Direct Excitation Research Articles

Ultrafast Intersystem Crossing in Naturally Occurring Plant Pigments 5-Hydroxyflavones under Direct UV Excitation.

Flavonoids, a group of natural pigments, have attracted notable attention for their intrinsic fluorescent bioactive properties and potential therapeutic implications. Recent studies have suggested that the photoexcitation of specific flavonoids can also lead to the formation of triplet states, thereby potentially enhancing their applications in photoactivated antioxidant mechanisms. However, the crucial mechanism details about triplet state formation are still poorly understood. In this Letter, the ultrafast excited state relaxation mechanism for a series of 5-hydroxyflavone derivatives was studied by femtosecond time-resolved spectroscopy combined with quantum chemical calculations. Our results reveal the ultrafast ISC (kISC ≈ 1011 s-1) channel, which is sensitive to molecular structure and solvent environment, in 5-hydroxyflavones for the first time. Notably, the triplet excited state quantum yield of 4',7-dimethoxy-5-hydroxyflavone can reach up to 8% in acetonitrile solution. These results are essential for understanding the triplet state generation mechanism in 5-hydroxyflavone derivatives and could help the further development of 5-hydroxyflavone scaffold antioxidant agents.

Direct excitation of Kelvin waves on quantized vortices

Direct excitation of Kelvin waves on quantized vortices

Luminescence Properties of Hoechst 33258 in Polyvinyl Alcohol Films.

We report a comprehensive investigation of the photophysical properties of Hoechst 33258 (HOE) embedded in polyvinyl alcohol (PVA) films. HOE displays a bright, highly polarized, blue fluorescence emission centered at 430 nm, indicating effective immobilization within the polymer matrix of PVA. Its fluorescence quantum yield is notably high (~0.74), as determined relative to a quinine sulfate standard. In addition, we observed that HOE-doped PVA films exhibit room temperature phosphorescence (RTP) that remains visible for several seconds after UV excitation ceases. The slightly negative phosphorescence anisotropy implies that the triplet-singlet radiative transition is orthogonal to the singlet-singlet transition governing fluorescence. Notably, we observed that direct triplet-state excitation at longer wavelengths (beyond the primary absorption band) produces highly polarized RTP. We believe this possibility of direct triplet-state excitation opens new avenues for studying RTP in polymer-immobilized molecules.

Electron temperature measurement from neutral atomic tungsten emission line ratio.

A method to determine electron temperature within a plasma by the spectral analysis of atomic tungsten emission has been explored. The technique was applied to a post-discharge region immediately following a high voltage nanosecond pulsed discharge in air with tungsten electrodes. Atomic tungsten lines are readily observed in the weak emission spectrum within the post-discharge region for many microseconds. Intensity ratios were measured at various times after the pulsed discharge for a select pair of neutral tungsten emission lines at 400.88 and 401.52nm, where the upper electronic levels of each transition are at 3.46 and 5.52eV respectively. This significant difference in upper state energy causes their line intensity ratio to vary as the electron temperature changes. In addition to the emission spectra, the absolute electron temperature could be accurately measured in our lab using laser Thomson scattering to calibrate the new tungsten emission line intensity ratio method. An analysis is presented that calculates electron temperature from these tungsten emission data assuming a Maxwellian electron energy distribution contributing to direct electron impact excitation to the upper states of each transition. The results included the derivation of a calibration factor between the two experimental methods representing a previously unreported ratio of Einstein A coefficients for the 400.88-401.52nm transitions. This derivation provides a method for future measurement of absolute electron temperature by the 400.88-401.52nm tungsten line intensity ratio without the need for laser Thomson scattering calibration.

Underwater low-power electromagnetic transducers with Central permanent magnet integration.

High-efficiency electromagnetic transducers are crucial for enabling the self-sustained operation of underwater electromagnetic sound sources under power-constrained conditions as noted by Hao, Xie, and Ma [Proceedings of the 2019 Western China Acoustics Academic Conference, Guangzhou, China (November 5-9, 2019)]. This paper proposes a permanent magnet drive technology to enhance the electromechanical conversion efficiency of can-type electromagnetic transducers under low-power driving conditions. The can-type transducers consist of coils, an armature, and a cylindrical magnetic core with a central pillar, similar to the pot core proposed by Cui, Xu, Xu, and Shui [Electr. Eng. 101, 911-919 (2019)]. The proposed driving technology involves adding a permanent magnet to the central pillar to introduce a direct current excitation bias, thereby adjusting the static operating point and the electromechanical conversion efficiency of the transducer. In this paper, a theoretical model of the electromagnetic driving force is established, and finite element simulations show that adding a permanent magnet increases the electromechanical efficiency of can-type transducers from less than 1% to 84.43% under low-power conditions. Furthermore, prototypes of underwater can-type electromagnetic transducers with and without a permanent magnet at the core center were fabricated in this study. The experimental results confirmed that the permanent magnet drive technology significantly improves electromechanical conversion efficiency by adding a permanent magnet to the core center.

Structural relaxation chirality transfer enhanced circularly polarized luminescence in heteronuclear CeIII-MnII complexes.

Circularly polarized luminescence (CPL) materials have developed rapidly in recent years due to their wide application prospects in fields like 3D displays and anti-counterfeiting. Utilizing energy transfer processes to transfer chirality has been proven as an efficient way to obtain CPL materials. However, the physics behind energy-transfer induced CPL is still not clear. Herein, in a well-designed heteronuclear CeIII-MnII complex system [(Ce((R/S)-L)Br(μ-Br))2]MnBr4 [(R/S)-L = (2R,3R)- or (2S,3S)-2,3-dimethyl-1,4,7,10,13,16-hexaoxacyclooctadecane] with intra energy transfer from CeIII to MnII, the luminescence dissymmetry factor of MnII obtained by excitation of CeIII is around 10 times higher than that obtained by direct excitation of MnII, while the CeIII center itself shows an almost negligible CPL. To address this unusual phenomenon, we proposed a new mechanism named structural relaxation chirality transfer (SRCT) where structural relaxation of the excited chiral donor amplified chirality transfer to the acceptor by intramolecular interactions. As an assistant proof, a mixture of CeIII-ZnII and LaIII-MnII complexes with inter energy transfer showed no CPL amplification. These results will inspire more breakthroughs in the physics nature and development of energy-transfer induced CPL.

Manipulation and Theoretical Modeling of the Luminescence Performance of Silver Clusters in LTA Zeolites by Adjusting Extra-framework Cations.

Luminescent silver clusters that are confined inside porous zeolites have attracted great research interest due to the combined advantages of high quantum yield, tunable emission, excellent chemical stability, and desirable adoption ability of this micronano-composite. In this research, a series of Ag-exchanged R-LTA/Ag (R = Li, Na, K) zeolites were synthesized, in which the silver clusters showed blue-shifted excitation in the UV region, red-shifted emission in the visible region, and decreased emission intensity when taking Li+, Na+, and K+ as the extra-framework cations, respectively. The [Ag4(H2O)xR4-sod]6- (x = 2, 4; R = Li, Na, K) structures were constructed, and the TD-DFT method was further carried out to study the influence of extra-framework cations on the luminescence performance of [Ag4]2+ clusters, in which the substitution of Al for Si and the reasonable hydrate levels were both taken into consideration. It is estimated that the S1 and S3 levels for the [Ag4(H2O)4Li4-sod]6- cluster, the S4 level for the [Ag4(H2O)4Na4-sod]6- cluster, and the S5 level for the [Ag4(H2O)2K4-sod]6- cluster can be effectively populated through direct excitation, and the S1 → S0 transition is assumed to be responsible for the radiative emission in the visible region for the [Ag4]2+ pseudo-tetrahedrons. This calculated energy level diagram and simulated absorption spectra of [Ag4(H2O)xR4-sod]6- (x = 2, 4; R = Li, Na, K) clusters explained the modified luminescence property of silver clusters in R-LTA/Ag (R = Li, Na, K) zeolites studied in this research and reported literature.

Defect-Mediated Exciton Localization and Relaxation in Monolayer MoS2.

Defects in chemical vapor deposition (CVD)-grown monolayer MoS2 are unavoidable and provide a powerful approach to creating single-photon emitters and quantum information systems through localizing excitons. However, insight into the A- trion and B/C exciton localization in monolayer MoS2 remains elusive. Here, we investigate defect-mediated A- trion and B/C exciton localization and relaxation in CVD-grown monolayer MoS2 samples via transient absorption spectroscopy. The localization rate of A- trions is five times faster than B excitons, which is attributed to the distinctions in the Bohr radius, diffusion rate, and multiphonon emission. Furthermore, we obtain unambiguous experimental evidence for the direct excitation of localized C excitons. Varying gap energy at the band-nesting region revealed by first-principles calculations explains the anomalous dependence of localized C exciton energy on delay time. We also find that the rapid dissociation of localized C excitons features a short characteristic time of ∼0.14 ps, while the measured relaxation time is much longer. Our results provide a comprehensive picture of the defect-mediated excitonic relaxation and localization dynamics in monolayer MoS2.

Design and Application of a Hemispherical XRF Secondary Target

ABSTRACTIn x‐ray fluorescence analysis (XRFA), the use of secondary targets to suppress measurement background is a common practice aimed at reducing detection limits. However, conventional polarized secondary targets often exhibit low conversion efficiency of primary x‐rays. In this paper, we propose a novel hemispherical secondary target designed to exploit a focusing‐like phenomenon for primary x‐rays, thereby enhancing the conversion efficiency and achieving improved background suppression. The optimal dimensions of the hemispherical secondary target are determined through theoretical calculations, and further validated through Monte Carlo simulations and experimental analysis. Our findings reveal a substantial enhancement in the peak‐to‐background ratio when compared to traditional polarized secondary targets. Specifically, for a 1000‐ppm Ce sample, the peak‐to‐background ratio increased from 8.45 to 9.78 with the implementation of the hemispherical secondary target. Similar improvements were observed when measuring a 1000‐ppm Ce sample, relative to direct excitation. Furthermore, the relative error between the system and the ICP‐MS measurement results of the rare earth sample remains within 5%, underscoring the effectiveness of the hemispherical secondary target in XRFA applications.

Whistling potential of throttling orifices and its connection to liquid rocket combustion instability

Orifice whistling is recently established as a cause of high frequency combustion instability in liquid rocket engines. The series of studies on the experimental thrust chamber, known as BKD, developed at the German Aerospace Center (DLR), were instrumental in identifying this phenomenon. BKD encountered an injector-driven thermo-acoustic instability caused by the self-excitation of the LOX injector. The throttle orifice in the LOX injector plays a vital role in the self-excitation through whistling. In this work, the impedance of the BKD throttling orifice is characterized with the motive to find its whistling range. The CFD approach based on solving the unsteady Reynolds Averaged Navier Stokes equations proposed by Lacombe et al. [1] is used for this characterization. We characterize the orifice at the weak and the strong coupling conditions reported in the literature. The orifice impedance is calculated in the range of 4 kHz to 15 kHz, covering the entire potential coupling range of LOX injector eigen modes. The impedance results show that the orifice character follows the whistling behavior of the “medium” orifices. The resistance turns negative in the Strouhal number range of St=0.89 - 1.2 for both the weak and strong operating conditions. When plotted against the frequency, the resistance at the strong coupling condition occurs close to the second longitudinal (2L) mode of the LOX injector, suggesting direct excitation of the injector through the orifice whistling. The coexistence of the 2L mode and the first transverse (1T) mode of the combustion chamber at the same frequency leads to combustion instability.

Triplet-Sensitized Switching of High-Energy-Density Norbornadienes for Molecular Solar Thermal Energy Storage with Visible Light.

Norbornadiene-based photoswitches have emerged as promising candidates for harnessing and storing solar energy, holding great promise as a viable solution to meet the growing energy demands. Despite their potential, the effectiveness of their direct photochemical conversion into the resulting quadricyclanes has room for improvement owing to (i) moderate quantum yields, (ii) poor overlap with the solar spectrum and (iii) photochemical back reactions. Herein, we present an approach to enhance the performance of such molecular solar thermal energy storage (MOST) systems through the triplet-sensitized conversion of aryl-substituted norbornadienes. Our study combines deep spectroscopic analyses, irradiation experiments, and quantum mechanical calculations to elucidate the energy transfer mechanism and inherent advantages of the resulting MOST systems. We demonstrate remarkable quantum yields using readily available sensitizers under both LED and solar light irradiation, significantly surpassing those achieved through direct excitation with photons of higher energy. In contrast to the conventional approach, light-induced back reactions of the high-energy products do not play any role, allowing quantitative switching within minutes. These results not only underscore the potential of triplet-sensitized MOST systems to leverage the high energy storage capabilities of multistate photoswitches but they might also stimulate the broader usage of sensitization strategies in photochemical energy conversion.

Resonant Excitation of Kelvin Waves by Interactions of Subtropical Rossby Waves and the Zonal Mean Flow

Abstract Equatorial Kelvin waves can be affected by subtropical Rossby wave dynamics. Previous research has demonstrated the Kelvin wave growth in response to subtropical forcing and the resonant growth due to eddy momentum flux convergence. However, the relative importance of the wave–mean flow and wave–wave interactions for the Kelvin wave growth compared to the direct wave excitation by the external forcing has not been made clear. This study demonstrates the resonant Kelvin wave excitation by interactions of subtropical Rossby waves and the mean flow using a spherical shallow-water model. The use of Hough harmonics as basis functions makes Rossby and Kelvin waves prognostic variables of the model and allows the quantification of terms contributing to their tendencies in physical and wave spaces. The simulations show that Kelvin waves are resonantly excited by interactions of Rossby waves and the balanced zonal mean flow in the subtropics, provided the Rossby and Kelvin wave frequencies, which are modified by the mean flow, match. The resonance mechanism is substantiated by analytical expressions. The Kelvin wave tendencies are caused by velocity and depth tendencies: The velocity tendencies due to the meridional advection of the zonal mean velocity can be outweighed by the zonal advection of the Rossby wave velocity or by the depth tendencies due to Rossby wave divergence. Identifying the resonant excitation mechanism in data should contribute to the quantification of Kelvin wave variability originating in the subtropics. Significance Statement This study seeks to understand how Kelvin waves, which are eastward-propagating disturbances in the tropical atmosphere, are connected to Rossby wave dynamics in the subtropics. Using idealized simulations, the Kelvin wave excitation is explained as a resonance effect due to interactions of Rossby waves and the zonal mean flow. The mechanism contributes to the understanding of atmospheric wave interactions and extratropical effects on the tropics. Further work searching for evidence of the new mechanism in atmospheric data may shed a new light on subseasonal variability in the tropics, such as the Madden–Julian oscillation.

Study on the design and application of the three-electrode ionization particle sensor

Accurate detection is the foundation of haze governance. The existing particle concentration detection methods and instruments have many shortcomings, such as complex structure, high cost, easy blockage, short life, and significant impact of environmental changes. Therefore, this paper proposes a silicon micropillar three-electrode ionization particle sensor based on the gas discharge principle and studies the sensor's sensitivity to PM2.5 particles. Three-electrode comprises a cathode with micro silicon pillars grown on the surface, an extracting electrode, and a collecting electrode. It shows that the cathode current of the sensor increases linearly with extraction voltage and nonlinearly with temperature. As the cathode material of the sensor, the silicon micropillar is compared with the carbon nanotube to solve the burn problem of the nanotube rising from positive ions bombardment during gas discharge. The collecting current of the sensor shows a decreasing trend with the increase in particle concentration. The collecting current and sensitivity of the sensor are much higher in radio frequency excitation than in direct current excitation. The sensor developed in this paper displays the advantages of simple technology, high integration, low cost, and high sensitivity, and it exhibits great application potential.

Employing Singlet-Singlet Energy Transfer for Boosting the Reactivity of Type I Photoinitiators in Radical Photopolymerization.

A remarkable and unexpected increase in the photopolymerization efficiency of an acrylic resin by a bisacylphosphine oxide photoinitiator was observed when an optical brightener was present in the medium. High values for the maximal rates of photopolymerization were obtained by RT-FTIR at 365 nm under a very low irradiance of 1 mW/cm2. Fluorescence studies revealed that the quenching process occurs through singlet-singlet energy transfer between the first singlet excited state of the optical brightener and the ground state photoinitiator. This mechanism acts as an additional pathway for the excitation of the photoinitiator, thereby increasing the total amount of initiating radicals. Using the Förster resonance energy transfer model, we calculated the relative efficiency of the photosensitization process with respect to the direct excitation efficiency of the photoinitiator. The results demonstrate that the photosensitization process can be predicted, paving the way for further improvements in photoinitiating systems.

Employing Singlet‐Singlet Energy Transfer for Boosting the Reactivity of Type I Photoinitiators in Radical Photopolymerization

AbstractA remarkable and unexpected increase in the photopolymerization efficiency of an acrylic resin by a bisacylphosphine oxide photoinitiator was observed when an optical brightener was present in the medium. High values for the maximal rates of photopolymerization were obtained by RT‐FTIR at 365 nm under a very low irradiance of 1 mW/cm2. Fluorescence studies revealed that the quenching process occurs through singlet‐singlet energy transfer between the first singlet excited state of the optical brightener and the ground state photoinitiator. This mechanism acts as an additional pathway for the excitation of the photoinitiator, thereby increasing the total amount of initiating radicals. Using the Förster resonance energy transfer model, we calculated the relative efficiency of the photosensitization process with respect to the direct excitation efficiency of the photoinitiator. The results demonstrate that the photosensitization process can be predicted, paving the way for further improvements in photoinitiating systems.

Harnessing the Natural Resonances of Time-Varying Dispersive Interfaces.

Space-time modulation of electromagnetic parameters offers novel exciting possibilities for advanced field manipulations. In this study, we explore wave scattering from a time-varying interface characterized by a Lorentz-type dispersion with a steplike temporal variation in its parameters. Our findings reveal a new process: an unconventional frequency generation at the natural resonances of the system. Remarkably, this phenomenon enables the coupling of propagating waves to evanescent ones, allowing the direct far-field excitation of surface-wave modes without the mediation of spatial gratings or prisms. These results suggest a novel strategy for designing compact and ultrafast photonic devices, eliminating the necessity for subwavelength spatial structuring or prolonged temporal modulations.

Measurement of differential collisional excitation cross sections for the Kα emission of He-like oxygen

We measure the energy-differential cross sections for collisional excitation of the soft x-ray electric-dipole Kα (x+y+w) emission from He-like oxygen (O ), using an electron beam ion trap. Values near their excitation thresholds were extracted from the observed emissivity by rapidly cycling the energy of the exciting electron beam. This allows us to subtract time-dependent contributions of the forbidden z-line emission to the multiplet. We develop a time-dependent collisional-radiative model to further demonstrate the method and predict all spectral features. We then compare the extracted x+y+w cross sections with calculations based on distorted-wave and R-matrix methods from the literature and our own predictions using the . All R-matrix results are validated by our measurements of direct and resonant excitation, supporting the use of such state-of-the-art codes for astrophysical and plasma physics diagnostics. Published by the American Physical Society 2024

Arylthianthrenium Salts for Triplet Energy Transfer Catalysis.

Sigma bond cleavage through electronically excited states allows synthetically useful transformations with two radical species. Direct excitation of simple aryl halides to form both aryl and halogen radicals necessitates UV-C light, so undesired side reactions are often observed and specific equipment is required. Moreover, only aryl halides with extended π systems and comparatively low triplet energy are applicable to synthetically useful energy transfer catalysis. Here we show the conceptual advantages of arylthianthrenium salts (ArTTs) for energy transfer catalysis with high energy efficiency compared to conventional aryl (pseudo)halides and their utility in arylation reactions of ethylene. The fundamental advance is enabled by the low triplet energy of ArTTs that may originate in large part from the electronic interplay between the distinct sulfur atoms in the tricyclic thianthrene scaffold, which is not accessible in either simple (pseudo)halides or other conventional sulfonium salts.

A new role for excitation in the retinal direction-selective circuit.

A key feature of the receptive field of neurons in the visual system is their centre-surround antagonism, whereby the centre and the surround exhibit responses of opposite polarity. This organization is thought to enhance visual acuity, but whether and how such antagonism plays a role in more complex processing remains poorly understood. Here, we investigate the role of centre and surround receptive fields in retinal direction selectivity by exposing posterior-preferring On-Off direction-selective ganglion cells (pDSGCs) to adaptive light and recording their response to globally moving objects. We reveal that light adaptation leads to surround expansion in pDSGCs. The pDSGCs maintain their original directional tuning in the centre receptive field, but present the oppositely tuned response in their surround. Notably, although inhibition is the main substrate for retinal direction selectivity, we found that following light adaptation, both the centre- and surround-mediated responses originate from directionally tuned excitatory inputs. Multi-electrode array recordings show similar oppositely tuned responses in other DSGC subtypes. Together, these data attribute a new role for excitation in the direction-selective circuit. This excitation carries an antagonistic centre-surround property, possibly designed to sharpen the detection of motion direction in the retina. KEY POINTS: Receptive fields of direction-selective retinal ganglion cells expand asymmetrically following light adaptation. The increase in the surround receptive field generates a delayed spiking phase that is tuned to the null direction and is mediated by excitation. Following light adaptation, excitation rules the computation in the centre receptive field and is tuned to the preferred direction. GABAergic and glycinergic inputs modulate the null-tuned delayed response differentially. Null-tuned delayed spiking phases can be detected in all types of direction-selective retinal ganglion cells. Light adaptation exposes a hidden directional excitation in the circuit, which is tuned to opposite directions in the centre and surround receptive fields.

Catalytic Dearomative Cycloaddition Reactions Enabled by Visible Light

AbstractThe synthesis of polycyclic compounds is highly valued due to the ubiquitous presence of these structures in pharmaceuticals and natural substances. Cycloaddition reactions are notable as one of the most important reaction classes in chemical synthesis due to their ability to generate polycyclic compounds in a single step using straightforward and easily accessible starting materials. In recent times, visible‐light‐mediated photocatalysis has emerged as a powerful tool for enabling a wide range of transformations. Significant advancements have been achieved in dearomative cycloaddition reactions induced by visible light over the past several years. This review offers an overview of visible‐light‐induced dearomative cycloaddition reactions categorized according to the method of substrate excitation ‐ a) Visible light induced dearomative photocatalytic cycloaddition by Energy Transfer (EnT); b) Visible light induced dearomative cycloaddition by direct excitation; c) Visible light induced dearomative radical cyclization.