metal–support interaction leads to a reactive Au δ+ –In 2 O 3– x interface for activation and hydrogenation of CO 2 to methanol. An effective gold-indium oxide bonding favors the dispersion of the noble metal and prevents its sintering under reaction conditions. The methanol selectivity reaches 100% at temperatures below 225 °C and is more than 70% at 275 °C over the Au δ+ –In 2 O 3– x catalyst. It is even 67.8% with a space time yield of methanol of 0.47 g MeOH /(h·g cat ) at 300 °C, 5 MPa, and 21,000 cm 3 h –1 g cat –1 . The results obtained here represent the highest selectivity and activity ever reported for CO 2 hydrogenation over supported gold catalysts. Our study shows that the strong Au/In 2 O 3 interaction and the intrinsic chemical activity of In 2 O 3 can be used to significantly improve the catalytic performance of Au catalysts, providing promising routes for the rational design and application of Au catalysts beyond CO 2 hydrogenation.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Commissioning of a Cask Enabling Characterization of Irradiated Nuclear Fuels with Pulsed Neutrons

Irradiation tests are a key component of nuclear fuel development and identifying typical and atypical regions in the irradiated fuel volume relies on very few characterization techniques. The goal of the effort reported here is to provide complementary measurements adding to the available parameter space for post irradiation examination as well as to inform subsequent hot cell PIE examinations by identifying typical and atypical regions with respect to microstructure, tomographic data, or isotope densities. Pulsed neutrons, enabling diffraction as well as energy-resolved neutron imaging and neutron absorption resonance spectroscopy, offer unique capabilities for this purpose. Time-of-flight neutron diffraction has the potential to offer efficient, non-destructive and non-contact microstructural characterization of irradiated fuel pins with spatial resolution of 1 mm 3 to 1 cm 3 while for energy-resolved neutron imaging (and by extension tomography) a resolution of 100 μm was demonstrated. The potential results include crystallinity vs. amorphous volumes and microstructural information such as phase compositions, lattice strains (indicative of residual stresses or chemistry variations) and textures from the diffraction data as well as distances (e.g. pellet to cladding), cracks, and isotopic distributions of minor actinides, fission products as well as fission gas partial pressures e.g. in the plenum from energy resolved neutron imaging.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2022

Progress Report on Mockup Irradiation Capsule Fuel Measurements at LANSCE

Irradiation tests are a key component of nuclear fuel development and identifying typical and atypical regions in the irradiated fuel volume relies on very few characterization techniques. The goal of the effort reported here is to provide complementary measurements adding to the available parameter space for post irradiation examination as well as to inform subsequent hot cell PIE examinations by identifying typical and atypical regions with respect to microstructure, tomographic data, or isotope densities. Pulsed neutrons, enabling diffraction as well as energy-resolved neutron imaging and neutron absorption resonance spectroscopy, offer unique capabilities for this purpose. Time-of-flight neutron diffraction has the potential to offer efficient, non-destructive and non-contact microstructural characterization of irradiated fuel specimen with spatial resolution of 1 mm 3 to 1 cm 3 while for energy-resolved neutron imaging (and by extension tomography) a resolution of 100 μm 3 was demonstrated. The potential results include crystallinity vs. amorphous volumes and microstructural information such as phase compositions, lattice strains (indicative of residual stresses or chemistry variations) and textures from the diffraction data as well as distances (e.g. pellet to cladding), cracks, and isotope densities of minor actinides, fission products as well as fission gas partial pressures e.g. in the plenum from energy resolved neutron imaging.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2024

Deciphering Charge Transfer Processes in Transition Metal Complexes from the Perspective of Ultrafast Electronic and Nuclear Motions

Chemical transformations in charge transfer states result from the interplay between electronic dynamics and nuclear reorganization along excited-state trajectories. Here, in this study, we investigate the ultrafast structural dynamics following photoinduced electron transfer from the metal-metal-to-ligand charge transfer state of an electron donor, a Pt dimer complex, to a covalently linked electron acceptor group using ultrafast time-resolved wide-angle X-ray scattering and optical transient absorption spectroscopy methods to disentangle the interdependence of the excited-state electronic and nuclear dynamics. Following photoexcitation, Pt-Pt bond formation and contraction takes up to 1 ps, much slower than the corresponding process in analogous complexes without electron acceptor groups. Because the Pt-Pt distance change is slow with respect to excited-state electron transfer, it can affect the rate of electron transfer. These results have potential impacts on controlling electron transfer rates via structural alterations to the electron donor group, tuning the charge transfer driving force.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Free Space Ramsey Spectroscopy in Rubidium with Noise below the Quantum Projection Limit

In this work, we demonstrate the utility of optical cavity generated spin-squeezed states in free space atomic fountain clocks in ensembles of 390 000 87 Rb atoms. Fluorescence imaging, correlated to an initial quantum nondemolition measurement, is used for population spectroscopy after the atoms are released from a confining lattice. For a free fall time of 4 milliseconds, we resolve a single-shot phase sensitivity of 814(61) microradians, which is 5.8(0.6) decibels (dB) below the quantum projection limit. We observe that this squeezing is preserved as the cloud expands to a roughly 200 μ m radius and falls roughly 300 μm in free space. Ramsey spectroscopy with 240 000 atoms at a 3.6 ms Ramsey time results in a single-shot fractional frequency stability of 8.4(0.2) × 10 –12 , 3.8(0.2) dB below the quantum projection limit. The sensitivity and stability are limited by the technical noise in the fluorescence detection protocol and the microwave system, respectively.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗