Time-resolved Emission Spectra Research Articles

The contribution of electrode spacing and operating voltage on the time-resolved optical emission spectra of atmospheric-pressure air and Ar DBD plasma

This paper reports the optical characterization of a controlled atmospheric-pressure dielectric barrier discharge (DBD) plasma source in air and Ar environments. In this study, Ar flow rate is held constant at 1 L/min while the operating voltages varied from 6.5 kV to 12.7 kV. Under these conditions, a calibrated commercial compact spectrometer covering the range from 200 to 850 nm was used to record the emission spectra of DBD plasma operating at different voltages, gap distances, and gas types. Using the Boltzmann distribution, we calculated the electron density ( n e ) and excitation temperatures in the plasma of air and Ar DBD at 2 mm gap distance . The emission spectra lines of the hydroxyl radicals (OH), N 2 , and Ar ions are acquired at different operating conditions. According to the findings, the light intensity of Ar DBD is significantly higher than that of air DBD under the same conditions of operation. The intensity of the generated plasma spectrum is also shown to rise with both an increase in the input voltage and a decrease in the gap distance. The measurements also showed that the average temperature of the excited electrons is around 1.1 eV for air DBD and 0.11 eV for Ar DBD, while n e for both is on the order of 10 16 cm − 3 . These results show that as the working voltage increases, the plasma changes from a filamentary state to a homogeneous mode. Knowing the DBD parameters increases its chance to compete with traditional methods in many fields, such as water, polymers, and biomedical treatments.

Time-resolved characteristics of a nanosecond pulsed multi-hollow needle plate packed bed dielectric barrier discharge

In this paper, self-designed multi-hollow needle electrodes are used as a high-voltage electrode in a packed bed dielectric barrier discharge reactor to facilitate fast gas flow through the active discharge area and achieve large-volume stable discharge. The dynamic characteristics of the plasma, the generated active species, and the energy transfer mechanisms in both positive discharge (PD) and negative discharge (ND) are investigated by using fast-exposure intensified charge coupled device (ICCD) images and time-resolved optical emission spectra. The experimental results show that the discharge intensity, number of discharge channels, and discharge volume are obviously enhanced when the multi-needle electrode is replaced by a multi-hollow needle electrode. During a single voltage pulse period, PD mainly develops in a streamer mode, which results in a stronger discharge current, luminous intensity, and E/N compared with the diffuse mode observed in ND. In PD, as the gap between dielectric beads changes from 0 to 250 μm, the discharge between the dielectric bead gap changes from a partial discharge to a standing filamentary micro-discharge, which allows the plasma to leave the local area and is conducive to the propagation of surface streamers. In ND, the discharge only appears as a diffusion-like mode between the gap of dielectric beads, regardless of whether there is a discharge gap. Moreover, the generation of excited states and is mainly observed in PD, which is attributed to the higher E/N in PD than that in ND. However, the generation of the radical in ND is higher than in PD. It is not directly dominated by E/N, but mainly by the resonant energy transfer process between metastable and . Furthermore, both PD and ND demonstrate obvious energy relaxation processes of electron-to-vibration and vibration-to-vibration, and no vibration-to-rotation energy relaxation process is observed.

Environmentally Driven Symmetry Breaking Quenches Dual Fluorescence in Proflavine.

Nonadiabatic couplings between several electronic excited states are ubiquitous in many organic chromophores and can significantly influence optical properties. A recent experimental study demonstrated that the proflavine molecule exhibits surprising dual fluorescence in the gas phase, which is suppressed in polar solvent environments. Here, we uncover the origin of this phenomenon by parametrizing a linear-vibronic coupling Hamiltonian from spectral densities of system-bath coupling constructed along molecular dynamics trajectories, fully accounting for interactions with the condensed-phase environment. The finite-temperature absorption, steady-state emission, and time-resolved emission spectra are then computed using powerful, numerically exact tensor network approaches. We find that the dual fluorescence in vacuum is driven by a single well-defined coupling mode but is quenched in solution due to dynamic solvent-driven symmetry breaking that mixes the two low-lying electronic states. We expect the computational framework developed here to be widely applicable to the study of non-Condon effects in complex condensed-phase environments.

Evidence of Short-Lived High-Energy Emissive State and Triplet Character of the Self-Trapped Exciton in Cs3Cu2I5 Perovskite.

Cs3Cu2I5 perovskite displays a Stokes-shifted photoluminescence (PL) at 445 nm, attributed to the self-trapped excitons (STEs). Unlike that observed in other perovskite materials, the free-exciton emission is not evidenced in this case. Herein, we reveal the existence of a short-lived high-energy emission centered around 375 nm through the reconstruction of time-resolved emission spectra (TRES), which is independent of the shape/size of Cs3Cu2I5 perovskite. This high-energy emission is proposed to originate from the free-exciton-derived distorted S1 state of the 0D Cs3Cu2I5 moiety. Moreover, STE PL (∼445 nm) was found to have phosphorescence characteristics. Theoretical calculation confirms a facile intersystem crossing at the Franck-Condon geometry, indicating the high lifetime of the STE and its triplet nature. The existence of a high-energy emissive state and the phosphorescent nature of the STE PL band provide valuable insights that could advance our understanding of the photophysics in these materials.

Triplet Excited-State Dynamics in Benzothiadiazole-Based Thermally Activated Delayed Fluorescence Compound.

The "hot exciton" thermally activated delayed fluorescence (TADF) materials have attracted considerable research interest for their utilization of high-lying triplet excitons. In this work, we reported the mechanism of photoluminescence by revealing the spectral evolution from singlet to triplet states in "hot exciton" TADF molecules by transient absorption (TA) spectra and triplet sensitization experiments. The internal conversion and intersystem crossing are much faster than reverse intersystem crossing (RISC), so that high-lying triplet states (Tn) are difficult to accumulate to be observed in the transient absorption spectra. In contrast, the emergence of delayed fluorescence in time-resolved emission spectra demonstrates the existence of a high-lying RISC process (hRISC) from Tn to S1. Triplet sensitization experiments successfully identified the spectral features of the T1 state in the TA spectra. This work sheds light on critical factors for the systematic design of these materials to achieve a high emission quantum yield.

Properties and characteristics of the nanosecond discharge developing at the water–air interface: tracking evolution from a diffused streamer to a spark filament

The characteristics of nanosecond discharge propagating along the water–air interface in a unique dielectric-barrier discharge (DBD)-like configuration with coplanar electrodes submerged in deionised (DI)/tap water are studied by combining ultrafast imaging and emission spectra with electrical characteristics. Time-resolved images provide a clear signature of streamer channels excited on the water surface at either side of the blade (insulated plastic separating the anode/cathode) and propagating perpendicularly away from it towards the anode/cathode with different velocities. Later on, the streamer channels convert into a few discrete and bright discharge channels due to ionisation instability (spark phase). There is no distinctive dependence on water conductivity in the streamer phase, as the optical emission spectroscopy and images of discharges only showed an increase of the luminosity and no significant changes in morphology. However, in the spark phase, more numerous, brighter, and thicker filaments form in tap water. The time-resolved emission spectra reveal the dominance of the first and second positive system of N2 molecular bands in the streamer phase, followed by the appearance of atomic lines of hydrogen, nitrogen, and oxygen in the spark phase. The emission spectra are utilised to estimate several important parameters (gas temperature, reduced electric field ( E/N ), and electron density ( ne )). The streamer phase is characterised by a low gas temperature and a peak E/N amplitude between 700 and 850 Td. On the other hand, the subsequent spark phase is characterised by a gas temperature of ∼400/1100 K and a free electron density up to ne∼1017 –1018 cm−3 in DI/tap water.

Solute dynamics of a hydrophobic molecule in a menthol-thymol based type-V deep eutectic solvent: effect of composition of the components.

Type-V deep eutectic solvents (DESs) are a newly emerging unique class of solvents obtained by physical mixing and heating of non-ionic components. These solvents show deviation from the thermodynamic ideality. Compared to type-I to IV DESs, type-V DESs are less explored and their physical chemistry is in its nascent stage. In this work, we have chosen a type-V DES based on menthol-thymol (MT) for our working media. Solvent and rotational dynamics were studied with varying temperature using a well-known solvatochromic probe, Coumarin 153 (C153). We prepared the MT-based DES using a reported procedure at three molar ratios: 1 : 1 (M1T1), 1 : 1.5 (M1T1.5), and 2 : 1 (M2T1) of menthol (M) and thymol (T). Time-resolved emission spectra (TRES) were constructed with varying temperature. Utilizing TRES, the decay of the solvent correlation function (C(t)) was plotted. We have correlated the solvent relaxation time in these DESs as a function of viscosity. The time-resolved anisotropy decays were also collected to perceive the rotational relaxation dynamics of C153 as a function of temperature. The decay of solvent relaxation was found to be bi-exponential, and the average solvation time (〈τs〉) in M2T1 was found to be longer than those of M1T1.5 and M1T1. The rotational reorientation times (〈τrot〉) also follow the same trend. We have analysed the rotational dynamics of C153 in type-V DESs employing the Stokes-Einstein-Debye (SED) hydrodynamic model. The rotational dynamics in DESs demonstrate a good correlation with the SED model with a little deviation. In MT-based DESs, the solute's rotational relaxation times approach hydrodynamic stick boundary condition at low viscosity (or at high temperatures) for all molar compositions. Using the Arrhenius-type equations, we have correlated the activation energies for the rotational motion of C153, along with the viscous flow and non-radiative pathways for all the DESs.

Особливості розпаду щільної плазми високовольтних імпульсних розрядів у воді, ініційованих електричним вибухом залізного провідника

The article presents the results of investigations of the decay coefficients of dense plasma in impulse discharges in water initiated by an electric explosion of an iron conductor. An analysis of time-resolved and spatial emission spectra of the discharges was performed, along with the study of absorption spectra dynamics with a 5 μs interval. The electron concentration of the dense plasma was determined at different time points, allowing the determination of its decay coefficients at electron concentrations ranging from 1020 to 1021 cm-3. It was found that under equal electron concentrations in the plasma, the decay coefficients of plasma with iron impurities are consistent with the decay coefficients of plasma with tungsten impurities.

Impact of Surface Trap States on Electron and Energy Transfer in CdSe Quantum Dots Studied by Femtosecond Transient Absorption Spectroscopy.

The presence of surface trap states (STSs) is one of the key factors to affect the electronic and optical properties of quantum dots (QDs), however, the exact mechanism of how STSs influence QDs remains unclear. Herein, we demonstrated the impact of STSs on electron transfer in CdSe QDs and triplet-triplet energy transfer (TTET) from CdSe to surface acceptor using femtosecond transient absorption spectroscopy. Three types of colloidal CdSe QDs, each containing various degrees of STSs as evidenced by photoluminescence and X-ray photoelectron spectroscopy, were employed. Time-resolved emission and transient absorption spectra revealed that STSs can suppress band-edge emission effectively, resulting in a remarkable decrease in the lifetime of photoelectrons in QDs from 17.1 ns to 4.9 ns. Moreover, the investigation of TTET process revealed that STSs can suppress the generation of triplet exciton and effectively inhibit band-edge emission, leading to a significant decrease in TTET from CdSe QDs to the surface acceptor. This work presented evidence for STSs influence in shaping the optoelectronic properties of QDs, making it a valuable point of reference for understanding and manipulating STSs in diverse QDs-based optoelectronic applications involving electron and energy transfer.

Temporal evolution of laser-induced ionization and recombination processes in argon-helium mixture

Preparation of metastable atoms (1s5) through laser-induced preionization holds the potential to mitigate the electromagnetic interference (EMI) issues associated with the large volume, atmospheric pressure discharge of traditional optically pumped rare-gas metastable laser (OPRGL). In this work, we conducted experimental investigations into the temporal evolution of the Ar 763.5 nm (2p6→1s5) spectral line in Ar-He mixture. These experiments unveiled the intricate interaction mechanism involving the laser, Ar atoms, He atoms, and free electrons within the laser-induced plasma. Our findings highlight the dual contributions of the multiphoton ionization and the inverse bremsstrahlung process to the initial plasma formation. Notably, the time-resolved atomic emission spectrum at 763.5 nm reveals two distinct regimes, namely Regime1 and Regime2. Regime1 primarily arises from the “excitation + radiation + collisional relaxation” process, wherein excited states Ar atoms, populated via multiphoton excitation and electron impact excitation, accumulate on the 2p6 level. Conversely, Regime2 is predominantly a result of the “ion-electron recombination” process. In this regime, highly excited states Ar atoms are generated through the recombination of ion and electron, subsequently populating the 2p6 level through a combination of radiation and collisional relaxation channels. The differences in the temporal evolution between 763.5 nm and 811.5 nm spectral lines can be attributed to the distinct radiation and collisional relaxation channels in the two aforementioned processes.

Tuning the Emission of Homometallic DyIII, TbIII, and EuIII 1-D Coordination Polymers with 2,6-Di(1H-1,2,4-triazole-1-yl-methyl)-4-R-phenoxo Ligands: Sensitization through the Singlet State.

This work reports the structural characterization and photophysical properties of DyIII, TbIII, and EuIII coordination polymers with two phenoxo-triazole-based ligands [2,6-di(1H-1,2,4-triazole-1-yl-methyl)-4-R-phenoxo, LRTr (R = CH3; Cl)]. These ligands permitted us to obtain isostructural polymers, described as a 1D double chain, with LnIII being nona-coordinated. The energies of the ligand triplet (T1) states were estimated using low-temperature time-resolved emission spectra of YIII analogues. Compounds with LClTr present higher emission intensity than those with LMeTr. The emission of TbIII compounds was not affected by the different excitation wavelengths used and was emitted in the pure green region. In contrast, DyLMeTr emits in the blue-to-white region, while the luminescence of DyLClTr remains in the white region for all excitation wavelengths. On the other hand, EuIII compounds emit in the blue (ligand) or red region (EuIII) depending on the substituent of the phenoxo moiety and excitation wavelength. Theoretical calculations were employed to determine the excited states of the ligands by using time-dependent density functional theory. These calculations aided in modeling the intramolecular energy transfer and rationalizing the optical properties and demonstrated that the sensitization of the LnIII ions is driven via S1 → LnIII, a process that is less common as compared to T1 → LnIII.

Excited-State Dynamics Simulations of a Light-Driven Molecular Motor in Solution.

Molecular motors, where light can be transformed into motion, are promising in the design of nanomechanical devices. For applications, however, finding relationships between molecular motion and the environment is important. Here, we report the study of excited-state dynamics of an overcrowded alkene in solution using a hybrid quantum mechanics/molecular mechanics (QM/MM) approach combined with excited-state molecular dynamics simulations. Using QM/MM surface-hopping trajectories, we calculated time-resolved emission and transient absorption spectra. These show the rise of a short-lived Franck-Condon state, followed by the formation of a dark state in the first 150 fs before the molecular motor relaxes to the ground state in about 1 ps. From the analysis of radial distribution functions, we infer that the orientation of the solvent with respect to the molecular motor in the electronic excited state is similar to that in the ground state during the photoisomerization.

Fast and inexpensive emission-time in-situ setup for upconversion nanoparticle characterization

In this work, a straightforward analytical approach is presented for the evaluation of the entire time-resolved emission spectrum of Er3+-doped upconversion nanoparticles (UCNP) by the acquisition of bidimensional emission wavelength-time signals. Employing diffractive optics and fast optomechanical sweeping, the entire emission wavelength-time matrix (EtM) of UCNP is registered. An optic fibre probe allows the measurement of micrometric-size domains in surfaces, either in solid form or in suspension. Further chemometric analysis was also performed to gain further insight into the behaviour of the emission bands and their corresponding time curve profiles. To show the applicability of the developed procedure, changes either in the emission spectrum or the time behaviour by external perturbations were monitored and EtM at different temperatures was acquired as a proof of concept. The results demonstrated the effectiveness of the proposed procedure in obtaining fast and highly informative images and enlightening knowledge about the phosphorescence-based system.

Study on luminescent performance of Dy3+-, Tb3+-doped and Tb3+/Dy3+ co-doped TeO2-(Gd,Lu)2O3-WO3 glass scintillators

Three series of Dy3+-, Tb3+-doped and Tb3+/Dy3+ co-doped 40TeO2-20(Gd,Lu)2O3–40WO3 glasses with density of about 6.75 g/cm3 were successfully prepared by high temperature melt-quenching method. The luminescence properties of these rare earth ions doped 40TeO2-20(Gd,Lu)2O3–40WO3 glass were systematically studied by transmittance, photoluminescence (excitation and emission spectra), time-resolved emission spectra (TRES) and X-ray excited luminescence (XEL) spectra, together with the luminescence decay curves. The optimal doping concentration of Dy3+ and Tb3+ ions in 40TeO2-20(Gd,Lu)2O3–40WO3 glasses was determined under UV and X-ray excitation, respectively. It is found there is an efficient energy transfer from Dy3+ to Tb3+ ions in 40TeO2-20(Gd,Lu)2O3–40WO3 glass in photoluminescence spectra. However, it is not true under X-ray excitation due to the complicated physical process.

Non-Phenomenological Description of the Time-Resolved Emission in Solution with Quantum-Classical Vibronic Approaches-Application to Coumarin C153 in Methanol.

We report a joint experimental and theoretical work on the steady-state spectroscopy and time-resolved emission of the coumarin C153 dye in methanol. The lowest energy excited state of this molecule is characterized by an intramolecular charge transfer thus leading to remarkable shifts of the time-resolved emission spectra, dictated by the methanol reorganization dynamics. We selected this system as a prototypical test case for the first application of a novel computational protocol aimed at the prediction of transient emission spectral shapes, including both vibronic and solvent effects, without applying any phenomenological broadening. It combines a recently developed quantum-classical approach, the adiabatic molecular dynamics generalized vertical Hessian method (Ad-MD|gVH), with nonequilibrium molecular dynamics simulations. For the steady-state spectra we show that the Ad-MD|gVH approach is able to reproduce quite accurately the spectral shapes and the Stokes shift, while a ∼0.15 eV error is found on the prediction of the solvent shift going from gas phase to methanol. The spectral shape of the time-resolved emission signals is, overall, well reproduced, although the simulated spectra are slightly too broad and asymmetric at low energies with respect to experiments. As far as the spectral shift is concerned, the calculated spectra from 4 ps to 100 ps are in excellent agreement with experiments, correctly predicting the end of the solvent reorganization after about 20 ps. On the other hand, before 4 ps solvent dynamics is predicted to be too fast in the simulations and, in the sub-ps timescale, the uncertainty due to the experimental time resolution (300 fs) makes the comparison less straightforward. Finally, analysis of the reorganization of the first solvation shell surrounding the excited solute, based on atomic radial distribution functions and orientational correlations, indicates a fast solvent response (≈100 fs) characterized by the strengthening of the carbonyl-methanol hydrogen bond interactions, followed by the solvent reorientation, occurring on the ps timescale, to maximize local dipolar interactions.

Optical and electrical investigation of plasma generated by high-energy self-stabilized spark ignition system

Spark discharge plasma is commonly used for ignition in internal combustion engines. The environmental performance of internal combustion engines with forced ignition is improved when operating under lean mixture conditions. High-energy ignition systems are needed to ensure reliable ignition of lean mixtures. The ignition of a combustible mixture is influenced by several plasma parameters, such as the temperature of its various components, the size of the plasma, and the deposited energy. It is, therefore, beneficial to know these parameters. Here, we present optical and electrical investigation of plasma generated in ambient air by a novel high-energy self-stabilized spark ignition system. The electrical investigation showed two high current pulses, with the current amplitude of ∼40 and ∼150 A. The energy is deposited to the spark gap mainly during the second current pulse, and it is increasing from 213 to 541 mJ with the increasing gap size from 3 to 13 mm. The energy efficiency increases with the gap as well, from around 23% to 58%. Time-resolved emission spectra enabled us to estimate the evolution of the gas temperature, electron excitation temperature, and electron density in the generated plasma. It was found that the highest electron density, 3–4 × 1017 cm−3, correlates with the maximum of the second pulse current. We observed a specific plasma evolution between the two current pulses, with an increase in temperature from 4500 to 7500 K and a contraction of the plasma channel diameter from 3.3 to 0.5 mm.

Luminescence properties of Pr3+ doped Bi4Ge3O12 crystal fibers grown by the micro-pulling down technique

The luminescence properties of 0.5%Pr-doped BGO crystals grown by using the micro-pulling down technique were carefully investigated. Room temperature absorption spectra were registered and analyzed within the framework of the Judd-Ofelt formalism and the best fit to the data is obtained by using a modified treatment allowing for the admixture of intensity enabling wave-functions. The results indicate 3P0 and 1D2 radiative lifetimes of about 5.7 μs and 79 μs, in perfect agreement with the time-constants derived from the measured fluorescence decays at room and liquid-nitrogen temperatures. Registration of time-resolved emission and excitation spectra and calibration of the 3P0 emission spectrum in cross section unit clearly indicate that Pr-doped BGO could be the first isotropic crystalline material leading to efficient diode-pumped laser operation at room temperature with laser performances, at least in the deep-red spectral region, comparable to that found with the most important anisotropic Pr-doped fluoride and oxide crystals.

Influence of the pulse polarity on micro-hollow cathode helium plasma jet

Helium plasma jets generated by micro-hollow cathode discharge (MHCD) with the square-wave power source of different polarities are investigated in this work. The effects of positive and negative polarity pulses on the MHCD and plasma jet were compared, and the time-resolved optical emission spectra of excited species ( and O) were studied. The results confirm that the electric field is a key factor for the propagation of the jet during the rising edge of the positive current pulse, while the gas expansion is mainly important for the jet propagation during the current stable phase. The time-resolved spectra show that the generation of specie O in the jet-driven by the electric field is more efficient.

Ultrafast multiexciton dynamics in molecular systems: Inclusion of exciton-exciton annihilation

We report simulations of exciton-exciton annihilation (EEA) in dye molecular aggregates with explicit laser pulse excitation in the framework of reduced density matrix theory. We use a supramolecular complex model Hamiltonian, in which each monomer of an aggregate is described by three electronic levels. An expectation value approximation is used to avoid the large amount of matrix elements for various exciton configuration distributions in molecular aggregates. Quantum fluctuations truncation is applied to derive the closed equations of motion for single- and two-site functions. Using these equations, we simulate the laser pulse driven EEA dynamics, and compare it with rate equations to highlight the importance of quantum coherence effects. We also discuss the effects of the aggregate configuration, the molecular transition dipoles and the duration of laser pulses on EEA. Furthermore, we analyze the relation between the coherence time of fusion and the annihilation process. Our study indicates that EEA process is more efficient for coherent excitation at the lowest energy level in J aggregates. Our simulation of the normalized population of the first excited state for various field amplitudes can qualitatively reproduce time-resolved experimental emission spectra. This confirms that the fundamental description of laser pulse induced EEA is correct within our model.

Influence of Thermal Maturity on the Time-Resolved Emission Spectrum (TRES) Fluorescence Lifetime Characteristics of Crude Oils: A Preliminary Study Based on a Thermal Simulation Experiment of Crude Oils

The time-resolved emission spectrum (TRES) fluorescence lifetime characteristics of crude oil have great application potential to classify the oil type and reveal its origin in a multisource petroleum system. However, the influence of thermal maturity on the TRES fluorescence lifetime characteristics of crude oil is still unclear. This study collected typical crude oil samples from deep reservoirs in the Tabei area of the Tarim Basin and obtained a group of pyrolyzed oils with different maturities by a thermal simulation experiment. The TRES average fluorescence lifetime value (τA) and profile pattern (λ–τ) of both pyrolyzed and crude oil samples were investigated. The results indicate that, with increasing maturity, the τA values of pyrolyzed oil slightly increase when EasyRo < 0.90% but decrease when EasyRo > 0.90%. Meanwhile, the λ–τ pattern shows no obvious changes when EasyRo < 1.53% but changes significantly when EasyRo > 1.53%. Based on the TRES fluorescence lifetime characteristics, the deep crude oils in various blocks of the Tabei area are divided into TB-I and TB-II types, which correspond well with their physical properties, diagnostic biomarkers, and carbon isotopic values. The TB-I crude oils are widely distributed in the Shunbei, Hadexun, Repu, and Halahatang blocks. This type of crude oil has a short τA value and exhibits an obvious platform pattern in its λ–τ profile. The TB-II crude oils are distributed only in the Yuke block. This type of crude oil has a long τA value and shows a unimodal pattern in its λ–τ profile. The TRES fluorescence lifetime parameters can effectively classify crude oils with different sources and can further identify the thermal maturity of crude oils with the same sources by combining them with the physical and geochemical parameters of crude oils, which exhibits great application potential to crude oils and individual petroleum inclusions in deep multisource petroleum systems.