Excited Transitions Research Articles

Ultrafast Mapping of Electronic and Nuclear Structure in the Photo Dissociation of Nitrogen Dioxide.

We investigate the photoinduced dissociation reaction of NO2 → NO + O upon electronic excitation of the X̃2A1 (D0) to the Ã2B2 (D1) state by femtosecond X-ray absorption spectroscopy at the nitrogen K-edge. We obtain key insight into the chemical bond breaking event and its associated electronic structural dynamics. Calculations of the photoinduced reaction allow to assign the transient absorption features at time scales of 10-50 fs to wave packet motions in the excited D1 and ground D0 states, followed by the formation of the NO photoproduct with a 255 ± 23 fs time constant. Our analysis shows that there is no direct correlation between the 1s core levels and the electronic ground and excited states transition energies and the bond elongation of NO2, while en route to dissociation toward the NO + O photoproducts, in the transient nitrogen K-edge spectra. However, simulations predict that for a sufficiently short UV pump pulse, the early wave packet dynamics in the D1 electronic excited state occurring within the first 35 fs along the bending and symmetric stretching modes can be directly mapped in the transient X-ray absorption spectra.

Linear Response pCCD-Based Methods: LR-pCCD and LR-pCCD+S Approaches for the Efficient and Reliable Modeling of Excited State Properties.

In this work, we derive working equations for the linear response pair coupled cluster doubles (LR-pCCD) ansatz and its extension to singles (S), LR-pCCD+S. These methods allow us to compute electronic excitation energies and transition dipole moments based on a pCCD reference function. We benchmark the LR-pCCD+S model against the linear response coupled-cluster singles and doubles method for modeling electronic spectra (excitation energies and transition dipole moments) of the BH, H2O, H2CO, and furan molecules. We also analyze the effect of orbital optimization within pCCD on the resulting LR-pCCD+S transition dipole moments and oscillator strengths and perform a statistical error analysis. We show that the LR-pCCD+S method can correctly reproduce the transition dipole moments features, thus representing a reliable and cost-effective alternative to standard, more expensive electronic structure methods for modeling electronic spectra of simple molecules. Specifically, the proposed models require only mean-field-like computational cost, while excited-state properties may approach the CCSD level of accuracy. Moreover, we demonstrate the capability of our model to simulate electronic transitions with non-negligible contributions of double excitations and the electronic spectra of polyenes of various chain lengths, for which standard electronic structure methods perform purely.

Theoretical investigation of the structure-property relationships of [n]benzocyclobutadiene carbon nanobelt: Aromaticity, singlet fission, and (hyper)polarizability

We present a theoretical design for a series of multifunctional nanomaterials that exhibit superior performance in singlet fission (SF) as well as exceptional nonlinear optical (NLO) performance. Our study focuses on a systematic investigation into the aromaticity, diradical character, excited state transition and (hyper)polarizability of the [n]benzocyclobutadiene carbon nanobelt system. To provide further guidance for designing multifunctional nanomaterials, our quantum chemical calculations propose empirical rules regarding the influence of aromatics on SF and NLO: Through an exploration of SF properties, we observed significant diradical character with radical sites exclusively located at anti-aromatic positions. Furthermore, these anti-aromatic positions serve as transition sites for excited state transitions to achieve the necessary low triplet excited states energy for SF. Then, by conducting calculations on NLO properties, we ascertain that the main tensor components of first-order hyperpolarizability (βtotal) and second-order hyperpolarizability (γtotal) come from the anti-aromatic region. However, there exists a contrasting impact of anti-aromaticity on first- and second-order hyperpolarizability density. The positive and negative contributions to first-order hyperpolarizability density mainly come from both aromatic and anti-aromatic regions while for second-order hyperpolarizability density it is contradictory. This study elucidates the role of aromatic sites in multifunctional nanomaterials with respect to diradical sites, excited state transition regions, and (hyper)polarizability density distribution.

Relaxation Channels of Two Types of Hot Carriers in Gold Nanostructures.

A fundamental understanding of hot carrier relaxation in metal nanostructures is essential for realizing their application potential in energy conversion and photocatalysis. Despite previous investigations of the relaxation of hot carriers generated by surface plasmon resonance (SPR) excitation and interband transitions (IBTs), the hot carrier relaxation lifetimes and their associated mechanisms remain unclear. Herein, we demonstrate two distinct hot carrier relaxation channels in gold plasmonic nanostructures. The experimental observations reveal that the hot carrier relaxation is faster following SPR excitation than that from IBTs in gold nanoparticles and nanorods. The experimental results and theoretical calculations indicate that the numerous plasmon-induced hot carriers undergo surface-mediated carrier-carrier scattering in large gold nanostructures, whereas almost all IBT-induced hot carriers experience bulk carrier-carrier scattering. These findings advance our understanding of hot carrier relaxation and contribute to a clearer microscopic description of scattering channels in plasmonic nanostructures.

Unveiling the mechanism of reverse saturable absorption in zinc tetrapyridyl porphyrins

Reverse saturable absorption (RSA) is one of the important mechanism for a molecule showing optical-limiting property. The experiment found that zinc tetrapyridyl porphyrins (ZnTPyP) exhibit distinct RSA characteristics at both 470 nm and 532 nm wavelengths. To understand the origin of the RSA in ZnTPyP, a theoretical analysis based on the linear-response time-dependent density functional theory(LR-TDDFT) for the ground state absorption (GSA) and the pseudo-wavefunction formulation of LR-TDDFT(PWF-TDDFT) for excited states absorption (ESA) were conducted. Experimentally observed RSA in ZnTPyP are well predicted. It is shown that the transitions of S1→S13,14,15,18 are the origin of the observed RSA feature in ZnTPyP. We found that these excited states transitions exhibit intramolecular charge transfer character and the pyridyl groups play an important role in the charge transfer process. It is also noticed that the pyridyl groups results in a red shift for both the GSA and ESA of porphyrin. In addition, the central zinc atom causes the energy gap between the positions with prominent absorption in GSA and ESA increased. These observations indicate that pyridyl groups and central zinc atom play an important role in tuning the spectral window and strength of RSA in the porphyrin complexes. These observations provide guidance for designing novel optical-limiting materials based on the porphyrin derivatives.

Compact and high excitation molecular clumps in the extended ultraviolet disk of M83

Context. The extended ultraviolet (XUV) disks of nearby galaxies show ongoing massive-star formation, but their parental molecular clouds remain mostly undetected despite searches in CO(1–0) and CO(2–1). The recent detection of 23 clouds in the higher excitation transition CO(3–2) within the XUV disk of M83 thus requires an explanation. Aims. We test the hypothesis introduced to explain the non-detections and recent detection simultaneously: The clouds in XUV disks have a clump-envelope structure similar to those in Galactic star-forming clouds, having dense star-forming clumps (or concentrations of multiple clumps) at their centers, which predominantly contribute to the CO(3–2) emission and are surrounded by less dense envelopes, where CO molecules are photo-dissociated due to the low-metallicity environment there. Methods. We utilize new high-resolution ALMA CO(3–2) observations of a subset (11) of the 23 clouds in the XUV disk of M83. Results. We confirm the compactness of the CO(3–2)-emitting dense clumps (or their concentrations), finding clump diameters below the spatial resolution of 6–9 pc. This is similar to the size of the dense gas region in the Orion A molecular cloud, a local star-forming cloud with massive-star formation. Conclusions. The dense star-forming clumps are common between normal and XUV disks. This may also indicate that once the cloud structure is set, the process of star formation is governed by the cloud’s internal physics rather than by external triggers. This simple model explains the current observations of clouds with ongoing massive-star formation, although it may require some adjustment, for example including the effect of cloud evolution, to describe star formation in molecular clouds more generally.

Study of the electronic structure and electron impact excitation cross section of helium impurities in spherical quantum dots

This manuscript is devoted to explore the atomic structure and electron impact excitation process of atom impurities in quantum dots. To achieve this goal, a method that solves the fully relativistic Dirac equation within the framework of relativistic configuration interaction is proposed. The Gaussian potential is used, which can accurately describe the location of impurities in quantum dots and their local effects on the surrounding electron cloud. The coupled Dirac equation is modified to include a new central potential, providing solutions that include both the continuous and bound state wave functions. The process of electron impact excitation is elucidated using the distorted wave method, all within the framework of relativistic Dirac theory. For illustrative purposes, a detailed investigation of the excitation energies, transition rates, wave functions, and excitation cross sections is carried out for a wide range of confinement strengths of the potential and quantum dot radii, using the helium impurities in spherical quantum dots as an example. Our results reveal that for a given confinement strength of the potential, the bound state wave functions are initially pulled into the inner region by the attractive Gaussian potential well, but eventually reflect the free atom scenario at large quantum dot radii. In contrast, the continuous electron wave functions exhibit monotonic variations as a function of the quantum dot radii. Such behavior of the wave functions gives rise to distinctive phenomena in the variation of excitation energies, transition rates, and excitation cross sections in relation to the potential parameters. Good agreement between the present results and existing data, where available, is obtained. This work holds importance not only for basic research in atomic physics but also for the optical and electronic applications of quantum dots. I.e., in the design and optimization of quantum dot lasers and quantum dot sensors.

Analytical strong line diagnostics and their redshift evolution

Abstract The JWST is allowing new measurements of gas-phase metallicities in galaxies between cosmic noon and cosmic dawn. The most robust approach uses luminosity ratios between the excited auroral transition, [O iii] 4364 Å, and the lower [O iii] 5008 Å/4960 Å lines to determine the gas temperature. The ratio of the luminosities in the latter transitions to those in hydrogen Balmer series lines then yield relatively clean metallicity estimates. In the absence of detection of the [O iii] auroral line, the ratios of various [O iii], [O ii], [N ii], and Balmer lines are used to determine metallicities. Here we present a refined approach for extracting metallicities from these “strong line diagnostics”. Our method exploits empirical correlations between the temperature of O iii/O ii regions and gas-phase metallicity. We then show, from first principles, how to extract metallicities and break degeneracies in these estimates using traditional strong line diagnostics, R2, R3, R23, and O3O2, and N2O2. We show that these ratios depend also on volume correction factors, i.e. on accounting for the fraction of the volume of HII regions that are in O iii and O ii, but that these can be determined self-consistently along with the metallicities. We quantify the success of our method using metallicities derived from galaxies with auroral line determinations and show that it generally works better than previous empirical approaches. The scatter in the observed line ratios and redshift evolution are largely explained by O3O2 variations. We provide publicly available routines for extracting metallicities from strong line diagnostics using our methodology.

Electron spin resonance impact on the longitudinal resistance in the quantum Hall regime

The electron spin resonance impact on the longitudinal resistance in the quantum Hall regime near the integer filling factors is considered. For the odd filling factors extra spin excitons are created in the process of the microwave absorption, and the resistance increase due to the effective temperature rise. For the even filling factors in the absorption process depending on the temperature inspired spin polarization leads to transition of the spin-flip excitations to the higher energy short-lived excitations, and as a result the resistance is decreased due to effective temperature decrease.

Computing linear optical spectra in the presence of nonadiabatic effects on graphics processing units using molecular dynamics and tensor-network approaches.

We compare two recently developed strategies, implemented in open source software packages, for computing linear optical spectra in condensed phase environments in the presence of nonadiabatic effects. Both approaches rely on computing excitation energy and transition dipole fluctuations along molecular dynamics (MD) trajectories, treating molecular and environmental degrees of freedom on the same footing. Spectra are then generated in two ways: in the recently developed Gaussian non-Condon theory, the linear response functions are computed in terms of independent adiabatic excited states, with non-Condon effects described through spectral densities of transition dipole fluctuations. For strongly coupled excited states, we instead parameterize a linear vibronic coupling Hamiltonian directly from spectral densities of energy fluctuations and diabatic couplings computed along the MD trajectory. The optical spectrum is then calculated using powerful, numerically exact tensor-network approaches. Both the electronic structure calculations to sample system fluctuations and the quantum dynamics simulations using tensor-network methods are carried out on graphics processing units, enabling rapid calculations on complex condensed phase systems. We assess the performance of the approaches using model systems in the presence of a conical intersection and the pyrazine molecule in different solvent environments.

Energy level spectrum and lattice polarization of polaron in metal halide perovskite parabolic quantum wells

In this study, polaron effect in 2D metal halide perovskite materials (TMHPMs), which naturally form quantum well (QW) structures, has been investigated. Polarization of carriers and lattice within these structures was explored based on unitary transformation and variational methods. Specifically, lattice polarization and energy level transition spectra of strong coupling polaron in parabolic potential metal halide perovskite QWs (MAPbCl3, MAPbBr3, and MAPbI3) were studied. Also, we calculated ground and first excited state energies, excitation energy, transition frequency, and vibration frequency of a TMHPM QWs. Our results indicated that parabolic potential confinement intensity affected related physical quantities, which provided insight to investigate metal halide perovskite materials and polaron effect.

Rational Design of 3D Space Connected Donor–Acceptor System in Covalent Organic Frameworks for Enhanced Photocatalytic Performance

AbstractHerein, a rational strategy is presented to reduce the energy barrier of singlet ground state to singlet excited state transitions, whilst simultaneously reducing energy losses in populating triplet excited states. The approach relies on constructing 3D space connected donor–acceptor systems in COFs. The 3D space connected D–A system in 8‐connected 3D COFs (denoted as COF‐1 and COF‐2) allows the efficient transfer of electrons, overcoming the traditional electron transport limitations of 2D COFs and significantly boosting the solar energy utilization efficiency under visible light irradiation. COF‐2, possessing an extended π‐conjugated structure relative to COF‐1, demonstrated high selectivity for the photocatalytic generation of H2O2 (6.93 mmol g−1 h−1) in natural seawater without the need for sacrificial reagents, exceeding the performance of most previously reported COF‐based photocatalysts. The 3D space connected D–A system reported in this work offers a new approach for optimizing electron and energy transfer in COF‐based photocatalysts for H2O2 production and other applications.

Advantages of Ultrathin AlN/GaN/AlN Quantum Wells for Excitonic Population Distribution and Transition Features Studied by Phononic–Excitonic–Radiative Model

Excitons are expected to be a high‐efficiency emission source in UV light‐emitting devices. However, the damping of the excitonic laser oscillation has been reported under conditions where the excitonic states are expected to be populated in the conventional theory. In order to understand the exciton dynamics under the thermal nonequilibrium state, a theoretical model including various energy species in semiconductors such as electrons, phonons, and photons is required. Herein, a 2D phononic–excitonic–radiative model is constructed to analyze the exciton dynamics in a 2D system. 2D excitons with four principal quantum number states and the continuum in the lowest energy level of the AlN/GaN/AlN quantum wells are considered. It is found that the 2D phonon significantly augments the excitation transition rate. When the high recombination rate corresponding to stimulated emission is considered, the exciton binding energy of 108 meV is not enough to reduce the population in the high‐order discreet states and the continuum states, while the binding energy of 215 meV corresponding to the one monolayer GaN has an advantage of reducing these populations. The analysis of population flux has an advantage in discussing the increase in the kinetic energy transfer to the 1S exciton.

Structure and photoluminescence properties of coordination compound [Eu2(o-MBA)6(phen)2] with high quantum yield

An europium(III) coordination binuclear compound [Eu2(o-MBA)6(phen)2] (o-MBA = ortho-methylbenzoate, phen = 1,10-phenantroline) has been synthesized and investigated by X-ray diffraction method, thermogravimetric analysis (TGA), infrared (IR) and photoluminescence (PL) spectroscopy. The single crystal X-Ray diffraction measurements show that the compound crystallizes in the triclinic centrosymmetric space group P-1. The PL excitation spectrum displays a broad band between 300 and 420 nm and a number of sharp transitions between 384 and 590 nm, which can be attributed to the Eu3+ 4f6 intra-shell excitation transitions. The complex exhibits characteristic narrow, metal-centred emission bands, which are determined by the radiative transitions of the Eu3+ ion, from the first excited non-degenerate 5D0 level to the 7FJ (J = 0–4) manifold. The compound shows intense hypersensitive transition 5D0→7F2 which dominates the PL emission spectrum. The dipole strengths of the transitions were determined from the emission spectra in terms of Judd-Ofelt (JO) intensity parameters Ωλ (λ = 2, 4). The high intensity bright-red emission of the complex can be attributed to highly polarizable ligands environment and the low site symmetry (Ci) around the lanthanide ions. We suppose that the J-mixing effect in the Eu(III) ions is responsible for enhanced peak maximum of the 5D0→7F0 transition.

Electronic dynamics through conical intersections via non-Markovian stochastic Schrödinger equation with complex modes.

Conical intersections (CIs) play a crucial role in photochemical reactions, offering an efficient channel for ultrafast non-adiabatic relaxation of excited states. This significantly influences the reaction pathways and the resulting products. In this work, we utilize the non-Markovian stochastic Schrödinger equation with complex modes method to explore the dynamics of electronic transitions through conical intersections (CIs) in pyrazine. The linear vibronic coupling model serves as the foundational framework, incorporating both intra-state and inter-state electron-vibrational interactions. The dynamics of the excited electronic transitions are analyzed across varying strengths of system-bath coupling and different bath relaxation times. The accuracy of this method is demonstrated by comparing its predictions with those from the hierarchical equations of motion method.

Preparation of oil-soluble carbon-dot based room temperature phosphorescent materials by surface modification

The strategy of preparing carbon dots-based room temperature phosphorescent materials by using inorganic substrate to promote the triplet excited transition of carbon dots (CDs) has been extensively studied. The stable existence of room temperature phosphorescent materials prepared by inorganic substrate materials in the organic phase is still a great challenge. In this study, a new strategy for obtaining oil-soluble CDs@SiO2 room temperature phosphorescent materials by surface modification were proposed. The hydrophobic long hydrocarbon chain was introduced on the surface of CDs@SiO2 composites by esterification reaction to obtain oil-soluble CDs@SiO2 room temperature phosphorescent materials (SSA composites). The decay lifetime of SSA composites is 1.263 s, SSA composites phosphors have a quantum yield (QY) of 48 %. Finally, the application of SSA composites in electrostatic spinning successfully improves the hydrophobicity of nano spun films and expands the new function of electrostatic spinning in anti-counterfeiting.

InAs/GaAs SK quantum dots stacking: Impact of spacer layer on optical properties

In this paper, we report the optical properties of vertically stacked multilayers of self-organized InAs/GaAs quantum dots QDs with different GaAs spacer layer thicknesses of 10 and 15 nm, using photoluminescence PL measurement. The 10 K PL spectrum exhibits double-emission peaks ambiguously identified in PL spectra where the excitation power dependence reveals that these emission peaks are attributed to fundamental ground state GS of large quantum dots in the low energy side, and first excited state 1ES transitions of large quantum dots in coincidence with fundamental ground state GS of smaller ones, in the high energy side. Both the excitation power dependence and the temperature dependence measurement results exhibited competition between the latter's optical transitions in the QDs, which becomes more prevalent at higher excitation powers. The main PL peak is quenched above 190 K, giving rise to “a 1D miniband”, ascribed to the electronic coupling between QDs along the stacking direction and tuning the emission wavelength around 1.3 μm.Our results emphasize the Key role of the vertically stacked InAs/GaAs QDs structures with thin GaAs spacer layers in optimizing design for long-wavelength devices.

Boric Acid Matrix‐Activated nπ* Transition of Guest Chromophores: from Pure Fluorescence to Efficient Afterglow

AbstractEl‐Sayed rule highlights the important role of heteroatoms (e.g., N, O, and S) and the corresponding nπ* transition in designing efficient organic phosphors. Nevertheless, for some heteroatom‐rich fluorophores, their phosphorescence is quite weak (e.g., fluorescein), since the nπ* components are absent in the whole transitions. Here, these chromophores (mainly with n electron‐containing twisted structure) are found doping into the boric acid (BA) matrix can activate the nπ* transition for efficient afterglow luminescence. For example, doping purely green‐emitting fluorescein into BA yielded a high afterglow quantum yield (≈24%) and a long‐lasting, blue‐shifted cyan afterglow (>10 s). The covalent/non‐covalent interactions between the BA matrix and the guests resulted in twisting the n electron‐containing structure to promote the spin‐orbit coupling process, leading to the generation of new excited state triplet transition pathways. Since the guest chromophores feature visible light absorption, a series of multi‐color afterglow phosphors with visible or white light excitation are successfully constructed.

Spacing distribution for quantum Rabi models * *Dedicated to Mr. Hiromichi Shinohara, who was instrumental in establishing NTT-IFM, on his 70th birthday.

The asymmetric quantum Rabi model (AQRM) is a fundamental model in quantum optics describing the interaction of light and matter. Besides its immediate physical interest, the AQRM possesses an intriguing mathematical structure which is far from being completely understood. In this paper, we focus on the distribution of the level spacing, the difference between consecutive eigenvalues of the AQRM in the limit of high energies, i.e. large quantum numbers. In the symmetric case, that is the quantum Rabi model (QRM), the spacing distribution for each parity (given by the Z2 -symmetry) is fully clarified by an asymptotic expression derived by de Monvel and Zielinski, though some questions remain for the full spectrum spacing. However, in the general AQRM case, there is no parity decomposition for the eigenvalues. In connection with numerically exact studies and recent theoretical results, we describe the spacing distribution for the AQRM which is characterized by a new type of periodicity and symmetric behavior of the distribution with respect to the bias parameter, reflecting the hidden symmetry of the AQRM. In addition, we observe in the AQRM the excited state quantum phase transition for large values of the bias parameter, analogous to the QRM with large qubit energy, and an internal symmetry of the level spacing distribution for fixed bias. This novel symmetry is independent from the symmetry for half-integer bias and not explained by current theoretical knowledge.

Unveiling the Multifaceted Nature of 4‐(4‐Methylphenyl Thio)benzophenone: Electronic Structure and Excited States in Gas Phase and Solvents

AbstractBenzophenone and its derivatives are widely used as UV filters and UV‐ink photoinitiators. The photoinitiating properties of benzophenones depend on their degree of conjugation and delocalization within the molecule. By understanding how conjugation, delocalization, and different substituents affect these properties, benzophenone derivatives can be customized for specific applications. Using quantum mechanical calculations based on B3LYP/6‐311++G(d, p) density functional theory (DFT), chemical reactivity, stability, and photoinitiating capabilities of 4‐(4‐methylphenylthio)benzophenone are analyzed. This includes studying its physical and chemical properties in the gas phase, as well as its excited state electronic transitions, vibrational characteristics, and spectroscopic properties both in gas phase and in various solvents. The DFT‐computed infrared spectra match experimental results. The UV/Visible spectra shows absorption towards longer wavelengths due to extended delocalization of π‐electrons. In different solvents with varying polarity, the absorption spectra exhibit high‐intensity peaks, shift in excitation energy and wavelengths based on the polarity of the solvent. This knowledge allows for the development of novel initiators with customized light absorption, excited state lifetimes, and reaction selectivities, which can enhance processes like UV‐curing, photopolymerization, and other light‐driven reactions.