Excitation Process Research Articles

Large negative thermal quenching of upconversion luminescence through selective excitation in a YAG:Yb/Er transparent ceramics

This study has demonstrated a salient negative thermal quenching behavior of upconversion (UC) luminescence in YAG:Yb/Er transparent ceramic phosphor in a wide temperature range from 100 to 500 K. With the rise of temperature, UC luminescence deteriorates under the excitation at λex = 932 nm, whereas it intensifies remarkably at λex = 974 nm (i.e., the integrated emission intensity increased by nearly 30 folds from T = 100 K to T = 500 K), indicating a dynamic UC luminescence-temperature relation dependent upon the incident excitation energy. Our later photophysical analysis shows that the negative thermal quenching behavior was mainly ascribed to a phonon-assisted thermal excitation process which occurs at low excitation energy. More interestingly, it is found that despite a final decline at high temperature, the UC luminescence lifetime increases as the temperature rises before certain degrees. A modified equation for luminescence lifetime was proposed and used for our interpretation. This work not only demonstrates large negative thermal quenching of UC luminescence in the ceramic phosphor for the first time, but also provides a deep insight into the photophysical behavior of UC luminescence.

Impact of multipath interference in Si/SiO2 substrate on ultrafast relaxation processes in two-dimensional transition metal dichalcogenides

At present, two-dimensional transition metal dichalcogenides (2D TMDs) are of great importance due to their unique properties. These materials attracted considerable attention due to the ultrafast dynamics of excitation and relaxation processes in charge carriers. Recently, many researchers have estimated the characteristic relaxation times using standard optical pump-probe methods. The impact of multipath interference in underlying substrates on the determination of the characteristic relaxation times is generally ignored, although it can be significant due to the high transparency of 2D TMDs. In this work, we propose an approach to estimating the impact of a substrate on relaxation processes in 2D TMDs. We experimentally studied the dynamics of ultra-fast relaxation processes in monolayer, bilayer, and bulk MoS2 on Si/SiO2 substrate using time-resolved spectroscopy. It was shown that the multipath interference in substrate has a strong effect on the determination of the slow (several hundred ps) relaxation times which are associated with interband carrier recombination and can lead to the up to 20% distortion of obtained results.

The photophysics of protonated cytidine and hemiprotonated cytidine base pair: A computational study.

We here study the effect that a lowering of the pH has on the excited state processes of cytidine and a cytidine/cytidine pair in solution, by integrating time-dependent density functional theory and CASSCF/CASPT2 calculations, and including solvent by a mixed discrete/continuum model. Our calculations reproduce the effect of protonation at N3 on the steady-state infrared and absorption spectra of a protonated cytidine (CH+ ), and predict that an easily accessible non-radiative deactivation route exists for the spectroscopic state, explaining its sub-ps lifetime. Indeed, an extremely small energy barrier separates the minimum of the lowest energy bright state from a crossing region with the ground electronic state, reached by out-of-plane motion of the hydrogen substituents of the CC double bond, the so-called ethylenic conical intersection typical of cytidine and other pyrimidine bases. This deactivation route is operative for the two bases forming an hemiprotonated cytidine base pair, [CH·C]+ , the building blocks of I-motif secondary structures, whereas interbase processes play a minor role. N3 protonation disfavors instead the nπ* transitions, associated with the long-living components of cytidine photoactivated dynamics.

Influence of the magnetic field on the extension of the ionization region in high power impulse magnetron sputtering discharges

The high power impulse magnetron sputtering (HiPIMS) discharge brings about increased ionization of the sputtered atoms due to an increased electron density and efficient electron energization during the active period of the pulse. The ionization is effective mainly within the electron trapping zone, an ionization region (IR), defined by the magnet configuration. Here, the average extension and the volume of the IR are determined based on measuring the optical emission from an excited level of the argon working gas atoms. For particular HiPIMS conditions, argon species ionization and excitation processes are assumed to be proportional. Hence, the light emission from certain excited atoms is assumed to reflect the IR extension. The light emission was recorded above a 100 mm diameter titanium target through a 763 nm bandpass filter using a gated camera. The recorded images directly indicate the effect of the magnet configuration on the average IR size. It is observed that the shape of the IR matches the shape of the magnetic field lines rather well. The IR is found to expand from 10 and 17 mm from the target surface when the parallel magnetic field strength 11 mm above the racetrack is lowered from 24 to 12 mT at a constant peak discharge current.

Research of features of oscillating process’ behavior in the nonlinear system of individual traction drive of an electrobus

BACKGROUND: When a vehicle is in motion, self-oscillations which properties are dependent on slip rate in a contact patch may occur in the area of tire interaction with ground surface. Oscillations frequency will vary in dependence with value of wheel slip relative to ground surface. Soft self-oscillations are excited by variable set of initial conditions at full slip in traction and driven wheel rolling modes as well as in mixed braking mode with partial slip. Hard mode of self-oscillations occurs at full wheel slip in braking mode. These processes have a negative impact on the processes in electric drive and mechanical drivetrain reducing their efficiency and may cause damage of components. Oscillations in the system are excited by interaction forces of an elastic tire with ground surface featuring vertical oscillations due to elastic behavior of its interaction with road unevenness. AIMS: Research of features of oscillating process behavior in the nonlinear system of individual traction drive of an electrobus. METHODS: Simulation of self-oscillation excitation processes in the area of contact interaction of a wheel and road was carried out in the MATLAB/Simulink software package. RESULTS: The article features the results of simulation and experimental studies of self-oscillation excitation processes of the KAMAZ 6282 electrobus moving on asphalt-concrete surface. It was found that vertical wheel displacement when moving through unevenness lead to oscillating behavior of vertical reaction forces in contact patches and, as a consequence, to oscillating behavior of longitudinal reaction forces, torque and rotation velocity of the shaft of the traction electric motor of the individual drive. It was defined that tire oscillation frequency is 67 Hz that coincides with electric motor shaft rotation oscillation frequency and this value is the same for both experiment and simulation. CONCLUSIONS: Practical value of the study lies in ability of using the study results at development of self-oscillation processes exclusion algorithms as a part of vehicle control system.

Excited State Photophysics of Curcumin and its Modulation in Alkaline Non-Aqueous Medium.

Curcumin, a well-known medicinal pigment, has seen limited applications in biology despite having great potential as a therapeutic drug. Deprotonation is one of the possible ways to enhance solubility of curcumin in polar solvent. Here, we have explored the effect of deprotonation on the ultrafast dynamics of this biomolecule with the help of the time-resolved fluorescence spectroscopic measurements using the femtosecond fluorescence upconversion technique. The excited state photophysics of fully deprotonated curcumin significantly differs from that of neutral curcumin. We have observed that the completely deprotonated curcumin not only has higher quantum yield, but also higher excited state lifetime and slower solvation dynamics in comparison to neutral curcumin. We propose solvation dynamics and intramolecular charge transfer as the excited state processes associated with the radiative decay of the completely deprotonated molecule, while ruling out the possibility of excited state proton exchange or proton transfer. Our results are well supported by time-dependent density-functional theory calculations. Lastly, we have also demonstrated the possibility of modulating the ultrafast dynamics of fully deprotonated curcumin using non-aqueous alkaline binary solvent mixtures. We believe our results will provide significant physical insight towards unveiling the excited state dynamics of this molecule.

Influence of atmospheric and vacuum plasma processing on the organic composition of araça-boi (Eugenia stipitata) juice

Cold plasma is a novel non-thermal technology that is still under development. Cold plasma treatment of food products induces a wide range of chemical changes in food compounds, several of which can improve nutritional quality. Understanding the chemical changes caused during plasma treatment is essential to enhance food quality and avoid the formation of undesired compounds. This investigation addressed the influence of two cold plasma technologies, glow discharge plasma and dielectric barrier discharge plasma, on the organic composition of araça-boi (Eugenia stipitata) juice, which is rich in sugars, organic and amino acids. The chemical composition of the juice was assessed by 1H nuclear magnetic resonance coupled with chemometrics. The effects of excitation frequency, processing time, and plasma flow rate were investigated. Glow discharge plasma and dielectric barrier discharge plasma application tended to decrease the concentration of glucose (33%), fructose (31%), malic acid (25%), and amino acids while increasing sucrose content (23%). Overall, plasma processing changed the juices’ alcohol, sugar, and amino acids content, primarily by reducing the concentration of cysteine (33%), alanine (42%), and methionine (43%). However, the dielectric barrier discharge plasma tended to slightly increase the amino acid content (up to 9%) when operating at 200 Hz.

Protective action of graphene oxide against singlet oxygen generation by cosmetic dye methylene blue

Nanohybrid methylene blue/graphene oxide (MB/GO) was prepared via non-covalent functionalization with the focus on spectroscopic characterization of MB molecules adsorbed on GO. Fluorescence was not detected for the MB/GO hybrid which indicates that other competitive fast singlet excited state deactivation process must take place. It was demonstrated by measuring singlet oxygen (1O2) phosphorescence at 1270 nm that MB functionalized with GO does not generate 1O2. Nanosecond transient absorption spectroscopy experiments pointed to a lack of population of triplet state by MB/GO hybrid material. Femtosecond pump–probe experiments revealed much faster dynamics of the singlet excited state of MB/GO in comparison to free MB, which was attributed to the photoinduced electron transfer from MB to GO. Suppressing of the singlet oxygen quantum efficiency by MB upon functionalization to GO might be useful for designing graphene-based dye additives to color cosmetics with a reduced risk of the 1O2 generation on the skin.

Ultrafast Laser Control of Antiferromagnetic-Ferrimagnetic Switching in Two-Dimensional Ferromagnetic Semiconductor Heterostructures.

Realizing ultrafast control of magnetization switching is of crucial importance for information processing and recording technology. Here, we explore the laser-induced spin electron excitation and relaxation dynamics processes of CrCl3/CrBr3 heterostructures with antiparallel (AP) and parallel (P) systems. Although an ultrafast demagnetization of CrCl3 and CrBr3 layers occurs in both AP and P systems, the overall magnetic order of the heterostructure remains unchanged due to the laser-induced equivalent interlayer spin electron excitation. More crucially, the interlayer magnetic order switches from antiferromagnetic (AFM) to ferrimagnetic (FiM) in the AP system once the laser pulse disappears. The microscopic mechanism underpinning this magnetization switching is dominated by the asymmetrical interlayer charge transfer combined with a spin-flip, which breaks the interlayer AFM symmetry and ultimately results in an inequivalent shift in the moment between two FM layers. Our study opens up a new idea for ultrafast laser control of magnetization switching in two-dimensional opto-spintronic devices.

Reversible conversion between skyrmions and skyrmioniums

Skyrmions and skyrmioniums are topologically non-trivial spin textures found in chiral magnetic systems. Understanding the dynamics of these particle-like excitations is crucial for leveraging their diverse functionalities in spintronic devices. This study investigates the dynamics and evolution of chiral spin textures in [Pt/Co]3/Ru/[Co/Pt]3 multilayers with ferromagnetic interlayer exchange coupling. By precisely controlling the excitation and relaxation processes through combined magnetic field and electric current manipulation, reversible conversion between skyrmions and skyrmioniums is achieved. Additionally, we observe the topological conversion from a skyrmionium to a skyrmion, characterized by the sudden emergence of the skyrmion Hall effect. The experimental realization of reversible conversion between distinct magnetic topological spin textures represents a significant development that promises to expedite the advancement of the next generation of spintronic devices.

Green emission characteristics of Er3+ -doped TeO2-WO3-GeO2 glasses through up and down conversion processes

Different concentrations of Er3+ions doped TeO2-WO3-GeO2 transparent oxyfluoride glassy materials were fabricated by conventional melt quench method and analyzed through X-ray diffraction, Fourier Transform infrared, scanning electron microscope and energy dispersive X-ray, photoluminescence-excitation, visible emission and luminescence decay studies. Various physical, structural, spectroscopic and laser characteristic parameters were evaluated following the Judd-Ofelt theory. The studied glasses were capable of emitting intense green luminescence through (2H11/2,4S3/2) → 4I15/2 transitions. The emission of green luminescence was studied through down-conversion process at 377 nm excitation and up-conversion process at 980 nm excitation. The color coordinates were found to fall in the green region of chromaticity diagram and close to the European Broadcosting Union green illuminant. The evaluated branching ratio, stimulated emission cross-section, gain linewidth, figure of merit and the quantum efficiency corresponding to 4S3/2 → 4I15/2 transition suggests that the studied glasses find wide suitability to design green emitting solid state lighting devices.

Mechanisms of low frequency pressure oscillation occurred in oxygen jet condensation based on height function method

In a propulsion system of cryogenic liquid rockets, low frequency pressure oscillation is an important topic on the oxygen jet condensation. It is a challenge to capture the numerical characteristic of pressure oscillation precisely, especially the frequency and amplitude. Aiming at the mechanism and excitation process of pressure oscillation, an oxygen jet condensation simulation is conducted by a novel mass transfer model based on the height function method. The height function method is used to calculate the interfacial curvature between gas-liquid, which is the key factor to determine the frequency of pressure oscillation. The new model is validated by a steam jet condensation simulation and the corresponding result is in great agreement with the experimental data. The research shows that the low frequency pressure oscillation is caused by the periodic condensation mass transfer rate of the continuous oxygen vapor plume, along with a GOX suck-back flow phenomenon in the gas inlet of condenser pipe. The excitation frequency of pressure oscillation is 10.6 Hz, consistent with the experiment data of engine test run. It is close to the natural frequency of propulsion system. It can be inferred that oxygen jet condensation might be one of the excitation source of self-excited Pogo vibration. These conclusions provide a theoretical basis for overcoming the Pogo vibration eventually.

A method to evaluate the inductance properties of REBCO excitation process based on magnetic energy density and T–A formula

The REBCO high-temperature superconducting tape has a pronounced electromagnetic anisotropy. The critical current is strongly dependent on the value of the magnetic field, the magnetic field direction, the temperature, and the stress, and has a significant nonlinear relationship. It is also important to note that screening current effects in superconducting magnets at elevated temperatures lead to a non-uniform distribution of current densities, and thus the inductance of superconducting magnets at high-temperatures is calculated differently from that of normal magnets. This is essential for the design of superconducting energy storage magnets at high-temperatures, especially for the accurate evaluation of the inductance values. To further analyze the variation of the inductance value due to the REBCO screening current, the T–A formulation combined with the magnetic energy density is considered to evaluate the energy storage and inductance values during the excitation of the superconducting coil. The effect of excitation rate, operating temperature, and excitation current on the inductance value is analyzed based on the design of a 5 T insulating superconducting coil. The mathematical relation between excitation voltage, contact resistance, and inductance in the magnetic excitation experiment is presented, and the accuracy of the calculated method is verified by comparison with simulation results.

Singly Reduced Iridium Chromophores: Synthesis, Characterization, and Photochemistry.

One-electron reduced photosensitizers have been invoked as crucial intermediates in photoredox catalysis, including multiphoton excitation and electrophotocatalytic processes. However, such reduced chromophores have been less investigated, limiting mechanistic studies of their associated electron transfer processes. Here, we report a total of 11 different examples of isolable singly reduced iridium chromophores. Chemical reduction of a cyclometalated iridium complex with potassium graphite affords a 19-electron species. Structural and spectroscopic characterizations reveal a ligand-centered reduction product. The reduced chromophore absorbs a wide range of light from ultraviolet to near-infrared and exhibits photoinduced bimolecular electron transfer reactivity. These studies shed light on elusive reduced iridium chromophores in both ground and excited states, providing opportunities to investigate a commonly invoked intermediate in photoredox catalysis.

Дослідження реакції таеквондистів на об’єкт, що рухається

Purpose: to define features of reaction to a moving object in taekwondo. Material and Methods. The following research methods were used: analysis of scientific and methodological literature and Internet sources; pedagogical observation; psychophysiological measurements; methods of mathematical statistics. Results: the use of non-parametric Mann-Whitney criterion confirmed that the average reaction time for the whole test in older sportsmen is significantly better than in junior sportsmen by 31,39 % (U=5, p<0,01). When analyzing the directionality of the processes of excitation or inhibition, it was found that older athletes had better reaction accuracy than junior athletes (U=10,5, p<0,05). In both groups sportsmen reacted mainly prematurely, but junior taekwondoists reacted prematurely more often by 17,66 % (U=4, p<0,01). According to Spearman's correlation coefficient between the age of sportsmen and the obtained indicators of measurement the following results were obtained: between reaction time and age in the 1st group r=0,26, in the 2nd group r=-0,37 (p>0,05), between exact reactions and age in the 1st group r=0,32, in the 2nd group r=0,68 (p<0,05). According to the analysis of the results of the second measurement, there is no sufficient evidence that the average values of reaction time in both groups before and after the competition are different. After the competition, the accuracy of reactions in sportsmen from group 1 is lower by 31,1 % than before the competition (U=11, p<0,05). In group 2 there were no statistically significant differences between the indicators of reaction accuracy before and after competitions. The U-criterion did not show statistically significant difference between the direction of processes of excitation and inhibition in group 1 and group 2 before and after competitions. In the 1st group Spearman's coefficient testifies to the absence of statistically significant (p>0,05) correlation between the performance of sportsmen at competitions and average reaction time (r=-0,31). The similar correlation was established in group 1 between results at competitions and accuracy of reactions, r=-0,25 (p>0,05). In group 2, there was also no statistically significant (p>0.05) correlation between the results of the competition and the average reaction time (r=0,52). There is a statistically significant strong inverse correlation (p<0,05) in group 2 between the results of athletes in competitions and their reaction accuracy (r=-0,83). Conclusions. The results of the analysis of the reaction to a moving object indicate that there are significant differences between the groups. Highly skilled, older athletes have better ability to adjust nervous processes and shorter reaction times to a moving object. There is a statistically significant correlation between athletes' performance in competitions and the accuracy of their reactions. Indicators such as reaction speed and accuracy are important in taekwondo competitions and should be studied in future research.

Two-Photon Excitation Photoredox Catalysis Enabled Atom Transfer Radical Polymerization.

In recent years, metal-free photoredox-catalyzed atom transfer radical polymerization (O-ATRP) has gained wide attention because of its advantages (e.g., no metal contamination and mild reaction conditions). However, this traditional one-photon excitation catalysis has thermodynamic limits. Most photocatalysts cannot effectively reduce the initiators and drive the polymerization under visible light. Herein, we investigate the two-photon excitation-catalyzed O-ATRP, in which the catalyst can absorb two photons to accumulate energy. Compared to one-photon excitation catalysis, this method not only has distinct advantages in the controllability, reaction rate, and catalyst loading but also can chemically reduce the various initiators (e.g., aryl halides) to initiate the polymerization. Density functional theory (DFT) calculation reveals that the two-photon excitation process reached a higher energy end state with stronger reduced ability via a thermodynamically more stable intermediate. We believe that this work will provide a new strategy for photoredox-catalyzed O-ATRP.

Influence of plasma fluctuation on the electronic structures and electron collisional excitation and ionization processes

Studies demonstrated significant discrepancies between the experimental observations of Stillman et al. [Phys. Rev. E 95, 063,204 (2017)] and almost all recent state-of-the art theoretical predictions for the He-α (1s2p→1s2) line shift of Al11+ ions placed in a hot dense plasma environment. Here, we suggest a new distorted wave method to determine the electronic structure and collision dynamics of highly stripped ions placed in plasmas. The method incorporating the plasma fluctuation, which yields modification term to the calculation of the central-field potential, is applied to the above problem. By adjusting the parameter q, which characterizes the intensity of the plasma fluctuation, we reproduce the experimental line shifts well over the whole range (of temperature and density) measured. In addition, we perform a calculation by incorporating the screened potential of Stanton and Murillo [Phys. Rev. E 91, 033,104 (2015)] and the current results (from ab-initio calculations) are in much better agreement with the experimental data than most previous theoretical results. Finally, we perform the calculation of electron collisional excitation and ionisation dynamic processes, where the continuous electron wave function is obtained by numerically solving the modified coupled Dirac equation. A careful investigation of the effect of plasma fluctuation on the integrated cross sections is made. The present work not only leads to further stimulation of the development of shielding models in extreme conditions, but also has important applications in laboratory and space plasmas.

Interfacial self-assembly of Cd2+-tetraphenylethylene coordination polymers and their photophysical properties in Langmuir-Blodgett films

Molecular arrangement and orientation of tetraphenylethylene (TPE) unit in the highly ordered thin films and aggregates can effectively limit its conformation that results in strong effect on the excited energy transfer process and luminescent behaviors under irradiation. We report here an interfacial self-assembly of metal-TPE coordination polymer (CP) ultrathin films and their aggregate-induced emission (AIE) behaviors. After connected with pyridyl or benzimidazole substituents, the tetradentate TPEs as synthesized are used as the multidentate linkers to coordinate with the Cd2+ ions to produce aggregates in the solutions, and to form well-defined two- and three-dimensional organized Cd-TPE CP multilayers at the air-water interfaces by the Langmuir-Blodgett (LB) method. It is revealed that, in addition to the aggregation-induced emission, the following external factors have also large effects on the luminescence of TPEs, including the coordination bonding in the CPs, in plane and interlayer molecular interaction in the LB films; both of them can restrict the intramolecular motion or rotation of TPE unit. Because of these external effects, the emission lifetime of TPEs reaches to approximately 5–10 times longer in some Cd-TPE CP LB films than that in the solutions. Further, the luminescent emission wavelengths and colors of LB films can be tuned to some extent by the in-plane and interlayer molecular interaction. Finally, it is demonstrated that the luminescent emissions and colors of the Cd-TPPE CP LB films is selectively sensing to methanol and formic acid in the gas phase with good reversibility and reproductivity.

N-Protonation as a Switch of the Twisted Excited States with ππ* or nπ* Character and Correlation with the π-Electrons Characteristic of Rotatable Bonds.

N-protonation for numerous fluorophores is widely known as an efficient switch for the fluorescence turn-on/off in acidic conditions, which has been applied in various scenarios that involve pH monitoring. Yet the universal mechanism for fluorescence regulation through N-protonation is still elusive. Herein, the excited state deactivation processes are systematically investigated for a series of nitrogen-containing fluorescent probes through theoretical approaches. Two types of mechanisms for the complex fluorescent phenomena by N-protonation are concluded: one is through the regulation for the transition to a ππ* twisted intramolecular charge transfer (TICT) state; the other one applies for the case when nonradiative decay pathway is predominant by a dark nπ* state, which is also accompanied by an evident structural twisting and can be regarded as another kind of TICT state. More generally, the formation of the TICT state is closely related to the conjugated π-electrons on the single bond that links the acceptor and donor part of fluorophores, which provides a simple strategy for evaluating the occurrence of the TICT process. The current contributions can bring novel insights for the rational design of functional fluorophores that involve TICT process in the excited states.

Plasmon Effects in Non-Diagram X- Ray Spectra of Calcium (Ca) and Cobalt (Co)

In recent years the whole area of high energy excitations in solid has flourished well, both experimentally and theoretically. Their spectroscopy and theoretical understanding have important implications for the knowledge of the structure and properties of materials, both in bulk and near the surface. The theory used in the present work, relates to origin of Non-diagram X-ray spectra in various excitation processes where a deep as well as the surface electron is excited. The specific contributions of the intrinsic as well as the extrinsic ‘Plasmon-coupling’ processes have been obtained theoretically in terms of “Non-diagram-to-parent-diagram X-ray line yield ratios” for the high energy plasmon satellite profiles in the X-ray line spectra of Kα L(0) main lines in Calcium (Ca) and Cobalt (Co) by extending the Bohm-Pines Hamiltonian and using many body theory. It has been shown that the high-energy X-ray satellite line shapes of Calcium (Ca) and Cobalt (Co) observed by Mauron and Douss are due to the Plasmon Oscillations. The theoretically calculated results have been probed by comparing to the relative satellite yields obtained by Mauron and Dousse and are found to agree well with their experimentally observed values.