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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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DOE OSTI2023

Excited-State Dynamics during Primary C–I Homolysis in Acetyl Iodide Revealed by Ultrafast Core-Level Spectroscopy

In typical carbonyl-containing molecules, bond dissociation events follow initial excitation to $nπ_{C=O}$$^*$ states. However, in acetyl iodide, the iodine atom gives rise to electronic states with mixed $nπ_{C=O}$$^*$ and $nπ_{C–I}$$^*$ character, leading to complex excited-state dynamics, ultimately resulting in dissociation. Using ultrafast extreme ultraviolet (XUV) transient absorption spectroscopy and quantum chemical calculations, we present an investigation of the primary photodissociation dynamics of acetyl iodide via time-resolved spectroscopy of core-to-valence transitions of the I atom after 266 nm excitation. The probed I 4d-to-valence transitions show features that evolve on sub-100-fs time scales, reporting on excited-state wavepacket evolution during dissociation. These features subsequently evolve to yield spectral signatures corresponding to free iodine atoms in their spin–orbit ground and excited states with a branching ratio of 1.1:1 following dissociation of the C–I bond. Calculations of the valence excitation spectrum via equation-of-motion coupled cluster with single and double substitutions (EOM-CCSD) show that initial excited states are of spin-mixed character. From the initially pumped spin-mixed state, we use a combination of time-dependent density functional theory (TDDFT)-driven nonadiabatic ab initio molecular dynamics and EOM-CCSD calculations of the N$_{4,5}$ edge to reveal a sharp inflection point in the transient XUV signal that corresponds to rapid C–I homolysis. Here, by examining the molecular orbitals involved in the core-level excitations at and around this inflection point, we are able to piece together a detailed picture of C–I bond photolysis in which d → σ* transitions give way to d → p excitations as the bond dissociates. We also report theoretical predictions of short-lived, weak 4d → 5d transitions in acetyl iodide, validated by weak bleaching in the experimental transient XUV spectra. This joint experimental–theoretical effort has thus unraveled the detailed electronic structure and dynamics of a strongly spin–orbit coupled system.

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

Electronic Structure of the Complete Series of Gas-Phase Manganese Acetylacetonates by X-ray Absorption Spectroscopy

Metal centers in transition metal–ligand complexes occur in a variety of oxidation states causing their redox activity and therefore making them relevant for applications in physics and chemistry. The electronic state of these complexes can be studied by X-ray absorption spectroscopy, which is, however, due to the complex spectral signature not always straightforward. Here, we study the electronic structure of gas-phase cationic manganese acetylacetonate complexes Mn(acac) 1–3 + using X-ray absorption spectroscopy at the metal center and ligand constituents. The spectra are well reproduced by multiconfigurational wave function theory, time-dependent density functional theory as well as parameterized crystal field and charge transfer multiplet simulations. This enables us to get detailed insights into the electronic structure of ground-state Mn(acac) 1–3 + and extract empirical parameters such as crystal field strength and exchange coupling from X-ray excitation at both the metal and ligand sites. By comparison to X-ray absorption spectra of neutral, solvated Mn(acac) 2,3 complexes, we also show that the effect of coordination on the L 3 excitation energy, routinely used to identify oxidation states, can contribute about 40–50% to the observed shift, which for the current study is 1.9 eV per oxidation state.

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

Kinetics of Shear-Induced Structural Ordering in Dense Colloids

The macroscopic rheological response of a colloidal solution is highly correlated with the local microscopic structure, as revealed by an in situ Rheo-SAXS experiment with a high temporal resolution. Oscillatory shear can induce a strain-controlled ordering-to-disorder transition, resulting in a shear-thickening process that is different from the normal shear-thickening behavior that is driven by hydrodynamics and particle friction. Here, we reveal that there is a complex time-dependent kinetics toward structural ordering under different applied strains. When the strain amplitude reaches a critical value that starts to induce disordering in the system, the pathway toward the dynamic equilibrium can also become highly non-monotonic. Within the same oscillatory cycle, there is a strong correlation of ordering with different phases of the oscillation, with the system oscillating between two dynamic metastable states.

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

Modulating the Electron Affinity of Small Bipyridyl Molecules on Single Gold Nanoparticles for Plasmon-Driven Electron Transfer

Developing controlled platforms for plasmon-driven chemistry is of great importance in catalytic reactions at the nanoscale. We report anion radical formation for five bipyridyl complexes of varying degrees of electron affinity utilizing optically fo-cused intraband (594 nm) and interband (532 nm) pump excitation of single gold nanoparticles. The surface-enhanced Raman scattering (SERS) of anion radicals for the five non-resonant adsorbed molecules 2,2’-bipyridine (22BPY), 4,4’-bipyridine (44BPY), trans-1,2-bis(4-pyridyl)ethylene (BPE), 1,2-bis(4-pyridyl)acetylene (BPA), and 1,2-bis(4-pyridyl)ethane (BPEt) were detected using localized surface-plasmon resonance (LSPR) excitation with 785 nm. The electron affinity of the five bipyridyl complexes were determined using electrochemistry. Molecules with low electron affinity experienced high-er instances of radical anion formation under a plasmon-coupled intraband electron transfer excitation (594 nm) whereas molecules with high electron affinity showed a preference for anion radical formation under direct interband electron trans-fer excitation (532 nm). The lowest unoccupied molecular orbital (LUMO) energy levels for low electron affinity surface-bound molecules (22BPY, BPEt) are on average ~0.43 eV higher for high electron affinity surface-bound molecules (BPA, BPE, 44BPY) as calculated using time-dependent density functional theory, elucidating the importance of plasmon coupling to energy levels that facilitate charge transfer pathways. We also show the ability to ‘activate’ high vs low electron affinity single nanoparticles with the choice of pump excitation wavelength. The findings show the complex interplay between molecular electron affinity, orbital overlap with the density of states of the plasmonic metal, and excitation energetics of the pump laser wavelength. Potential applications of this work include enhanced control over molecular scale catalysis, biosensor design, and solar energy capture.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Integrated Experimental and Computational K-Edge X-ray Absorption Near-Edge Structure Analysis of Vanadium Catalysts

X-ray Absorption Near-Edge Structure (XANES) spectroscopy is a powerful tool to reveal key structural and electronic features of isolated catalytic sites, yet insights into molecular structure and more detailed orbital analysis through a combination of experimental and computed XANES analysis are necessary for accurate interpretation of the spectra, especially when significant heterogeneity exits among the catalytic sites. Herein, we present an integrated computational and experimental strategy to determine both primary and secondary bonding interactions within the XANES pre-edge region for organovanadium complexes, which was developed using a series of well-defined molecular vanadium complexes and then applied to the characterization of a supported organovanadium olefin hydrogenation catalyst. Time-dependent density functional theory is used to predict the energy of pre-edge XANES features for a series of vanadium complexes with a variety of oxidation states and local coordination environments. Further, a calibration scheme incorporating different density functionals and basis sets is established, resulting in an optimized scheme that accurately predicts pre-edge energies with a mean absolute error of 0.40 eV. Second-shell coordination (e.g., V---V) effects within XANES are identified through the analysis of the computed dominant orbital contributions for multi-vanadium complexes. Orbital analysis also provided confirmation that the vanadium-hydride formation combined with the heterogeneity of the catalytic active species in ole-fin hydrogenation caused the energy shift and broadening of the pre-edge peak after hydrogen treatment of the silica-supported organovanadium pre-catalyst. This work further elucidates computational XANES simulations and techniques potentially guiding characterization in surface organometallic chemistry.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

NO Degradation on the Anatase TiO 2 (001) Surface in the Presence of Water

Nitric oxide (NO) is known to degrade to nitric acid (HNO 3 ) on anatase TiO2 facets under visible light irradiation in the presence of water. However, the exact role that water plays in this photoreaction is not fully understood. Here, by employing the density functional theory (DFT) and time-dependent density functional tight binding (TD-DFTB), we show the viability of two suggested degradation pathways involving water. Both reaction pathways are triggered by a charge transfer excitation from the NO molecule to the TiO 2 surface. In one of them, NO interacts with dissociated water molecules adsorbed on the surface to form HONO + , whereas for the second pathway, NO is oxidized to NO 2 + by capturing one oxygen atom from the substrate. Both HONO and NO 2 are known byproducts in the photodegradation of NO, which further react with adsorbed water to finally produce HNO 3 . We also demonstrate by means of nudged elastic band calculations that these reactions are unlikely to happen without illumination.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

O 2 Oxidation and Sublimation Kinetics of Single Silicon Nanoparticles at 1200–2050 K: Variation of Reaction Rates, Evolution of Structural and Optical Properties, and the Active-to-Passive Transition

Sublimation and O 2 etching kinetics for a series of individual silicon (Si) nanoparticles (NPs) were studied for NP temperatures (T NP ) from 1200 to 2050 K, using a single NP mass spectrometry technique. Sublimation was significant for T NP > 1700 K, with rates reasonably well fit to Arrhenius kinetics, but evolving, particularly during initial heating. O 2 etching efficiencies varied from NP-to-NP and with changing T NP , but also evolved dramatically over time. For T NP ≤ 1500 K, NPs were observed to passivate after losing 30 to 50% of the initial NP mass. At higher T NP , etching efficiency decreased over time, but never passivated. Interestingly, bulk Si passivation has not been observed for the range of T NP and O 2 pressures used here, and a model was developed to test the effects of several NP-specific mechanistic parameters on both the initial and time-dependent etching behavior. As a result, the optical properties of the hot NPs were also found to evolve as the NPs etched, particularly during the initial fast mass loss, and correlations between emission intensities and etching kinetics were examined.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2024

Electronic Structure of Excitons in Hematite Fe 2 O 3

We present a computational study of the electronic structure of excitons in hematite Fe 2 O 3 using density functional theory (DFT) and time-dependent DFT theory. Excitons resulting from solar light absorption are precursors to photogenerated charge carriers, which are the active species in solar-to-fuel redox conversions. Upon vertical excitation, the lowest energy exciton is found to have a multideterminantal wave function, describing an electron excited from O 2p states to Fe 3d states of the nearest basal plane, possibly supporting a recent XUV-derived exciton structure with a radius of a single Fe–O bond. Subsequently, the exciton self-traps into stable electron–hole pair structures whose wave functions are well-approximated by single excitations conveniently labeled “HOMO–LUMO”. Self-trapping yields electron–hole pair structures whereby the electron state (electron polaron-like) is separated from the hole state (hole polaron-like) by 3, 5, 7, 9, ... basal planes in structures referred to as Exc-3, Exc-5, Exc-7, Exc-9, ... The natural transition orbitals (electron–hole states) exhibit a strong “localized” character. The hole state is best described as a (FeO 6 ) + octahedral moiety carrying ~0.65 h + charge, and overall ~70% of the hole is assigned to O 2p atomic states, mostly on the four (4) equatorial O atoms of the moiety. The electron state is best described as a (FeFe) – moiety carrying ~0.80 e – charge in Fe 3d atomic states. The lattice distortions around the hole site exhibit Fe–O bond shortening due to the removal of electron density from an orbital state with O–Fe–O antibonding character. The electron site shows Fe–Fe distance shortening due to excess electron density added to an orbital state with in-phase Fe–Fe interactions. Furthermore, these excitonic structures with increasing electron–hole separation can be viewed as the onset of exciton separation into isolated electrons and hole polarons.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Modeling Surface-Enhanced Raman Scattering of Au-Pyrazine and Au-Pyrazine-Au Nanorod Dimer Systems with the TD-DFTB Method

This study investigates Raman scattering from a pyrazine molecule adsorbed on gold nanorod surfaces using the time-dependent density functional tight-binding (TD-DFTB) method. We analyze surface enhanced Raman scattering (SERS) of the molecule for two configurations: one where the molecule is adsorbed on a single gold nanorod forming Au-pyrazine complex, and another where it forms a nano-junction Au-pyrazine-Au dimer system. These two configurations offer distinct chemical environments for the molecule and different local field enhancements, with the dimer systems generating enhanced hotspot regions at the junctions. We present results for nanojunction, Au-pyrazine-Au dimer structures, for variable nanogap sizes that lead to shift in the plasmon energy. The study identifies contributions to the SERS enhancements from chemical, electromagnetic, and resonance charge transfer mechanisms. Our results show that the chemical mechanism provides enhancement factors in the range of 10 2 –10 3 , while the electromagnetic enhancements are on the order of 10 3 –10 5 for monomer systems and 10 6 –10 8 for dimer structures. We present results for a nanorod with a length of 5.3 nm and a width of 0.52 nm, consisting of 121 gold atoms.

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

An Atomistic Study of Reactivity in Solid-State Electrolyte Interphase Formation for Li/Li7P3S11

Lithium metal batteries offer superior volumetric and gravimetric specific capacities compared to those based on traditional graphite anodes. Although advancements in solid-state electrolytes address safety concerns, challenges remain, particularly regarding interphase formation in lithium metal anodes. This work presents a computational framework based on high-throughput first-principles density functional theory and machine-learning interatomic potentials (MLIPs) including automated iterative, active learning to enable robust computational exploration of interphase formation between lithium metal anodes and an inorganic solid-state electrolyte. As a demonstration, we apply the framework to a Li/Li7P3S11 interface and find that it accurately identifies the experimentally observed, thermodynamically stable interphase products as well as their overall spatial arrangement within a heterogeneous, amorphous layered structure, with Li2S domains of nanocrystallinity. Our simulations show two stages, a fast and slow diffusion reaction regime, that corroborate the relative phase formation rate of Li x P, Li2S, and Li3P. Using the Onsager transport theory, we capture time-dependent ionic diffusion within the reacting interface, including cross-correlation effects. We found that cross-correlation effects between Li-P and P-S ionic motion significantly influence P-ion diffusion, making it highly sensitive to the local environment and potentially leading to "kinetic trapping" of Li-P phases. The passivation of the interface is shown as the ionic fluxes all approach zero, effectively halting interphase growth.

Diffusion↗
DOE OSTI2026

Theoretical Study of High Harmonic Generation in Monolayer NbSe 2

High harmonic generation (HHG) is a powerful probe of electron dynamics on attosecond to femtosecond time scales and has been successfully used to detect electronic and structural changes in solid-state quantum materials, including transition-metal dichalcogenides (TMDs). Among TMDs, bulk NbSe 2 exhibits charge density wave (CDW) order below 33 K and becomes superconducting below 7.3 K. Monolayer NbSe 2 also has superconducting and CDW behavior and is therefore interesting as a material whose different structural and electronic properties could be probed via HHG. Here, we predict the HHG response of the pristine 2H and CDW phases of monolayer NbSe 2 using real-time time-dependent density functional theory under the application of a simulated laser pulse excitation. We find that due to the lack of inversion symmetry in both monolayer phases, it is possible to excite even harmonics and that the even harmonics appear as the transverse components of the current response under excitations polarized along the zigzag direction of the monolayer, while odd harmonics arise from the longitudinal current response in all excitation directions. This suggests that the even and odd harmonic responses can be controlled via the polarization of the probing field, opening an avenue for potentially useful applications in optoelectronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2025

The Kinetics of the Coherent Stage of Hydroxide Corrosion on Li 2 O-Covered LiH

The capability to model LiOH growth on vacuum-baked LiH is a necessary precursor to making kinetic predictions of undesirable hydrogen outgassing from corroded LiH materials when placed in sealed-system applications. Here, in this work, LiH samples with initial LiOH surface corrosion layers less than 1.1 μm were outgassed by vacuum baking at high temperature to convert LiOH to Li 2 O. Diffuse-reflectance infrared Fourier transform (DRIFT) spectroscopy was used to measure the subsequent LiOH regrowth during exposure to 25–375 ppm of H 2 O vapor at room temperature. Logarithmic kinetics best model the growth of this coherent hydroxide corrosion layer (up to 1.5 μm) on previously vacuum-baked LiH samples. The logarithmic kinetics are attributed to the dissociation of H 2 O on the corrosion layer and the establishment of a H + /OH – electric field across the thickening LiOH corrosion layer. A time-dependent model of LiOH corrosion growth as a function of both initial LiOH thickness before vacuum baking and moisture partial pressures during re-exposure was developed to help researchers better assess the unwanted hydrogen outgassing potential from LiH/LiD materials.

Matt, Sarah M. [Lawrence Livermore National Labora↗
DOE OSTI2026

When Do Band Gap Calculations Agree with Experiments in Monolayer-Protected Cu 14 and Au 20 Atomically Precise Nanoclusters? A (TD)-DFT Comparison of HOMO–LUMO, Fundamental, Optical, and Electrochemical Energy Gaps

In view of the tremendous progress in atomically precise metal nanoclusters where electrochemical and optical energetics are routinely supported by computations to establish structure−function correlations, we explore the relationship between different protocols for measuring and computing band gaps of two distinct organic ligand-protected nanoclusters: [Cu 14 H 10 (MBN) 3 (PPh 3 ) 8 ] + and Au 20 (TBBT) 16 . Through UV/visible spectroscopy and differential pulse voltammetry, we measure optical and electrochemical band gaps in those systems. We then compare these experimentally determined gaps to HOMO−LUMO gaps, fundamental gaps, vertical excitation energies, and E o ox − E o red potentials computed using different density functional theory (DFT) or time-dependent DFT (TDDFT) methods. Specifically, in both copper and gold nanoclusters, we test the effect of truncating inert ligands from the model and compare density functionals with varying degrees of Hartree−Fock (HF) exchange from 0 to 50%, range-separated hybrids with a varying long-range tuning parameter, different correlation functionals, basis sets, and (equilibrium and nonequilibrium) continuum solvation models. Despite having different frontier orbital characters (the copper nanocluster has a metal-to-ligand charge transfer character while the gold nanocluster has metal-centered frontier orbitals), both nanoclusters display a similar sensitivity of the HOMO−LUMO gap to the HF exchange that is partially mitigated when computing the fundamental, optical, and electrochemical gaps. Other factors, such as the nature of the correlation functional, basis set, and geometry relaxation, have a considerably smaller effect on computed band gaps in these systems. Overall, this work provides guidelines for factors of varied importance for correlating computed and experimental band gap values.

Chemical calculations↗
DOE OSTI2020

First-Principles Approach for Assessing Cold Electron Injection Efficiency of Dye-Sensitized Solar Cell: Elucidation of Mechanism of Charge Injection and Recombination

The adequacy of the inclusion of spacer units in the metal-free D-π–A organic dyes pertaining to the augmentation of dye-sensitized solar cell (DSSC) efficiency has been examined through the excited-state simulations of the charge injection and recombination processes at the dye–semiconductor interface. Within the framework of the time-dependent density functional theory, the proposed computational studies focus on the precise evaluation of pivotal factors controlling the rates of photoinduced charge-transfer and energy-transfer processes, including electronic coupling, reorganization energy, and threshold energy barrier in the semiclassical Marcus formalism. The estimation of the fluorescent state appears to be the crucial step while explaining the ultrafast electron injection process and the charge recombination at the Marcus inverted region, as revealed by the obtained results. The retardation of charge recombination is facilitated by the insertion of a thiophene moiety between the π-bridge and the acceptor units. The estimated cold electron injection efficiencies deploying the Onsager–Braun theory, that rely on the computations of cold electron injection lifetime and cold electron lifetime, show a linear correlation with the experimental photovoltaic parameters of the DSSC comprising short-circuit current density, open-circuit voltage, and power conversion efficiency. The outcomes of the present investigation establish a basis for unraveling the mechanism of intricate dynamical processes upon photoexcitation of the sensitizers, as well as devising plausible routes for functional DSSC materials.

14 SOLAR ENERGY↗
DOE OSTI2020

Frequency and Time Domain Nuclear-Electronic Orbital Equation-of-Motion Coupled Cluster Methods: Combination Bands and Electronic-Protonic Double Excitations

The accurate description of excited vibronic states is important for modeling a wide range of photoinduced processes. The nuclear-electronic orbital (NEO) approach, which treats specified protons on the same level as the electrons, can describe excited electronic-protonic states. Herein the multicomponent equation-of-motion coupled cluster with singles and doubles (NEO-EOM-CCSD) method and its time-domain counterpart, TD-NEO-EOM-CCSD, are developed and implemented. The application of these methods to the HCN molecule highlights their capabilities. These methods predict qualitatively reasonable energies and intensities for a combination band corresponding to simultaneous excitation of two vibrational modes, as well as an overtone. These methods also describe states with double excitation character, such as excited electronic-protonic states corresponding to the simultaneous excitation of an electron and a proton. The ability of the NEO-EOM-CCSD method and its time-dependent counterpart to describe combination bands, overtones, and double excitations will enable a wide range of photochemical applications.

Pavosevic, Fabijan↗
DOE OSTI2021

Excited-State Charge Distribution of a Donor–π–Acceptor Zn Porphyrin Probed by N K-Edge Transient Absorption Spectroscopy

Zinc porphyrin solar cell dyes with donor-π–acceptor architectures combine light absorber (π), electron-donor, and electron-acceptor moieties inside a single molecule with atomic precision. The donor-π–acceptor design promotes the separation of charge carriers following optical excitation. Here, we probe the excited-state electronic structure within such molecules by combining time-resolved X-ray absorption spectroscopy at the N K-edge with first-principles time-dependent density functional theory (TD-DFT) calculations. Furthermore, customized Zn porphyrins with strong-donor triphenylamine groups or weak-donor tri-tert-butylbenzene groups were synthesized. Energetically well-separated N K-edge absorption features simultaneously probe the excited-state electronic structure from the perspectives of the macrocycle and triphenylamine N atoms. New absorption transitions between the macrocycle N atoms and the excited-state HOMO vacancy are observed, and the triphenylamine associated absorption feature blue-shifts, consistent with partial oxidation of the donor groups in the excited state.

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

X-ray Free Electron Laser-Induced Synthesis of ε -Iron Nitride at High Pressures

The ultrafast synthesis of ε-Fe 3 N 1+x in a diamond-anvil cell (DAC) from Fe and N 2 under pressure was observed using serial exposures of an X-ray free electron laser (XFEL). When the sample at 5 GPa was irradiated by a pulse train separated by 443 ns, the estimated sample temperature at the delay time was above 1400 K, confirmed by in situ transformation of α- to γ-iron. Ultimately, the Fe and N 2 reacted uniformly throughout the beam path to form Fe 3 N 1.33 , as deduced from its established equation of state (EOS). We thus demonstrate that the activation energy provided by intense X-ray exposures in an XFEL can be coupled with the source time structure to enable exploration of the time-dependence of reactions under high-pressure conditions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Relationship Between Hydrogen-Bonding Motifs and the 1b1 Splitting in the X-ray Emission Spectrum of Liquid Water

The split of the 1??! peak observed in the X-ray emission (XE) spectrum of liquid water has been the focus of intense research over the last two decades. Although several hypotheses have been proposed to explain the origin of the 1??! splitting, a general consensus has not yet been reached. In this study, we introduce a novel theoretical/computational approach which, combining pathintegral molecular dynamics (PIMD) simulations carried out with the MB-pol potential energy function and time-dependent density functional theory (TD-DFT) calculations, correctly predicts the split of the 1??! peak in liquid water and not in crystalline ice. A systematic analysis in terms of the underlying local structure of liquid water at ambient conditions indicates that several different hydrogen-bonding motifs contribute to the overall XE lineshape in the energy range corresponding to emissions from the 1??! orbitals, which suggests that it is not possible to unambiguously attribute the split of the 1??! peak to only two specific structural arrangements of the underlying hydrogen-bonding network.

Cruzeiro, Vinicius↗