Low State Research Articles

Electron-donor-impurity polarizability in a semiconductor nanostructure under electric field: Geometrical effects in ellipsoidal quantum dots

The features of electron-impurity polarizability in a quantum nanostructure are investigated from the knowledge of allowed electron states coupled to a shallow-donor impurity. The influence of geometry on the carrier confinement and the presence of external probes such as electric fields are explicitly considered. For the sake of illustration, the example of CdS/ZnSe-based prolate ellipsoidal core/shell quantum dot (PECSQD) configuration is chosen. This problem allows to consider a finite confinement potential model, employing both the finite element method and the effective mass approximation. Within this framework, impurity-related energies and associated polarizability for the lowest states were determined as functions of the shell thickness, core dimensions, impurity position, and applied electric field strength - while keeping the shell dimensions fixed. Electron-impurity energies are largely affected by electric field values. Under thinner shell conditions, polarizability shows variations with field strength that include “anomalous” negative values for certain states. Then, the increase in shell thickness causes a significant increment in the polarizabilities, regardless impurity position.

Independent Mutation of Two Bridging Carboxylate Ligands Stabilizes Alternate Conformers of the Photosynthetic O2-Evolving Mn4CaO5 Cluster in Photosystem II.

The O2-evolving Mn4CaO5 cluster in photosystem II is ligated by six carboxylate residues. One of these is D170 of the D1 subunit. This carboxylate bridges between one Mn ion (Mn4) and the Ca ion. A second carboxylate ligand is D342 of the D1 subunit. This carboxylate bridges between two Mn ions (Mn1 and Mn2). D170 and D342 are located on opposite sides of the Mn4CaO5 cluster. Recently, it was shown that the D170E mutation perturbs both the intricate networks of H-bonds that surround the Mn4CaO5 cluster and the equilibrium between different conformers of the cluster in two of its lower oxidation states, S1 and S2, while still supporting O2 evolution at approximately 50% the rate of the wild type. In this study, we show that the D342E mutation produces much the same alterations to the cluster's FTIR and EPR spectra as D170E, while still supporting O2 evolution at approximately 20% the rate of the wild type. Furthermore, the double mutation, D170E + D342E, behaves similarly to the two single mutations. We conclude that D342E alters the equilibrium between different conformers of the cluster in its S1 and S2 states in the same manner as D170E and perturbs the H-bond networks in a similar fashion. This is the second identification of a Mn4CaO5 metal ligand whose mutation influences the equilibrium between the different conformers of the S1 and S2 states without eliminating O2 evolution. This finding has implications for our understanding of the mechanism of O2 formation in terms of catalytically active/inactive conformations of the Mn4CaO5 cluster in its lower oxidation states.

Shapes and structure for the lowest states of the 42,44Ca isotopes

The shape and the associated dynamics of the 42,44Ca isotopes are investigated within the Bohr-Mottelson Model and the Covariant Density Functional Theory for the presence of the shape coexistence and mixing phenomena. The corresponding experimental energy spectrum and most of the electromagnetic transitions are well reproduced only by taking into account such phenomena. New possible developments of the models are indicated where improvements in agreement with the experimental data are needed.

Ion-core switching in Rydberg series of XeKr

We investigate XeKr in high Rydberg states by mass-resolved optical-optical double resonance excitation spectroscopy. We excite XeKr via two-photon absorption to the v* = 0 and v* = 2 vibrational levels of an intermediate electronically excited state of Xe*Kr correlated to Xe*6p[5/2]2 and excite Xe*Kr further via one-photon absorption to the high Rydberg states of Xe**Kr in the energy range of 93200–97400 cm−1. The potential energy curves and the quantum defects of the d-Rydberg and s-Rydberg series of Xe**Kr, converging the A 2Π3/2 state of XeKr+ are determined. From the kinetic energy release in the predissociation process, we conclude that the ion-core switching occurs by the interaction between the bound electronic states of Xe**Kr converging to the A 2Π3/2 state of XeKr+ and the repulsive electronic states of low-lying electronic states of XeKr**. Based on numerical simulations, we interpret the dependence of the yield of the ion-core switching on the excitation energy in terms of the population transfer from the bound electronic states of the high Rydberg states Xe**Kr, converging to the A 2Π3/2 state of XeKr+, to those converging to the electronic ground X2 Σ 1 / 2 + state of XeKr + .

Structural Insights into Linkage-Specific Ubiquitin Chains Using Ion Mobility Mass Spectrometry.

The structural characterization and differentiation of four types of oligoubiquitin conjugates [linear (Met1)-, Lys11-, Lys48-, Lys63-linked di-, tri-, and tetraubiquitin chains] using ion mobility mass spectrometry are reported. A comparison of collision cross sections for the same linkage of di-, tri-, and tetraubiquitin chains shows differences in conformational elongation for higher charge states due to the interplay of linkage-derived structure and Coulombic repulsion. For di- and triubiquitin chains, this elongation results in a single narrow feature representing an elongated conformation type for multiple higher charge state species. In contrast, higher charge state tetraubiquitin species do not form a single conformer type as readily. A comparison of different linkages in tetraubiquitin chains reveals greater similarity in conformation type at lower charge states; with increasing charge state, the four linkage types diverge in the relative proportions of elongated conformer types with Met1- ≥ Lys11- > Lys63- > Lys48-linkage. These differences in conformational trends could be discussed with respect to biological functions of linkage-specific polyubiquitinated proteins.

Stabilization of Charge-Transfer Excited States in Biological Systems: A Computational Focus on the Special Pair in Photosystem II Reaction Centers.

Charge-transfer (CT) excited states play an important role in many biological processes. However, many computational approaches often inadequately address the equilibration effects of nuclear and environmental degrees of freedom on these states. One prominent example of systems in which CT states are of utmost importance is reaction centers (RC) in photosystems. Here we use a multiscale approach combined with time-dependent density functional theory to explore the lowest CT excited state of the special pair PD1-PD2 in the Photosystem II-RC of a cyanobacterium. We find that the nonequilibrium CT excited state resides near the Soret band, making an exciton the lowest-energy excited state. However, accounting for nuclear and state-specific dielectric equilibration along the CT potential energy surface (PES), the CT state PD1--PD2+ stabilizes energetically below the excitonic state. This underscores the crucial role of state-specific solvation in mapping the PES of CT states, as demonstrated in a simplified dimer model.

Dendritic copper oxide catalyst engineering weak-polarity Cu-O bond for high-efficiency nitrate electroreduction

Nitrate reduction reaction (NO3RR) is deemed a promising pathway for both ammonia synthesis and water purification. Developing a high-efficiency catalyst with excellent NH3 selectivity and catalytic stability is desirable but remains challenging. In this work, a dendritic copper oxide catalyst (Cu-B2) has been developed to efficiently catalyze NO3RR for ammonia production, the Cu-B2 exhibits excellent catalytic performance, achieving an NH3 Faradaic efficiency as high as 94 % and an NH3 yield of 16.9 mg h−1 cm−2 with a current density of 192.3 mA cm−2 at − 0.6 V (vs. RHE, reversible hydrogen electrode). During NO3RR testing, the Cu-B2 catalysts are reduced in situ to form highly active Cu0/Cu+ sites, while retaining its dendritic morphology. Compared with other catalysts, the Cu-O bond in Cu-B2 catalyst has weaker polarity, resulting in Cu0/Cu+ sites in lower oxidation states. In situ attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) studies reveal the Cu-B2 catalyst exhibits a potential-independent capability for *NO3- adsorption and high conversion efficiency of NO2- intermediate into ammonia, DFT calculations reveal that Cu-B2 exhibts higher NO3- adsorption energy and lower NO3- adsorption energy barrier than Cu-B1, thus endowing it with a remarkably improved catalytic activity and durability.

Thermodynamic stability of the meniscus oil film between air/solid/water and its relevance to flotation

The behavior of oil is an important aspect in the area of mineral flotation or oil-water separation. This paper proposes a specific phenomenon that a meniscus oil film may form between the air bubble and the solid surface under water surroundings. A thermodynamic system containing an air bubble, an oil droplet, a solid matrix, and water surroundings was constructed. The mechanical equilibrium state of the oil meniscus film was solved. The free energy difference ∆B between the oil film-formed state and a reference state based on the Gibbs surface thermodynamic theory is applied to characterize the stability of the meniscus oil film. The impacts of the oil-gas volume ratio and the interfacial tensions are taken into consideration. The results show that the equilibrium morphologies of the system are directly correlated with the interfacial tensions. A minimal ∆Bmin can be found at a certain volume ratio and the case ∆B<0 generally exists. It indicates the formation of oil film leads the system to a lower energy state which is thermodynamically stable. The batch flotation using glass beads and n-alkane collectors was conducted. The flotation results show that the oil can improve the particle recovery and the results can be well interpreted by the mechanism correlated to meniscus oil film.

Modified t-butyl in tetradentate platinum (II) complexes enables exceptional lifetime for blue-phosphorescent organic light-emitting diodes

In blue phosphorescent dopants, the tetradentate platinum(II) complex is a promising material showing high efficiency and stability in devices. However, metal-metal-to-ligand charge transfer (MMLCT) formation leads to low photo-luminescence quantum yields (PLQYs), wide spectra, and intermolecular interaction. To suppress MMLCT, PtON-tb-TTB and PtON-tb-DTB are designed using theoretical simulation by modifying t-butyl in PtON-TBBI. Both materials effectively suppress MMLCT and exhibit high PLQYs of 99% and 78% in 5 wt% doped film, respectively. The PtON-tb-TTB and PtON-tb-DTB devices have maximum external quantum efficiencies of 26.3% and 20.9%, respectively. Additionally, the PtON-tb-DTB device has an extended lifetime of 169.3 h with an initial luminescence of 1200 nit, which is 8.5 times greater than the PtON-TBBI device. Extended lifetime because of suppressed MMLCT and smaller displacement between the lowest triplet and triplet metal-centered states compared to other dopants. The study provides an effective approach to designing platinum(II) complexes for long device lifetimes.

Nonlinear optical response of 1H-indene-based donor-acceptor chromophores. Influence of the higher-lying states on the first hyperpolarizability

The hyperpolarizabilities often fall short of the fundamental quantum limits. However, in case of 5-(diphenylamino)thiophen-2-yl substituted 1H-indene based donor-π-acceptor (D-π-A) chromophores, in spite of the preponderance of twisted intramolecular charge transfer state, we find that the first hyperpolarizability is modulated (limited!) due to the possible interference of the higher-lying excited states of significant oscillator strength with the lower-lying excited state.

Excitonic Thermalization Bottleneck in Twisted TMD Heterostructures.

Twisted van der Waals heterostructures show intriguing interface exciton physics, including hybridization effects and emergence of moiré potentials. Recent experiments have revealed that moiré-trapped excitons exhibit remarkable dynamics, where excited states show lifetimes that are several orders of magnitude longer than in monolayers. The origin of this behavior is still under debate. Based on a microscopic many-particle approach, we investigate the phonon-driven relaxation cascade of nonequilibrium moiré excitons in the exemplary MoSe2-WSe2 heterostructure. We track exciton relaxation pathways across different moiré mini-bands and identify the phonon-scattering channels assisting the spatial redistribution of excitons into low-energy pockets of the moiré potential. We unravel a phonon bottleneck in the flat band structure at low twist angles preventing excitons from fully thermalizing into the lowest state, explaining the measured enhanced emission intensity and lifetime of excited moiré excitons. Overall, our work provides important insights into exciton relaxation dynamics in flat-band exciton materials.

Electronic and optical properties of tetrapod quantum dots under applied electric and magnetic fields

In this work, the ground and low-lying excited states in GaAs tetrapod-shaped nanocrystals and in CdTe/CdS core/shell tetrapod-shaped nanocrystals are computed in the case of a finite confinement potential model using the finite element method and the effective mass approximation. For the single material tetrapod, electron energy levels are studied as a function of size parameters and the intensity of externally applied electric and magnetic fields, while for the core/shell tetrapod the electron and hole states are analyzed relative to size parameters of the core and shell thickness. The results show that the energies are strongly influenced by the tip-arm geometry, arm diameter, shell thickness and the intensity of the electric and magnetic fields.

Observation of Dark States in Two-Dimensional Electronic Spectra of Chlorosomes.

Observations of low-lying dark states in several photosynthetic complexes challenge our understanding of the mechanisms behind their efficient energy transfer processes. Computational models are necessary for providing novel insights into the nature and function of dark states, especially since these are not directly accessible in spectroscopy experiments. Here, we will focus on signatures of dark-type states in chlorosomes, a light-harvesting complex from green sulfur bacteria well-known for uniting a broad absorption band with very efficient energy transfer. In agreement with experiments, our simulations of two-dimensional electronic spectra capture the ultrafast exciton transfer occurring in 100s of femtoseconds within a single chlorosome cylinder. The sub-100 fs process corresponds to relaxation within the single-excitation manifold in a single chlorosome tube, where all initially created populations in the bright exciton states are quickly transferred to dark-type exciton states. Structural inhomogeneities on the local scale cause a redistribution of the oscillator strength, leading to the emergence of these dark-type exciton states, which dominate ultrafast energy transfer. The presence of the dark-type exciton states suppresses energy loss from an isolated chlorosome via fluorescence quenching, as observed experimentally. Our results further question whether relaxation to dark-exciton states is a leading process or merely competes with transfer to the baseplate within the photosynthetic apparatus of green sulfur bacteria.

The Excited-State Lifetime of Poly(NDI2OD-T2) Is Intrinsically Short.

Conjugated polymers composed of alternating electron donor and acceptor segments have come to dominate the materials being considered for organic photoelectrodes and solar cells, in large part because of their favorable near-infrared absorption. The prototypical electron-transporting push-pull polymer poly(NDI2OD-T2) (N2200) is one such material. While reasonably efficient organic solar cells can be fabricated with N2200 as the acceptor, it generally fails to contribute as much photocurrent from its absorption bands as the donor with which it is paired. Moreover, transient absorption studies have shown N2200 to have a consistently short excited-state lifetime (∼100 ps) that is dominated by a ground-state recovery. In this paper, we investigate whether these characteristics are intrinsic to the backbone structure of this polymer or if these are extrinsic effects from ubiquitous solution-phase and thin-film aggregates. We compare the solution-phase photophysics of N2200 with those of a pair of model compounds composed of alternating bithiophene (T2) donor and naphthalene diimide (NDI) acceptor units, NDI-T2-NDI and T2-NDI-T2, in a dilute solution. We find that the model compounds have even faster ground-state recovery dynamics (τ = 45, 27 ps) than the polymer (τ = 133 ps), despite remaining molecularly isolated in solution. In these molecules, as in the case of the N2200 polymer, the lowest excited state has a T2 to NDI charge-transfer (CT) character. Electronic-structure calculations indicate that the short lifetime of this state is due to fast nonradiative decay to the ground state (GS) promoted by strong CT-GS electronic coupling and strong electron-vibrational coupling with high-frequency (quantum) normal modes.

Uncertainty quantification for high-temperature flame sensing using wavelength modulation spectroscopy

In this study, we explore the uncertainty of the wavelength modulation spectroscopy (WMS) technique applied to water profile measurement in a flat flame. The scanned WMS-2f/1f method is used to measure the water profile in a C2H4-O2-Ar flame, using an absorption transition line near 1391.7 nm. The Monte Carlo sampling method is used for the uncertainty quantification (UQ) analysis. The results show a non-negligible uncertainty on the water concentration with an uncertainty range from 10% to 30%, depending on the uncertainty level considered for the input parameters of the spectroscopic model. Further sensitivity analyses show that the lower state energy (E′′), the line strength, and the temperature are the most important uncertainty sources. When multiple transition lines are involved, the line strength value controls the overall influence of the other parameters associated with these specific lines. We also compared the uncertainty obtained when applying the direct absorption spectroscopy (DA) and WMS techniques for a similar flame configuration. Only a slight advantage is observed for the WMS.

218 Cost-Effectiveness and Quality-Adjusted Life Years Analysis for Endovascular Management of Acute Stroke With a Large Ischemic Region

INTRODUCTION: Mechanical thrombectomy (MT) for acute ischemic stroke is generally avoided when the infarction is large (defined as ASPECT score of &lt; 5). However, a recent trial (RESCUE-Japan) published in NEJM, showed that patients with large cerebral infarctions had better functional outcomes, measured as mRS of 0-3 at 90 days, with MT compared to best medical management (BMM) alone. METHODS: Treeage, a decision-analytic software, was used to construct a cost effectiveness analysis model that compares the cost-effectiveness of MT vs. BMM alone based on the RESCUE-Japan study. The model includes a short-term decision model that examines costs and clinical outcomes of patients with acute ischemic stroke during the initial three months of treatment. Additionally, a long-term Markov state transition model is created to estimate expected outcomes and costs annually over a 20-year timeframe, with patients potentially experiencing recurrent strokes, transitioning to lower health states, and death from stroke. The success of each treatment was evaluated using QALYs. Then, to assess the robustness of this model, a sensitivity analysis was performed. RESULTS: In the base–case analysis of the total study population using a time horizon of 20 years, MT and BMM alone were found to have a 2.86 QALY and 1.44 QALY, respectively. This increase in the number of QALY by MT came at a lower cost of $275,805 compared to BMM alone, which had a cost $282,476. The incremental cost-effectiveness ratio (ICER) was determined at $-4697/QALY. The results were further supported by the 1-way deterministic and probabilistic sensitivity analyses. CONCLUSIONS: This study shows that, besides having a better clinical outcome, MT was more cost effective and associated with higher QALYs compared to BMM alone, when accounting for health care cost associated with treatment outcome.

Theoretical investigation of the silver chalcogenide molecules AgSe and AgTe electronic structure with dipole moment calculation

The spectroscopic constants (Re, Te, ωe, Be) are determined for 46 doublet and quartet low-lying states of the molecules AgTe and AgSe using CASSCF/MRCI (single and double excitations plus Davidson correction) calculations. The equilibrium bond length Re, and the vibrational frequency ωe of the ground state of both molecules have also been investigated using the density functional theory (DFT) incorporating eight different functionals along with different basis sets. The potential energy and permanent dipole moment curves of 23 states of each molecule have been investigated and depicted. This study represents the first theoretical investigation of the molecules AgSe and AgTe.

Quantum-Number Projected Generator Coordinate Method for 21Ne with a Chiral Two-Nucleon-Plus-Three-Nucleon Interaction

In this paper, we report a study of the low-lying states of deformed 21Ne within the framework of the quantum-number projected generator coordinate method (PGCM), starting from a chiral two-nucleon-plus-three-nucleon (NN+3N) interaction. The wave functions of states are constructed as a linear combination of a set of axially deformed Hartree–Fock–Bogliubov (HFB) wave functions with different quadrupole deformations. These HFB wave functions are projected onto different angular momenta and the correct neutron and proton numbers for 21Ne. The results of the calculations based on the effective Hamiltonians derived by normal-ordering the 3N interaction with respect to three different reference states, including the quantum-number projected HFB wave functions for 20Ne, 22Ne, and an ensemble of them with equal weights, are compared. This study serves as a key step towards ab initio calculations of odd-mass deformed nuclei with the in-medium GCM.

Gas-Phase Reactivity of Quinoline-Based Singlet Oxenium Cations.

Isomeric quinolyloxenium cations were generated in the gas phase in an ion trap mass spectrometer to explore their reactions. The structures of some products were identified via collision-activated dissociation experiments involving model compounds to demonstrate that they have the expected heavy atom connectivity. The lack of radical reactions suggests that the cations have closed-shell singlet electronic ground states. Calculations (CASPT2/CASSCF(16,14)/cc-pVTZ//CASSCF(16,14)/cc-pVTZ) predict that their closed-shell singlet (1A') ground states are lower in energy by ca. 25 kcal mol-1 than their lowest-lying excited states. All cations are reactive toward dimethyl disulfide, dimethyl sulfide, and allyl iodide and most toward water and moderately reactive toward cyclohexane, reflecting their strongly electrophilic nature. They form adducts with nucleophiles in exothermic reactions (ca. 50 kcal mol-1 for dimethyl sulfide) that can fragment or be stabilized via IR emission. Most water adducts spontaneously isomerize to lower-energy tautomers. The nucleophiles preferentially add to those carbon atoms in the benzene ring that have the greatest positive charge (but not the carbonyl carbon). The cations react with cyclohexane via hydride abstraction by the oxygen atom. This is the only reaction that initially involves the oxygen atom and hence reflects the formally positively charged, monovalent oxygen atom in these cations.

Impact of Packing Geometry on Excimer Characteristics and Mobility in Perylene Bisimide Polycrystalline Films.

Efficient exciton transport is essential for high-performance optoelectronics. Considerable efforts have been focused on improving the exciton mobility in organic materials. While it is feasible to improve mobility in organic systems by forming well-ordered stacks, the formation of trap states, particularly the lower-lying states referred to as excimers, remains a significant challenge to enhancing mobility. The mobility of excimer excitons intricately depends on the strength of excitonic coupling in terms of Förster-type diffusive exciton transfer processes. Given that the formation and mobility of excimer excitons are highly sensitive to molecular arrangements (packing geometries), conducting comprehensive investigations into the structure-property relationship in organic systems is crucial. In this study, we prepared three types of polycrystalline films of perylene bisimide (PBI) by varying substituents at the imide and bay positions, which allowed us to tailor the properties of excimer excitons and their mobility based on packing geometries and excitonic coupling strengths. By utilizing femtosecond transient absorption spectroscopy, we observed ultrafast excimer formation in the higher coupling regime, while in the lower coupling regime, the transition from Frenkel to excimer excitons occurs with a time constant of 500 fs. Under high pump-fluence, exciton-exciton annihilation processes occur, indicating the diffusion of excimer excitons. Intriguingly, employing a three-dimensional diffusion model, we derived a diffusion constant that is 3000 times greater in the high coupling regime than in the low coupling regime. To investigate the optoelectronic properties in the form of a bulk system, we fabricated n-type organic field effect transistors and obtained 8000 times higher mobility in the high coupling regime. Furthermore, photocurrent measurements enable us to investigate the charge carrier transport by mobile excimer excitons, suggesting a 230-fold improvement in external quantum efficiency with tightly packing PBI molecules compared to the low coupling regime. These findings not only offer valuable insights into optimizing organic materials for optoelectronic devices but also unveil the intriguing potential of exciton migration within excimers.