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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 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Ab initio polaritonic potential-energy surfaces for excited-state nanophotonics and polaritonic chemistry

Advances in nanophotonics, quantum optics, and low-dimensional materials have enabled precise control of light–matter interactions down to the nanoscale. Combining concepts from each of these fields, there is now an opportunity to create and manipulate photonic matter via strong coupling of molecules to the electromagnetic field. Toward this goal, here we demonstrate a first principles framework to calculate polaritonic excited-state potential-energy surfaces, transition dipole moments, and transition densities for strongly coupled light–matter systems. In particular, we demonstrate the applicability of our methodology by calculating the polaritonic excited-state manifold of a formaldehyde molecule strongly coupled to an optical cavity. This proof-of-concept calculation shows how strong coupling can be exploited to alter photochemical reaction pathways by influencing avoided crossings with tuning of the cavity frequency and coupling strength. Therefore, by introducing an ab initio method to calculate excited-state potential-energy surfaces, our work opens a new avenue for the field of polaritonic chemistry.

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

Long-living excited states of a 2D diamagnetic exciton

Hydrogenic excited states of a 2D exciton are degenerate. In the presence of a weak magnetic field, the -states with a zero momentum of the center of mass get coupled to the -states with finite momentum of the center of mass. This field-induced coupling leads to a strong modification of the dispersion branches of the exciton spectrum. Namely, the lower branch acquires a shape of a “mexican hat” with a minimum at a finite momentum. At certain magnetic field, exciton branches exhibit a linear crossing, similarly to the spectrum of a 2D electron in the presence of spin–orbit coupling. While spin is not involved, degenerate and states play the role of the spin projections. Lifting of degeneracy due to diamagnetic shifts and deviation of electron–hole attraction from purely Coulomb suppresses the linear crossing.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
NASA NTRS1985

Direct observation of the excited-state proton transfer and decay kinetics of internally hydrogen-bonded photostabilizers in copolymer films

The excited-state dynamics of a 2-hydroxyphenylbenzotriazole (HPB) photostabilizer copolymerized with polystyrene are reported. HPB fluorescence from these copolymer films is observed at approximately 630 nm, characteristic of the proton-transferred excited state of HPB, and it has a risetime of less than 10 ps and a decay time of 28 + or - 4 ps at room temperature. Measurement of the relative fluorescence quantum yield as a function of temperature gives the activation energy for nonradiative decay of this state to be E/hc = 259 + or 25/cm.

Oconnor, D. B.↗
DOE OSTI2020

Delayed fluorescence from a zirconium(iv) photosensitizer with ligand-to-metal charge-transfer excited states

Advances in chemical control of the photophysical properties of transition-metal complexes are revolutionizing a wide range of technologies, particularly photocatalysis and light-emitting diodes, but they rely heavily on molecules containing precious metals such as ruthenium and iridium. While the application of earth-abundant ‘early’ transition metals in photosensitizers is clearly advantageous, a detailed understanding of excited states with ligand-to-metal charge transfer (LMCT) character is paramount to account for their distinct electron configurations. Here we report an air- and moisture-stable, visible lightabsorbing Zr(iv) photosensitizer, Zr( Mes PDP Ph ) 2 , where [ Mes PDP Ph ] 2- is the doubly deprotonated form of [2,6-bis(5-(2,4,6- trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine]. This molecule has an exceptionally long-lived triplet LMCT excited state (τ = 350 μs), featuring highly efficient photoluminescence emission (Ф = 0.45) due to thermally activated delayed fluorescence emanating from the higher-lying singlet configuration with significant LMCT contributions. Zr( Mes PDP Ph ) 2 engages in numerous photoredox catalytic processes and triplet energy transfer. Our exploration offers a blueprint for future photosensitizer development featuring early transition metals and excited states with significant LMCT contributions.

14 SOLAR ENERGY↗
DOE OSTI2021

Lifetime measurements of excited states in 15 O

The CNO cycle is the main energy source in stars more massive than our Sun, it defines the energy production and the cycle time that lead to the lifetime of massive stars, and it is an important tool for the determination of the age of globular clusters. One of the largest uncertainties in the CNO chain of reactions comes from the uncertainty in the 14 N(p,γ) 15 O reaction rate. This uncertainty arises predominantly from the uncertainty in the lifetime of the subthreshold state in 15 O at E x = 6792 keV. Previous measurements of this state's lifetime are significantly discrepant. Here, we report on a new lifetime measurement of this state, as well as the excited states in 15 O at E x = 5181 keV and E x = 6172 keV, populated via the 14 N(p,γ) 15 O reaction at proton energies of E p = 1020 keV and E p = 1570 keV. Furthermore, the lifetimes have been determined with the Doppler-Shift Attenuation Method (DSAM) with three separate, nitrogen-implanted targets with Mo, Ta, and W backing. We obtained lifetimes from the weighted average of the three measurements, allowing us to account for systematic differences between the backing materials. For the 6792 keV state, we obtained a τ = 0.6 ± 0.4 fs. To provide cross-validation of our method, we measured the known lifetimes of the states at 5181 and 6172 keV to be τ = 7.5 ± 3.0 and τ = 0.7 ± 0.5 fs, respectively, in good agreement with previous measurements.

6 ≤ A ≤ 19↗
DOE OSTI2023

Efficient core-excited state orbital perspective on calculating x-ray absorption transitions in determinant framework

X-ray absorption spectroscopy (XAS) is an explicit probe of the unoccupied electronic structure of materials and an invaluable tool for fingerprinting various electronic properties and phenomena. Computational methods capable of simulating and analyzing such spectra are therefore in high demand for complementing experimental results and for extracting valuable insights therefrom. In particular, a recently proposed first-principles approach titled many-body XAS (MBXAS), which approximates the final (initial) state as a Slater determinant constructed from Kohn-Sham (KS) orbitals optimized in the absence (presence) of the relevant core electron has shown promising prospects in evaluating the transition amplitudes. In this article, we show that the MBXAS approach can be rederived using a transition operator expressed entirely in the basis of core-excited state KS orbitals and that this reformulation offers substantial practical and conceptual advantages. In addition to circumventing previous issues of convergence with respect to the number of unoccupied ground-state orbitals, the aforementioned representation reduces the computational expense by rendering the calculation of such orbitals unnecessary altogether. Furthermore, the reformulated approach also provides a direct pathway for comparing the many-body approximation with the so-called single-particle treatment and indicates the relative importance in observed XAS intensity of the relaxation of the valence occupied subspace induced by the core excitation. Finally, using the core-excited state basis, we define auxiliary orbitals for x-ray absorption and demonstrate their utility in explaining the spectral intensity by contrasting them with single-particle approximations to the excited state.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2025

An Accurate, Affordable Density Functional Tight-Binding Model for Excited State Hydrocarbon Polymer Molecular Dynamics

We have developed a density functional tight-binding model for hydrocarbon excited state dynamics by referencing to high-level electronic structure theory and incorporating a many-body repulsive energy. We then validate our model against n-octane geometry optimizations, bond dissociation scans, and vibrational frequencies. Our model is approximately 10 3 times more efficient than hybrid time-dependent density functional theory calculations with comparable accuracy. In conclusion, our efforts enable longer timescale excited state simulations of photochemistry and scattering of incident radiation.

Chemical calculations↗
DOE OSTI2026

Ground and excited state gradients with end-to-end differentiable semiempirical quantum chemistry

Accurate and efficient gradients of molecular energy with respect to nuclear degrees of freedom are essential for geometry optimization and molecular dynamics, including simulations that go beyond the Born–Oppenheimer regime. A common approach involves deriving analytical formulas for new electronic structure methods, which is often conceptually difficult and requires tedious coding. Here, we implement analytical, semi-numerical, and automatic differentiation (AD)-based gradient pathways for semiempirical Hamiltonian models in the PYSEQM software package, leveraging both graphics processing unit (GPU) and central processing unit (CPU) architectures. We further extend these capabilities to excited states calculated using the configuration interaction singles and time-dependent Hartree–Fock ansätze. We benchmark wall time, peak memory usage, and accuracy across three molecular families of varying chemical complexity, including systems of up to a thousand atoms. For ground-state simulations, analytical and AD gradients achieve near-identical GPU runtimes, while semi-numerical gradients are slower on GPU but remain competitive on CPU. For excited states, both analytical and custom AD approaches using implicit differentiation show similar performance and low memory requirements, whereas gradients with full AD are memory-limited. AD gradients match analytical ones in accuracy across all tested systems, aided by a quaternion-based diatomic frame rotation for two-center quantities that ensures smooth energy surfaces. Overall, automatic differentiation emerges as a practical alternative to analytical gradients in semiempirical quantum chemistry, offering high accuracy while allowing seamless integration in AI-driven workflows and popular packages, such as PyTorch and JAX. Our results provide actionable guidance for selecting optimal gradient strategies in large-scale ground- and excited-state molecular dynamics simulations.

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

Systematic trends of 0 2 + , 1 1 - , 3 1 - and 2 1 + excited states in even-even nuclei

Here, the spin and parity ( J π ) assignments in even-even nuclei were reviewed across the nuclear chart using the Evaluated Nuclear Structure Data File (ENSDF). The prevalence of 2$+\atop{1}$ first or lowest excited states is confirmed. The properties of 0$+\atop{2}$, 1$-\atop{1}$, and 3$-\atop{1}$ lowest excited states were reexamined using the ENSDF data evaluation procedures. The J π systematic trends and correlations between level quantum numbers and nuclear physics phenomena are discussed.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

3 d-d Excited States of Ni(II) Complexes Relevant to Photoredox Catalysis: Spectroscopic Identification and Mechanistic Implications

Synthetic organic chemistry has seen major advances due to the merger of nickel and photoredox catalysis. A growing number of Ni-photoredox reactions are proposed to involve generation of excited nickel species, sometimes even in the absence of a photoredox catalyst. To gain insights about these excited states, two of our groups previously studied the photophysics of Ni( t-Bu bpy)(o-Tol)Cl, which is representative of proposed intermediates in many Ni-photoredox reactions. This complex was found to have a long-lived excited state (τ = 4 ns), which was computationally assigned as a metal-to-ligand charge transfer (MLCT) state with an energy of 1.6 eV (38 kcal/mol). This work evaluates the computational assignment experimentally using a series of related complexes. Ultrafast UV–Vis and mid-IR transient absorption data suggest that a MLCT state is generated initially upon excitation but decays to a long-lived state that is 3 d-d rather than 3 MLCT in character. Dynamic cis,trans-isomerization of the square planar complexes was observed in the dark using 1 H NMR techniques, supporting that this 3 d-d state is tetrahedral and accessible at ambient temperature. Through a combination of transient absorption and NMR studies, the 3 d-d state was determined to lie ~0.5 eV (12 kcal/mol) above the ground state. Because the 3 d-d state features a weak Ni–aryl bond, the excited Ni(II) complexes can undergo Ni homolysis to generate aryl radicals and Ni(I), both of which are supported experimentally. Thus, photoinduced Ni–aryl homolysis offers a novel mechanism of initiating catalysis by Ni(I).

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

Probing coherent electronic superpositions of singly and doubly excited states of krypton with extreme-ultraviolet four-wave-mixing spectroscopy

Radiative nonlinear four-wave mixing can monitor the evolution of electronic wave packets, providing access to lifetimes and quantifying the light-induced couplings between excited states. In this article, we report the observation of quantum beats in an autoionizing electronic wave packet in krypton, probed using this technique. Analysis of the signal reveals that these beats originate from the contribution of previously unassigned, doubly excited states interacting with singly excited ones. We introduce a minimal theoretical model, based on multichannel quantum-defect theory, which quantitatively reproduces both the wave-packet dynamics and the static spectrum. This work combines a versatile, noncommensurate XUV-IR-based experimental scheme with a tractable model, establishing a powerful approach for the metrology and control of complex, correlated electronic states.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2025

Characterizing the Excited States and Electronic Absorption Spectra of Small Alkylperoxy (RO 2 •) and Hydroperoxy (•QOOH) Radicals

Organic peroxy (ROO•) and hydroperoxy (•QOOH) radicals are key reactive intermediates that are formed via the oxidation of volatile organic compounds during combustion or in the Earth’s atmosphere. Their primary fate is continued unimolecular decay or bimolecular chemistry, the relative branching for which is heavily structure- and temperature-dependent. This article outlines a combined single- and multi-reference quantum chemical study to characterize the near-UV accessible electronically excited states of the prototypical ROO• and •QOOH intermediates, tert-butyl peroxy and hydroperoxy-tert-butyl radicals—the ground-state chemistries of which have been well studied both experimentally and computationally. Additionally, we simulate the electronic absorption profiles of these ROO• and •QOOH intermediates with a variety of multi- and single-reference methods. The results show an interesting conformer dependence on the electronically excited-state character and electronic absorption maxima of •QOOH. The results show promise for electronic absorption spectroscopy to be used as a selected probe for determining •QOOH conformers. Additionally, electronic absorption may contribute to the daytime removal of long-lived •QOOH intermediates formed in the troposphere. We expect that our studies will motivate experiments on the electronic absorption spectra of experimentally achievable ROO• and •QOOH.

excited states↗
DOE OSTI2024

Steric and Electronic Influence of Excited-State Decay in Cu(I) MLCT Chromophores

For the past eleven years, a dedicated effort in our research group focused on fundamentally advancing the photophysical properties of cuprous bis-phenanthroline-based metal-to-ligand charge transfer (MLCT) excited states. We rationalized that by gaining control over the numerous factors limiting the more widespread use of Cu I MLCT photosensitizers, they would be readily adopted in numerous light-activated applications given the earth-abundance of copper and the extensive library of 1,10-phenanthrolines developed over the last century. Significant progress has been achieved by recognizing valuable structure-property concepts developed by other researchers in tandem with detailed ultrafast and conventional time-scale investigations, in-silico-inspired molecular designs to predict spectroscopic properties, and applying novel synthetic methodologies. Ultimately, we achieved a plateau in exerting cooperative steric influence to control Cu I MLCT excited state decay. This led to combining sterics with π-conjugation and/or inductive electronic effects to further exert control over molecular photophysical properties. The lessons gleaned from our studies of homoleptic complexes were recently extended to heteroleptic bis(phenanthrolines) featuring enhanced visible light absorption properties and long-lived room-temperature photoluminescence. Furthermore, this Account navigates the reader through our intellectual journey of decision-making, molecular and experimental design, and data interpretation in parallel with appropriate background information related to the quantitative characterization of molecular photophysics using Cu I MLCT chromophores as prototypical examples.

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

Complex and Unusual Excited-State Relaxation Dynamics of 9,9′-Bifluorenylidene Revealed by Comprehensive Time-Resolved Spectroscopy and MRSF-TDDFT Calculations

Design of novel photochemical molecular motors often requires molecular building blocks that exhibit rather unusual photoactivity, for which conventional analyses of spectroscopic data can lead to conflicting interpretations. We here systematically investigated the excited-state relaxation dynamics of one such molecule, 9,9′-bifluorenylidene (BF), through comprehensive and complementary integration of ultrafast transient absorption (TA) and femtosecond stimulated Raman (FSRS) spectroscopies and first-principles mixed-reference spin-flip time-dependent density functional theory (MRSF-TDDFT). TA and FSRS identified two sequentially formed transients following photoexcitation. The decay kinetics of the two intermediates differ in response to excitation wavelengths and the viscosity/polarity of solvents. MRSF-TDDFT calculations reveal a direct, barrierless internal-conversion pathway from the bright Franck−Condon state to a dark S1 minimum, where the excited-state population is transiently trapped, accounting for the first transient species observed in spectroscopic experiments. Further tracking down along the PES with MRSF-TDDFT mapped out two nonradiative relaxation pathways via conical intersections that connect the dark S 1 state to three configurations in the ground-state manifolds, within which a ring structure with a C8−C8′ bond and the vibrationally excited ground-state BF were identified from spectroscopic and kinetic data. The complexity of relaxation kinetics was attributed to the flexible torsional and twisting motions about the C9−C9′ bridge bond enabled by the diradical character of the S 1 state. These findings clarify unusual photoactive relaxation dynamics stemming from a novel correlation between structural flexibility and shifting electronic characteristics, and they demonstrate the importance of integrating spectroscopic and advanced electronic structure calculation studies for judicious clarification of complex, competing relaxation pathways of excited states.

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

An Ab Initio Electronic Structure Investigation of the Ground and Excited States of ScH + , YH + , and LaH +

Multireference configuration interaction (MRCI), Davidson-corrected MRCI (MRCI+Q), coupled-cluster singles, doubles, and perturbative triples [CCSD(T)], and frozen-core full configuration interaction (fcFCI) calculations were carried out using large, correlation-consistent basis sets to investigate the excited states of the Sc atom and the spin–free and spin–orbit coupled potential energy profiles, energetics, spectroscopic constants, and electron populations of low-lying states of MH + (M = Sc, Y, La). The core electron correlation effects, complete basis set effects, and spin–orbit coupling effects were also evaluated. The first four electronic states of all MH + are 1 2 Δ, 1 2 Σ + , 1 2 Π, and 2 2 Σ + with 1σ 2 1δ 1 , 1σ 2 2σ 1 , 1σ 2 1π 1 , and 1σ 2 3σ 1 single-reference electron configurations, respectively. These states of MH + can be represented by the M 2+ H – ionic structure. The ground states of ScH + , YH + , and LaH + are 1 2 Δ 3/2 , 1 2 Σ + 1/2 , and 1 2 Δ 3/2 with 55.45, 60.54, and 62.34 kcal/mol bond energies, respectively. The core electron correlation was found to be vital for gaining accurate predictions on the ground and excited state properties of MH + . The spin–orbit coupling effects are minor for ScH + but become substantial moving to YH + and LaH + . Overall, the results of this work are in good agreement with the limited set of experimental findings of MH + available in the literature and will be of use for future investigations. Furthermore, the theoretical approaches, findings, and trends reported here are expected to aid studies of similar species.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2022

Ab initio coupled-cluster calculations of ground and dipole excited states in He 8

We perform coupled-cluster calculations of ground- and dipole excited-state properties of the 8 He halo nucleus with nucleon-nucleon and three-nucleon interactions from chiral effective field theory, both with and without explicit delta degrees of freedom. By increasing the precision in our coupled-cluster calculations via the inclusion of leading-order three-particle three-hole excitations in the cluster operator, we obtain a ground-state energy and a charge radius that are consistent with experiment, albeit with a slight underbinding. We also investigate the excited states induced by the electric dipole operator and present a discussion on the Thomas-Reiche-Kuhn and cluster sum rules. Lastly, we compute the electric dipole polarizability, providing a theoretical benchmark for future experimental determinations that will study this exotic nucleus.

6 ≤ A ≤ 19↗