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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Benchmarking the semi-stochastic CC( P;Q ) approach for singlet–triplet gaps in biradicals

Here, we recently proposed a semi-stochastic approach to converging high-level coupled-cluster (CC) energetics, such as those obtained in the CC calculations with singles, doubles, and triples (CCSDT), in which the deterministic CC(P;Q) framework is merged with the stochastic configuration interaction Quantum Monte Carlo propagations [J. E. Deustua, J. Shen, and P. Piecuch, Phys. Rev. Lett. 119, 223003 (2017)]. In this work, we investigate the ability of the semi-stochastic CC(P;Q) methodology to recover the CCSDT energies of the lowest singlet and triplet states and the corresponding singlet–triplet gaps of biradical systems using methylene, (HFH) − , cyclobutadiene, cyclopentadienyl cation, and trimethylenemethane as examples.

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

Photoluminescence spectra of point defects in semiconductors: Validation of first-principles calculations

Optically and magnetically active point defects in semiconductors are interesting platforms for the development of solid state quantum technologies. Their optical properties are usually probed by measuring photoluminescence spectra, which provide information on excitation energies and on the interaction of electrons with lattice vibrations. We present a combined computational and experimental study of photoluminescence spectra of defects in diamond and SiC, aimed at assessing the validity of theoretical and numerical approximations used in first-principles calculations, including the use of the Franck-Condon principle and the displaced harmonic oscillator approximation. We focus on prototypical examples of solid state qubits, the divacancy centers in SiC and the nitrogen-vacancy in diamond, and we report computed photoluminescence spectra as a function of temperature that are in very good agreement with the measured ones. As expected we find that the use of hybrid functionals leads to more accurate results than semilocal functionals. Interestingly our calculations show that constrained density functional theory (CDFT) and time-dependent hybrid DFT perform equally well in describing the excited state potential energy surface of triplet states; our findings indicate that CDFT, a relatively cheap computational approach, is sufficiently accurate for the calculations of photoluminescence spectra of the defects studied here. Finally, we find that only by correcting for finite-size effects and extrapolating to the dilute limit can one obtain a good agreement between theory and experiment. Our results provide a detailed validation protocol of first-principles calculations of photoluminescence spectra, necessary both for the interpretation of experiments and for robust predictions of the electronic properties of point defects in semiconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Magnetic excitations from the hexagonal spin clusters in the 𝑆 = $\frac{1}{2}$ distorted honeycomb lattice antiferromagnet Cu 2 ⁢(pymca)⁢ 3 ⁢(ClO 4 )

Cu 2 ⁢(pymca) ⁢3 (ClO 4 ) (pymca: pyrimidine-2-carboxylate) consists of a slightly distorted honeycomb lattice of Cu 2+ spins, which shows no long-range magnetic order down to 0.6 K. A magnetization study revealed 1/3 and 2/3 plateau phases [A. Okutani et al., J. Phys. Soc. Jpn. 88, 013703 (2019)], which is not expected for regular honeycomb antiferromagnets. Inelastic neutron scattering experiments were performed using a powder sample to investigate the exchange interactions of this material. The spin excitations from the singlet ground state to the first three triplet states, predicted from the antiferromagnetic hexagonal spin cluster interacting with 3.9 meV, were observed. Using the exact diagonalization methods, the intercluster coupling was estimated from the excitation peak width to be about 20% of the intracluster interaction, which is consistent with the previously reported value. Finally, our exchange path model explains the anisotropic exchange interactions in the distorted honeycomb plane.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Splitting photons: Singlet fission in nanocrystal-molecule hybrid structures

The goal of this research is to enable all the energy contained in sunlight to be harvested by making full use of the energy contained in the blue and green wavelengths of light. Current systems are unable to extract all of the energy available from photons in this wavelength range due to rapid relaxation processes that dissipate a fraction of the energy as heat. In this work, inexpensive, earth-abundant components capable of supporting multi-excitonic processes involving more than one tightly bound excited state are investigated as a way to exceed the Shockley-Queisser limit. This research will examine hybrid organic-inorganic nanostructures capable of singlet fission, a process by which one high-energy spin-singlet state is converted into two lower-energy spin-triplet states, and subsequent triplet exciton transfer. Specifically, the organic molecules diphenylhexatriene and tetracene that maximally absorb blue light and are known to exhibit singlet fission will be bound to lead chalcogenide nanocrystals. A variety of steady-state and time-resolved spectroscopic techniques will be used to study the transfer of energy from spin-triplet excitons that are created in the organic molecules to the nanoparticle acceptors. The hybrid nanostructures here will be fully characterized both in solution or thin film via electronic absorption and photoluminescence spectroscopy, nuclear magnetic resonance spectroscopy, high-resolution mass spectrometry, transmission electron microscopy, photoelectron spectroscopy, time correlated single photon counting and transient absorption experiments. The effect of molecular and nanocrystal structure on the electronic coupling between the hybrid components will be examined to establish fundamental relationships between structure and triplet energy transfer efficiency. The findings will be directly applicable to a potential tetracene-silicon platform that may ultimately enhance the power conversion efficiency of silicon solar cells.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2020

Enhanced intersystem crossing of boron dipyrromethene by TEMPO radical

Radical enhanced intersystem crossing (EISC) of organic chromophores is an important approach to generate a long-lived triplet state for various electronic and optoelectronic applications. However, structural factors and design rules to promote EISC are not entirely clear. In this work, we report a series of boron dipyrromethene (BODIPY) derivatives covalently linked with a 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) radical with varying distances and topologies. We show that the incorporation of the TEMPO radical to BODIPY results in strong fluorescence quenching by up to 85% as a result of EISC and enhanced internal conversion. In BDP-2AR [2-(4-methyleneamino-TEMPO) BODIPY], a dyad with the shortest BODIPY–TEMPO through-bond distance, we observe the fastest EISC rate (τ isc = 1.4 ns) and the longest triplet excited state lifetime (τ T = 32 μs) compared to other distance and geometry variations. Contrary to previous reports and a general presumption, the BODIPY–TEMPO through-bond distance in this system does not play a significant role on the triplet formation rate and yield. Finally, density functional theory suggests a folding of the TEMPO radical to form a sandwich-like structure with a BODIPY ring that leads to a decrease in the through-space distance, providing a new and an interesting insight for the radical enhanced intersystem.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1994

The Low-Lying States of AlCu and AlAg

The singlet and triplet states of AlCu and AlAg below about 32 000/cm are studied using the internally contracted multireference configuration-interaction method. A more elaborate study of the X(sup 1)Sum(sup +) ground state of AlCu is undertaken using extended Gaussian basis sets, including the effect of inner-shell correlation and including a perturbational estimate of relativistic effects. Our best estimate of the spectroscopic constants (r(sub 0), DeltaG(sub 1/2), and D(sub 0)) for the X(sup 1)Sum(sup+) state with the experimental values in parentheses are: 4.416(4.420) a(sub 0), 295 (294) /cm, and 2.318 (2.315) eV. The calculations definitively assign the upper state in the observed transition at 14 892/cm to the lowest (sup 1)Prod state. The calculated spectroscopic constants and radiative lifetime for the (sup 1)Prod state are in good agreement with experiment. The calculations support the tentative assignments of Behm et al. for three band systems observed in the visible region between 25 000 and 28 000 / cm. However, the computed spectroscopic constants are in very poor agreement with those deduced from an analysis of the spectra. Analogous theoretical results for AlAg suggest that the (2)(sup 3)Prod, (3)(sup 3)Prod, and (3)(sup 1)Sum(sup +) states account for the bands observed, but not assigned, by Duncan and co-workers.

Bauschlicher, Charles W., Jr.↗
DOE OSTI2020

Complete collision data set for electrons scattering on molecular hydrogen and its isotopologues: I. Fully vibrationally-resolved electronic excitation of H 2 $X^1Σ^+_g)$

Here, we present a comprehensive set of vibrationally-resolved cross sections for electron-impact electronic excitation of molecular hydrogen suitable for implementation in collisional-radiative models. The adiabatic-nuclei molecular convergent close-coupling method is used to calculate cross sections for excitation of all bound vibrational levels and dissociative excitation of the B 1 Σ u + , C 1 Π u , E F 1 Σ g + , B ′ 1 Σ u + , G K 1 Σ g + , I 1 Π g , J 1 Δ g , D 1 Π u , H 1 Σ g + , b 3 Σ u + , c 3 Π u , a 3 Σ g + , e 3 Σ u + , d 3 Π u , h 3 Σ g + , g 3 Σ g + , i 3 Π g , and j 3 Δ g electronic states from all –14 bound vibrational levels of the ground electronic ($\text{X}$ $^1&#x3A3^+_g$) state. The data set consists of cross sections from threshold to 500 eV for over 5000 transitions, representing all possible electronic and vibrational transitions between the state and the –3 singlet and triplet states (where refers to the united-atoms-limit principle quantum number). The cross sections are presented in graphical form and provided as both numerical values and analytic fit functions in supplementary data files.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2024

Spin-dependent electron transfer in electrochemically transparent van der Waals heterostructures for oxygen evolution reaction

Spin selective catalysis is an emerging approach for improving the thermodynamics and kinetics of reactions. The role of electron spins has been scarcely studied in catalytic reactions. One exception is the oxygen evolution reaction (OER) where strongly correlated metals and oxides are used as catalysts. In OER, spin alignment facilitates the transition of singlet state of the reactant to the triplet state of O 2 . However, the influence of strong correlations on spin exchange mechanism and spin selective thermodynamics of most catalytic reactions remain unclear. Here we decouple the strongly correlated catalyst from the electrolyte to study spin exchange in two-dimensional (2D) magnetic iron germanium telluride (FGT) heterostructure. We demonstrate that transmission of spin and electrochemical information between the catalyst and the reactant can occur through quantum exchange interaction despite the catalyst of FGT being completely encapsulated by graphene or hexagonal boron nitride (hBN). The strong correlations in FGT that lead to enhanced spin exchange in OER are observed in graphene or hBN layers with thicknesses of up to 6 nm. We demonstrate that spin alignment in FGT leads to a lowering of thermodynamic barrier for adsorption of hydroxide ion and electron transfer to the catalyst. This results in up to fivefold enhancement in OER performance and improved kinetics. Our results provide clear evidence that transmission of both quantum mechanical and electrochemical information through quantum spin exchange interaction in FGT leads to an enhancement in catalytic performance.

2D materials↗
DOE OSTI2021

Potent strategy towards strongly emissive nitroaromatics through a weakly electron-deficient core

Nitroaromatics seldom fluoresce. The importance of electron-deficient (n-type) conjugates, however, has inspired a number of strategies for suppressing the emission-quenching effects of the strongly electron-withdrawing nitro group. Here, we demonstrate how such strategies yield fluorescent nitroaryl derivatives of dipyrrolonaphthyridinedione (DPND). Nitro groups near the DPND core quench its fluorescence. Conversely, nitro groups placed farther from the core allow some of the highest fluorescence quantum yields ever recorded for nitroaromatics. This strategy of preventing the known processes that compete with photoemission, however, leads to the emergence of unprecedented alternative mechanisms for fluorescence quenching, involving transitions to dark nπ* singlet states and aborted photochemistry. Forming nπ* triplet states from ππ* singlets is a classical pathway for fluorescence quenching. In nitro-DPNDs, however, these ππ* and nπ* excited states are both singlets, and they are common for nitroaryl conjugates. Understanding the excited-state dynamics of such nitroaromatics is crucial for designing strongly fluorescent electron-deficient conjugates.

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

Influence of substitution pattern on the dynamics of internal conversion and intersystem crossing in thiopyridone isomers

We report a combined experimental and theoretical investigation of the ultrafast internal conversion (IC) and intersystem crossing (ISC) dynamics of two thiopyridone (TP) isomers in solution. Our study used ultrafast transient X-ray absorption spectroscopy (XAS) at the sulfur K-edge, in conjunction with electronic excited state surface hopping molecular dynamics and simulations of the excited state XAS, to investigate the impact of the functional group substitution pattern and solvent on the dynamics of IC and ISC. The combination of the localized X-ray probe and the simulation results enables, in part, the differentiation between ππ* and nπ* character excited states, as well as singlet and triplet states. Access to nπ* character excitations has particular value since they often prove challenging to assess with optical spectroscopy. For 2-TP, the photoexcited S 2 (ππ*) state rapidly undergoes IC to the S 1 (nπ*) state below the instrument response time, followed by ISC to the T 1 (ππ*) state on a timescale of 600 fs in acetonitrile. For 4-TP, the timescale of S 2 to S 1 IC increases to 330 fs and the timescale of ISC increases to more than 10 ps. The differences between isomers are rationalized by considering the key role of the, nπ* intermediates in mediating the intersystem crossing of these systems. Varying the substitution pattern of the molecule can stabilize or destabilize these intermediates leading to the increase in ISC rate in the ortho isomer as compared to the para isomer, while changing the solvent from acetonitrile to water had minimal effect on the electronic excited state relaxation mechanism.

Garratt, Douglas [SLAC National Accelerator Labora↗
NASA NTRS1973

Positronium origin of 476 keV galactic feature.

Leventhal noticed that the gamma-ray spectrum due to the annihilation of positronium, which consists of two 511 keV photons from the singlet state and three photons from the triplet state, produces a spectral feature with an apparent peak at an energy less than 511 keV when viewed with a gamma-ray telescope having a Gaussian energy resolution. He calculated that the observed peak will lie at 490 keV. The author calculates that if the positronium spectrum sits atop a steeply falling continuum due to other sources, then the apparent peak can easily fall near 476 keV where it was observed. It is shown that explosive nucleosynthesis is a plausible source of the positrons.

Clayton, D. D.↗
DOE OSTI2023

Computational tools for the simulation and analysis of spin-polarized EPR spectra

The EPR spectra of paramagnetic species induced by photoexcitation typically exhibit enhanced absorptive and emissive features resulting from sublevel populations that differ from thermal equilibrium. The populations and the resulting spin polarization of the spectra are dictated by the selectivity of the photophysical process generating the observed state. Simulation of the spin-polarized EPR spectra is crucial in the characterization of both the dynamics of formation of the photoexcited state as well as its electronic and structural properties. EasySpin, the simulation toolbox for EPR spectroscopy, now includes extended support for the simulation of the EPR spectra of spin-polarized states of arbitrary spin multiplicity and formed by a variety of different mechanisms, including photoexcited triplet states populated by intersystem crossing, charge recombination or spin polarization transfer, spin-correlated radical pairs created by photoinduced electron transfer, triplet pairs formed by singlet fission and multiplet states arising from photoexcitation in systems containing chromophores and stable radicals. In this paper, we highlight EasySpin’s capabilities for the simulation of spin-polarized EPR spectra on the basis of illustrative examples from the literature in a variety of fields ranging across chemistry, biology, material science and quantum information science.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS2016

The Low-Lying Electronic States of MgO

The low-lying singlet and triplet states of MgO have been studied using a SA-CASCF/ICMRCI approach using the aug-cc-pV5Z basis set. The spectroscopic constants (r(sub e), W(sub e), and T(sub e)) are in good agreement with the available experimental data. The computed lifetime for the B state is in excellent agreement with two of the three experimental results. The d state lifetime is in good agreement with experiment, while the computed D state lifetime is about twice as long as experiment.

Charles W Bauschlicher↗
DOE OSTI2025

Spin-Vibronic Effect in Photoinduced Electron Transfer

The spin-vibronic effect (SVE) accelerates quantum-mechanically forbidden electronic transitions, but experimental manifestations of this phenomenon remain limited. In this contribution, the role of SVE in photoinduced electron transfer (PET) dynamics is probed by coherent vibrational wavepacket (CVWP) motions in Pt­(II) dimer-naphthalene diimide donor–acceptor complexes. Metal–metal-to-ligand charge-transfer (MMLCT) excitation of the donor triggers ballistic PET, driving the molecule away from equilibrium. The SVE then directs the PET trajectory from the 1 MMLCT excited state to an intermediate ligand-centered triplet state, modulating the CVWP dynamics along the reaction coordinates. Herzberg–Teller type oscillations indicate that the Pt–Pt stretching vibration, arising from the formation of the excited triplet intermediate, serves as the reaction coordinate for charge separation. Finally, our experimental findings advance the understanding of the interplay between electronic, vibrational, and spin degrees of freedom in ultrafast photoactivated processes.

Kim, Pyosang [Argonne National Laboratory (ANL), A↗
DOE OSTI2021

Direct coherent switching with decay of mixing for intersystem crossing dynamics of thioformaldehyde: The effect of decoherence

We evaluate the effect of electronic decoherence on intersystem crossing in the photodynamics of thioformaldehyde. First, we show that the state-averaged complete-active-space self-consistent field electronic structure calculations with a properly chosen active space of 12 active electrons in 10 active orbitals can predict the potential energy surfaces and the singlet–triplet spin–orbit couplings quite well for CH 2 S, and we use this method for direct dynamics by coherent switching with decay of mixing (CSDM). We obtain similar dynamical results with CSDM or by adding energy-based decoherence to trajectory surface hopping, with the population of triplet states tending to a small steady-state value over 500 fs. Without decoherence, the state populations calculated by the conventional trajectory surface hopping method or the semiclassical Ehrenfest method gradually increase. This difference shows that decoherence changes the nature of the results not just quantitatively but qualitatively.

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

High-resolution infrared spectroscopy of supersonically cooled singlet carbenes: Bromomethylene (HCBr) in the CH stretch region

First high-resolution spectra of cold (~35 K) singlet bromomethylene HCBr in the CH stretching (v 1 ) region from 2770 to 2850 cm-1 are reported using near quantum shot-noise limited laser absorption methods in a slit jet supersonic discharge expansion source. Three rovibrational bands are identified at high S/N (20:1 - 40:1) and rotationally assigned to i) the CH stretch fundamental (v 1 ) band $\tilde{X}$(1,0,0) ← $\tilde{X}$ (0,0,0) and ii) vibrational hot bands ($\tilde{X}$(1,1,0) ← $\tilde{X}$(0,1,0) and $\tilde{X}$(1,0,1) ← $\tilde{X}$(0,0,1)) arising from vibrationally excited HCBr populated in the discharge with single quanta in either the H–C–Br bend (v 2 ) or C–Br stretch (v 3 ) modes. Precision rotational constants are reported for a total of six states, with an experimentally determined CH stretch vibrational frequency (2799.38 cm -1 ) in good agreement with previous low-resolution fluorescence studies. Detailed analysis of the fundamental v 1 band highlights the presence of perturbations in the $\tilde{X}$(1,0,0) level, which we tentatively attribute to arise from the nearby triplet state $\tilde{a}$(0,0,1) through spin-orbit interaction or the multiple quanta $\tilde{X}$(0,2,1) singlet state via c-type Coriolis coupling. Reduced-Doppler resolution (60 MHz) in the slit-jet IR spectrometer permits clear observation of nuclear spin hyperfine structure, with experimental line shapes well reproduced by nuclear quadrupole/spin-rotation coupling constants from microwave studies. Lastly, the a-type to b-type transition intensity ratio for the fundamental CH stretch band is notably larger than predicted by a bond-dipole model, which from high level ab initio quantum calculations (CCSD(T)/PVQZ) can be attributed to vibrationally induced “charge-sloshing” of electron density along the polar C–Br bond.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗