Search Results (60)

Rare charm decays offer a clean arena for visible massive dark photon searches through narrow dielectron resonances. In the process $D^0\to \gamma A^{\prime }\to \gamma e^{+}{e}^{-}$, the observable signal rate is controlled simultaneously by the flavor changing charm transition, the radiative $D^0\to \gamma$ hadronic matrix element, the visible decay probability of $A^{\prime }$, and the experimental response of the reconstructed mass spectrum. We introduce a visible coupling formulation that combines these ingredients into a single search parameter while keeping the form factor and the $A^{\prime }\to e^{+}{e}^{-}$ branching fraction as explicit external inputs. This provides a direct bridge between branching fraction sensitivity and coupling reach, without assuming a specific experimental background model. The central novelty of this approach is that it separates the particle physics information entering the signal rate from the analysis-dependent ingredients that determine the observable mass spectrum. We apply this construction to the $D^0\to \gamma e^{+}{e}^{-}$ spectrum, including effective exposure, reconstruction efficiency, mass resolution, and the treatment of the $\rho /\omega$ and $\varphi$ regions. The $\rho /\omega$ and $\varphi$ dominated intervals are kept outside the coupling definition and left to experiment specific resonance treatment. This formulation gives a practical and reusable basis for comparing visible massive dark photon sensitivity across current and future charm data sets. Full article

There are some 300 naturally occurring nuclides. In addition, over 3000 radioactive isotopes have become known. The s(low) and r(apid) processes of neutron capture synthesize the nuclides heavier than iron. The synthesis, namely the increase in the atomic numbers Z, is actually governed by $\beta$ decays. A “flow” of successive neutron captures in the chart of the nuclides is intercepted by a nucleus whose $\beta$ decay half-life is short enough. In this review, I discuss the s-process exclusively. The neutron capture rate to be compared with the $\beta$ decay rate is represented by $\lambda =n_{\mathrm{n}}{v}_T<\sigma >$, where $n_{\mathrm{n}}$ is the neutron number density, $v_T$ is the neutron thermal velocity at the temperature T, and $<\sigma >$ is the Maxwellian averaged (around $v_T$) radiative neutron capture cross-section, which depends on the nucleus of interest. The classical analysis of the solar system abundances of nuclides leads to canonical combinations like $n_{\mathrm{n}}\sim {10}^8/{\mathrm{cm}}^3$ and $T\sim 3\times {10}^8$ K for the s-process. The s-process flow becomes intricate when the neutron capture and $\beta$ decay timescales are comparable, causing a branch of the flow. Subsequently, an evaluation of $\beta$ decay rates is required, which is difficult to do straightforwardly. In this review, I will discuss the historical developments and the current status of predicting $\beta$ decay rates under s-process environments (specified basically by temperature, density, and composition). Those conditions are inaccessible in the laboratory. Embedded in high-temperature environments, even a very massive atomic species could be highly ionized, and its atomic and nuclear excited states could be thermally populated. I will exemplify the consequent difficulties of $\beta$ decay rate evaluations for s-process studies. Full article

Fluorescence remains a foundational optical phenomenon underpinning applications in sensing, imaging, diagnostics, and catalysis. Among the strategies developed to modulate fluorescence, coupling fluorophores with plasmonic metals has emerged as a powerful route for both enhancement and quenching. The collective excitation and decay of surface plasmons can profoundly alter fluorophore excitation rates, radiative pathways, and emission efficiencies. This review provides a mechanistic and historical synthesis of metal–fluorophore interactions, unifying enhancement and quenching phenomena under the term Metal Manipulated Fluorescence (MMF). We summarize the fundamental principles of fluorescence and plasmon resonance, discuss theoretical and computational approaches for predicting metal–fluorophore coupling, and critically examine recent advances in plasmonic nanostructure synthesis that enable precise control over fluorophore behaviour. By integrating experimental observations with theoretical models, we highlight the opportunities and limitations of current MMF strategies and outline future directions in materials design, synthesis methodologies, and predictive modelling for next-generation optical and optoelectronic technologies. Full article

Although various phosphorescent organic light-emitting diodes (PhOLEDs) have been developed, their lifetimes remain shorter than those of fluorescent OLEDs. In this study, a novel Pt(II) complex featuring a tetradentate ligand composed of bipyridine and phenoxypyridine, referred to as LL-O, was synthesized and fully characterized to evaluate its potential as a dopant for PhOLEDs. Geometry-optimized calculations indicate that LL-O adopts a distorted square–planar structure around the Pt(II) center. The complex displays bluish-green emission with maxima at 490 and 518 nm. However, it exhibits a low photoluminescence quantum yield (4%), primarily due to a dominant non-radiative decay rate that surpasses the radiative decay rate. Natural transition orbital analysis reveals that the emission of LL-O originates from a combination of triplet ligand-centered (³LC), triplet ligand-to-ligand charge-transfer (³LL′CT), and triplet metal-to-ligand charge-transfer (³MLCT) transitions. This compound also demonstrates high thermal stability (decomposition temperature > 340 °C) and an appropriate HOMO energy level (−5.58 eV), making it suitable for use as a dopant in versatile PhOLEDs. Full article

To investigate the in situ irradiation effects of gallium nitride at varying temperatures, we combined ion beam-induced luminescence spectroscopy with variable-temperature irradiation using a home-built IBIL system and a GIC4117 2 × 1.7 MV tandem accelerator. Unlike previous static studies—limited to post-irradiation or single-temperature luminescence—we in situ tracked dynamic luminescence changes throughout irradiation, directly capturing the real-time responses of luminescent centers to coupled temperature-dose variations—a rare capability in prior work. To clarify how irradiation and temperature affect the luminescent centers of GaN, we integrated density functional theory (DFT) calculations with literature analysis, then resolved the yellow luminescence band into three emission centers via Gaussian deconvolution: 1.78 eV associated with C/O impurities, 1.94 eV linked to VGa, and 2.2 eV corresponding to CN defects. Using a single-exponential decay model, we further quantified the temperature- and dose-dependent decay rates of these centers under dual-variable temperature and dose conditions. Experimental results show that low-temperature irradiation such as at 100 K suppresses the migration and recombination of V_Ga/C_N point defects, significantly enhancing the radiation tolerance of the 1.94 eV and 2.2 eV emission centers; meanwhile, it reduces non-radiative recombination center density, stabilizing free excitons and donor-bound excitons, thereby improving near-band-edge emission center resistance. Notably, the 1.94 eV emission center linked to gallium vacancies exhibits superior cryogenic radiation tolerance due to slower defect migration and more stable free exciton/donor-bound exciton states. Collectively, these findings reveal a synergistic regulation mechanism of temperature and radiation fluence on defect stability, addressing a key gap in static studies, providing a basis for understanding degradation mechanisms of gallium nitride-based devices under actual operating conditions (coexisting temperature fluctuations and continuous radiation), and offering theoretical/experimental support for optimizing radiation-hardened gallium nitride devices for extreme environments such as space or nuclear applications. Full article

This paper investigates the validity of the long wavelength approximation in the calculation of two-photon decay of $2s_{1/2}$ level in hydrogen-like ions with nuclear charge $Z=1-100$ based on time-dependent second-order perturbation theory and angular momentum algebra. While the relativistic structure effects on the two-photon decay rates are highlighted in the literature, the role of slowing effects in the photon electric dipole operators are not discussed extensively. The rate is computed by the sum-over-states method, with bound-bound and bound-free electric dipole matrix elements obtained in the Babushkin and Coulomb gauges, which satisfy the Lorenz gauge condition, as well as their non-relativistic limits in the long-wavelength approximation (Length and Velocity forms, respectively). The present results explicitly show how this approximation breaks gauge invariance by overestimating the Babushkin values by ∼24%${\left(\alpha Z\right)}^2$ while underestimating the Coulomb rates by ∼$31\%{\left(\alpha Z\right)}^2$. Using analytical eigenfunctions of the Dirac equation, we found that the contributions of the negative continuum states to the rate scale are ∼$0.0134{\left(\alpha Z\right)}^4$ in the Babushkin gauge and ∼$1.46{\left(\alpha Z\right)}^4$ in the Coulomb gauge, making the latter gauge more susceptible to errors when attempting to achieve basis completeness in multiphoton calculations. The present results are useful in assessing the complexity requirements of radiative transition rates for atomic systems of interest. Full article

The widespread presence of antibiotic residues, particularly norfloxacin (NFX), in food products and the environment has raised concern, underscoring the need for sensitive and selective detection methods. In this study, a novel broccoli-derived nitrogen-doped carbon quantum dots (N-CQDs) was synthesized via a green hydrothermal approach, 4-dimethylaminopyridine (DMAP) as both a nitrogen dopant and a functionalizing agent. The synthesized N-CQDs exhibit an average diameter of approximately ~4.2 nm and emit bright blue fluorescence, with a maximum emission at 445 nm upon excitation at 360 nm. A “Turn-ON” response toward NFX was achieved with a detection limit of 0.30 nM, attributed to hydrogen bonding and π–π stacking interactions that suppressed non-radiative decay. Moreover, the sensor demonstrates high selectivity for NFX, effectively distinguishing it from common interfering substances, including other antibiotics, organic acids, and biomolecules. The N-CQDs also exhibit excellent stability under diverse conditions, such as varying pH levels, high ionic strength, and prolonged irradiation. Finally, the practical applicability of the developed sensor was validated by detecting NFX in spiked broccoli extract and milk samples, with recovery rates ranging from 98.2% to 100.1% and relative standard deviations of less than 2.0%. This work presents a sustainable and efficient N-CQD-based fluorescent sensing platform, offering significant potential for rapid and reliable detection of NFX in food safety and environmental monitoring. Full article

We report the development of highly luminescent, bovine serum albumin (BSA)-stabilized gold–silver bimetallic nanoclusters (Au-AgNCs@BSA) as a novel platform for high-sensitivity, ratiometric intracellular temperature sensing. Precise and non-invasive temperature sensing at the nanoscale is crucial for applications ranging from intracellular thermogenesis monitoring to localized hyperthermia therapies. Traditional luminescent thermometric platforms often suffer from limitations such as high cytotoxicity and low photostability. Here, we synthesized Au-AgNCs@BSA via a one-pot aqueous reaction, achieving significantly enhanced photoluminescence quantum yields (PL QYs, up to 18%) and superior thermal responsiveness compared to monometallic counterparts. The dual-emissive Au-AgNCs@BSA exhibit a linear ratiometric fluorescence response to temperature fluctuations within the physiological range (20–50 °C), enabling accurate and concentration-independent thermometry in live cells. Time-resolved PL and Arrhenius analyses reveal two distinct emissive states and a high thermal activation energy (E_a = 199 meV), indicating strong temperature dependence. Silver doping increases radiative decay rates while maintaining low non-radiative losses, thus amplifying fluorescence intensity and thermal sensitivity. Owing to their small size, excellent photostability, and low cytotoxicity, these nanoclusters were applied to non-invasive intracellular temperature mapping, presenting a promising luminescent nanothermometer for real-time cellular thermogenesis monitoring and advanced bioimaging applications. Full article

The quantum Mpemba effect refers to the counterintuitive phenomenon in which a system initially farther from equilibrium relaxes faster than one prepared closer to it. Several mechanisms have been identified in open quantum systems to explain this behavior, including the strong Mpemba effect, non-Markovian memory, and initial system–reservoir entanglement. Here, we unveil a distinct mechanism rooted in the non-normal nature of the Liouvillian superoperator in Markovian dynamics. When the Liouvillian’s eigenmodes are non-orthogonal, transient interference between decaying modes can induce anomalous early-time behavior—such as delayed thermalization or transient freezing—even though asymptotic decay rates remain unchanged. This differs fundamentally from strong Mpemba effects, which hinge on suppressed overlap with slow-decaying modes. We demonstrate this mechanism using a waveguide quantum electrodynamics model, where quantum emitters interact with the photonic modes of a one-dimensional waveguide. The directional and radiative nature of these couplings naturally introduces non-normality into the system’s dynamics. As a result, certain initial states—despite being closer to equilibrium—can exhibit slower relaxation at short times. This work reveals a previously unexplored and universal source of Mpemba-like behavior in memoryless quantum systems, expanding the theoretical framework for anomalous relaxation and opening new avenues for control in engineered quantum platforms. Full article

Currently, photothermal (PT) polymers are gaining increasing attention in water evaporation, photocatalysis and photothermal therapy. However, high-performance PT polymers often require conjugated backbones and/or large fused units, which can impede non-radiative decay and lead to suboptimal PT performance. The development of general strategies for preparing high-performance PT polymers remains a significant challenge. In this paper, the high-performance donor–acceptor (D–A) random copolymers, named PBT4T-BBT-x (x = 0, 5, 10, 20 and 100), were fabricated by cross-mixing bithiophene donors with benzothiadiazole (BT) and benzodithiadiazole (BBT) acceptors. Notably, when the ratios of BT and BBT are finely tuned, the polymers exhibit significantly controllable open-shell radical effects and twisted intermolecular charge transfer (TICT) states. The synergistic effects of radicals and TICT states notably enhanced the PT performance of random copolymers. Specifically, when the proper ratios of BBT units are used, the photothermal conversion efficiency (PTCE) is remarkably increased from 21.7% to 58.5%, and the PT temperature obviously increases from 150 °C to 232 °C under 808 nm laser irradiation. Furthermore, the random copolymers exhibit good water evaporation rates. We propose that this strategy provides a valuable synthesis pathway for generating high-performance photothermal therapy and water evaporation materials. Full article

Organic molecular fluorophores have been extensively utilized for biological imaging in the visible and the first near-infrared windows. However, their applications in the second near-infrared (NIR-II) window remain constrained, primarily due to the insufficient fluorescence brightness. Herein, we employ a theoretical protocol combining the thermal vibration correlation function with the time-dependent density functional theory method to investigate the mechanism of the planar-twisted strategy for developing fluorophores with balanced NIR-II emission and fluorescence brightness. Based on a planar donor–acceptor–donor molecular skeleton, various ortho-positioned alkyl side chains with steric hindrances are tactfully incorporated into the backbone to construct a series of twisted fluorophores. Photophysical characterizations of the studied fluorophores demonstrate that the emission spectra located in the NIR-II region exhibited a hypsochromic shift with the structural distortion. Notably, conformational twisting significantly accelerated the radiative decay rate while simultaneously suppressing the nonradiative decay rate, resulting in an improved fluorescence quantum efficiency (FQE). This enhancement can be mainly attributed to both the enlarged adiabatic excitation energy and reduced nonadiabatic electronic coupling between the first excited state and the ground state. Compared with the planar fluorophore, the twisted structures possessed a more than fivefold increase in FQE. In particular, the optimal twisted fluorophore BBTD-4 demonstrated a desirable fluorescence brightness (16.59 M⁻¹ cm⁻¹) on the premise of typical NIR-II emission (980 nm), making it a promising candidate for NIR-II fluorescence imaging in biomedical applications. The findings in this study elucidate the available experimental observations on the analogues, highlighting a feasible approach to modulating the photophysical performances of NIR-II chromophores for developing more highly efficient fluorophores toward optical imaging applications. Full article

Surface enhanced fluorescence (SEF) based on noble metal nanoparticles is an effective means to achieve high sensitivity in fluorescence detection. Currently, the physical mechanism behind enhanced fluorescence is not fully understood. This paper measures the fluorescence signals of Dihydroporphyrin f methyl ether (CPD4) under both single-photon and two-photon excitation based on submicrometer silver particles with rough morphologies, achieving enhancement factors of 34 and 45 times, respectively. On this basis, by combining the radiative field characteristics produced by the silver particles, a stimulated radiation model of molecules is established to elucidate the changes in the molecular photophysical process when influenced by silver particles. Moreover, the fluorescence lifetime of the molecules was measured, showing that the presence of silver particles induces an increase in the molecular radiative decay rate, causing the fluorescence lifetime to decay from 3.8 ns to 3 ns. The results indicate that the fluorescence enhancement primarily originates from the submicrometer silver particles’ enhancement effect on the excitation light. Additionally, the fluorescence signal emitted by the molecules couples with the silver particles, causing the local surface plasmon resonances generated by the silver particles to also emit light signals of the same frequency. Under the combined effect, the fluorescence of the molecules is significantly enhanced. The findings provide a theoretical foundation for understanding the fluorescence enhancement mechanism of silver particles, adjusting the enhancement effect, and developing enhanced fluorescence detection devices based on submicrometer silver particles, holding significant practical importance. Full article

Studies on estimating cumulative fire intensity from spreading wildland fires based on fire radiative energy density (FRED) have primarily been conducted through controlled experiments. The objective of this study was to assess the potential for estimating FRED for freely-burning wildfires at landscape scales. Airborne thermal infrared image sequences collected 8 and 9 December 2017 during the Thomas Fire were used for surface temperature derivation and FRED estimation. Sensitivity of varying ambient temperatures, and a newly developed method that adjusts for ash radiances on fire radiative flux density (FRFD) and FRED estimates were tested. Pixel-level image classification was run to identify FRFD time sequences that were complete or incomplete because of cloud obscuration and provided the basis for an obscuration gap filling technique. Variations in estimated ambient temperature used to estimate FRFD had little impact on FRED estimates, while our ash adjustment led to notable differences. An exponential decay model characterized FRFD time sequences well, providing a basis for gap filling irregular sequences caused by atmospheric obscuration. FRED estimates were regressed on rate of spread (ROS) magnitudes and found to be positively and significantly correlated. FRED magnitudes were higher on 9 December when the Thomas Fire burned under higher wind speeds and lower relative humidity levels (Santa Ana weather conditions) than on 8 December. Full article

The charge carrier formation and transport in the pristine polymers as well as in the polymer–fullerene blend is still a hot topic of discussion for the scientific community. In the present work, the carrier generation in some prominent organic molecules has been studied through ultrafast transient absorption spectroscopy. The identification of the exciton and polaron lifetimes of these polymers has led to device performance-related understanding. In the Energy Gap Law, the slope of the linear fit gradient (γ) of lifetimes vs. bandgap are subjected to the geometrical rearrangements experienced by the polymers during the non-radiative decay from the excited state to the ground state. The value of gradient (γ) for excitons and polarons is found to be −1.1 eV⁻¹ and 1.14 eV⁻¹, respectively. It suggests that the exciton decay to the ground state is likely to involve a high distortion in polymer equilibrium geometry. This observation supports the basis of Stokes shift found in the conjugated polymers due to the high disorder. It provides the possible reasons for the substantial variation in the exciton lifetime. As the bandgap becomes larger, exciton decay rate tends to reduce due to the weak attraction between the holes in the HUMO and electron in the LUMO. The precise inverse action is observed for the polymer–fullerene blend, as the decay of polaron tends to increase as the bandgap of polymer increases. Full article

We report recent enhancements to the online atomic database at the Ohio State University, NORAD-Atomic-Data, that provide various parameters for radiative and collisional atomic processes dominant in astrophysical plasma. NORAD stands for Nahar Osu RADiative. The database belongs to the data sources, especially for the latest works, of the international collaborations of the Opacity Project and the Iron Project. The contents of the database are calculated values for energies, oscillator strengths, radiative decay rates, lifetimes, cross-sections for photoionization, electron-ion recombination cross-sections, and recombination rate coefficients. We have recently expanded NORAD-Atomic-Data with several enhancements over those reported earlier. They are as follows: (i) We continue to add energy levels, transition parameters, cross-sections, and recombination rates for atoms and ions with their publications. (ii) Recently added radiative atomic data contain a significant amount of transition data for photo-absorption spectral features corresponding to the X-ray resonance fluorescence effect, showing prominent wavelength regions of bio-signature elements, such as phosphorus ions, and emission bumps of heavy elements, such as of lanthanides, which may be created in a kilonova event. We are including (iii) collisional data for electron-impact-excitation, (iv) experimental data for energies and oscillator strengths for line formation, (v) experimental cross-sections for photoionization that can be applied for benchmarking and other applications, and (vi) the introduction of a web-based interactive feature to calculate spectral line ratios at various plasma temperature and density diagnostics, starting with our recently published data for P II. We presented a summary description of theoretical backgrounds for the computed data in the earlier paper. With the introduction of experimental results in the new version of NORAD, we present a summary description of measurement of high-resolution photoionization cross-sections at an Advanced Light Source of LBNL synchrotron set-up and briefly discuss other set-ups. These additions should make NORAD-Atomic-Data more versatile for various applications. For brevity, we provide information on the extensions and avoid repetition of data description of the original paper. Full article