Ultrafast carrier dynamics in GeSn thin film based on time-resolved terahertz spectroscopy

We measure the time-resolved terahertz spectroscopy of GeSn thin film and studied the ultrafast dynamics of its photo-generated carriers.The experimental results show that there are photo-generated carriers in GeSn under femtosecond laser excitation at 2500 nm,and its pump-induced photoconductivity can be explained by the Drude–Smith model.The carrier recombination process is mainly dominated by defect-assisted Auger processes and defect capture.The firstand second-order recombination rates are obtained by the rate equation fitting,which are(2.6±1.1)×10^(-2)ps^(-1)and(6.6±1.8)×10^(-19)cm^(3)·ps^(-1),respectively.Meanwhile,we also obtain the diffusion length of photo-generated carriers in GeSn,which is about 0.4μm,and it changes with the pump delay time.These results are important for the GeSn-based infrared optoelectronic devices,and demonstrate that Ge Sn materials can be applied to high-speed optoelectronic detectors and other applications.

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 multihollow 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 to250μm,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 diffusionlike mode between the gap of dielectric beads,regardless of whether there is a discharge gap.Moreover,the generation of excited states N_(2)^(+)(B^(2)∑_(u)^(+))and N2(C^(3)Π_(u))is mainly observed in PD,which is attributed to the higher E/N in PD than that in ND.However,the generation of the OH(A^(2)∑^(+))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 N_(2)(A^(3)∑_(u)^(+))and OH(X^(2)Π).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.

Time-resolved K-shell x-ray spectra of nanosecond laser-produced titanium tracer in gold plasmas

A study of a nanosecond laser irradiation on the titanium-layer-buried gold planar target is presented. The timeresolved x-ray emission spectra of titanium tracer are measured by a streaked crystal spectrometer. By comparing the simulated spectra obtained by using the FLYCHK code with the measured titanium spectra, the temporal plasma states, i.e.,the electron temperatures and densities, are deduced. To evaluate the feasibility of using the method for the characterization of Au plasma states, the deduced plasma states from the measured titanium spectra are compared with the Multi-1D hydrodynamic simulations of laser-produced Au plasmas. By comparing the measured and simulated results, an overall agreement for the electron temperatures is found, whereas there are deviations in the electron densities. The experiment–theory discrepancy may suggest that the plasma state could not be well reproduced by the Multi-1D hydrodynamic simulation, in which the radial gradient is not taken into account. Further investigations on the spectral characterization and hydrodynamic simulations of the plasma states are needed. All the measured and FLYCHK simulated spectra are given in this paper as datasets. The datasets are openly available at http://www.doi.org/10.57760/sciencedb.j00113.00032.

Time-resolved photoluminescence of anatase/rutile TiO_2 phase junction revealing charge separation dynamics

Junctions are an important structure that allows charge separation in solar cells and photocatalysts. Here, we studied the charge transfer at an anatase/rutile TiO2 phase junction using time-resolved photoluminescence spectroscopy. Visible （-S00 nm） and near-infrared （NIR, -830 nm） emissions were monitored to give insight into the photoinduced charges of anatase and rutile in the junction, respectively, New fast photoluminescence decay components appeared in the visible emission of futile-phase dominated TiO2 and in the NIR emission of many mixed phase TiO2samples. The fast decays confirmed that the charge separation occurred at the phase junction. The visible emission intensity from the mixed phase TiO2 increased, revealing that charge transfer from rutile to anatase was the main pathway. The charge separation slowed the microsecond time scale photolumines- cence decay rate for charge carriers in both anatase and rutile. However, the millisecond decay of the charge carriers in anatase TiO2 was accelerated, while there was almost no change in the charge carrier dynamics of rutile TiO2. Thus, charge separation at the anatase/rutile phase junction caused an increase in the charge carrier concentration on a microsecond time scale, because of slower electron-hole recombination. The enhanced photocatalytic activity previously observed at ana- tase/rutile phase junctions is likely caused by the improved charge carrier dynamics we report here. These findings may contribute to the development of improved photocatalytic materials.

Ultrafast Dynamics Through Conical Intersections in 2,6-dimethylpyridine Studied with Time-resolved Photoelectron Imaging

The ultrafast dynamics through conical intersections in 2,6-dimethylpyridine has been studied by femtosecond time-resolved photoelectron imaging coupled with time-resolved mass spectroscopy. Upon absorption of 266 nm pump laser, 2,6-dimethylpyridine is excited to the S2 state with a ππ character from So state. The time evolution of the parent ion signals consists of two exponential decays. One is a fast component on a timescale of 635 fs and the other is a slow component with a timescale of 4.37 ps. Time-dependent photo- electron angular distributions and energy-resolved photoelectron spectroscopy are extracted from time-resolved photoelectron imaging and provide the evolutive information of S2 state. In brief, the ultrafast component is a population transfer from S2 to S1 through the S2/S1 conical intersections, the slow component is attributed to simultaneous IC from the S2 state and the higher vibrational levels of S1 state to So state, which involves the coupling of S2/S0 and S1/So conical intersections. Additionally, the observed ultrafast S2--＋S1 transition occurs only with an 18% branching ratio.

Distance-Dependent Long-Range Electron Transfer in Protein:a Case Study of Photosynthetic Bacterial Light-Harvesting Antenna Complex LH2 Assembled on TiO 2 Nanoparticle by Femto-Second Time-Resolved Spectroscopy

The function of protein in long-range biological electron transfer is a question of debate. We report some preliminary results in femtosecond spectroscopic study of photosynthetic bacterial light-harvesting antenna complex assembled onto TiO2 nanoparticle with an average size of 8 nm in diameter. Crystal structure shows that photosynthetic bacterial antenna complex LH2 has a ring-like structure composed by alpha- and beta-apoprotein helices. The alpha- and beta-transmembrance helices construct two concentric cylinders with pigments bacteriochlorophyll a (Bchl a) and carotenoid (Car) buried inside the protein. We attempt to insert TiO2 nanoparticle into the cavity of the inner cylindrical hollow of LH2 to investigate the nature of the electron transfer between the excited-state Bchl a and the TiO2 nanoparticle. A significant decrease in the ground state bleaching recovery time constant for Bchl a at 850 run (B850) in respect to that of the Bchl a in free LH2 has been observed. By using the relation of distance-dependent long-range electron transfer rate in protein, the distance between the donor B850 and the acceptor TiO2 nanoparticle has been estimated, which is about 0.6 nm. The proposed method of assembling proteins onto wide-gap semiconductor nanoparticle can be a promising way to determine the role of the protein playing in biological electron transfer processes.

A New kHz Velocity Map Ion/Electron Imaging Spectrometer for Femtosecond Time-Resolved M olecular Reaction Dynamics Studies

A new velocity map imaging spectrometer is constructed for molecular reaction dynamics studies using time-resolved photoelectron/ion spectroscopy method. By combining a kHz pulsed valve and an ICCD camera, this velocity map imaging spectrometer can be run at a repetition rate of 1 kHz, totally compatible with the fs Ti：Sapphire laser system, facilitating time-resolved studies in gas phase which are usually time-consuming. Time-resolved velocity map imaging study of NH3 photodissociation at 200 nm was performed and the time-resolved total kinetic energy release spectrum of H＋NH~ products provides rich information about the dissociation dynamics of NH3. These results show that this new apparatus is a powerful tool for investigating the molecular reaction dynamics using time-resolved methods.

Photochemical Reaction of Benzoin Caged Compound： Time-Resolved Fourier Transform Infrared Spectroscopy Study

The benzoin group caged compound has received strong interests due to its excellent photo- deprotection properties and wide use in chemical and biological studies. We used timeresolved infrared spectroscopy to investigate the photochemical reaction of the benzoin caged compound, o-（2-methylbenzoyl）-DL-benzoin under 266 nm laser irradiation. Taking advantage of the specific vibrational marker bands and the IR discerning capability, we have detected and identified the uncaging product 2-methylbenzoic acid, and two intermediate radicals of benzoyl and 2-methylbenzoate benzyl in the transient infrared spectra. Our results provide spectral evidence to support the homolytic cleavage reaction of C-C=O bond in competition with the deprotection reaction. Moreover, the product yields of 2-methylbenzoic acid and benzoyl radical were observed to be affected by solvents and a largely water contalning solvent can be in favor of the deprotection reaction.

Simultaneous determination of captopril and hydrochlorothiazide by time-resolved chemiluminescence with artificial neural network calibration

The combined use of chemometrics and chemiluminescence（CL）measurements,with the aid of the stopped-flow mixing technique,developed a simple time-resolved CL method for the simultaneous determination of captopril（CPL）and hydrochlorothiazide（HCT）.The stopped-flow technique in a continuous-flow system was employed in this work in order to emphasize the kinetic differences between the two analytes in cerium（IV）-rhodamine 6G CL system.After the flow was stopped,an initial rise of CL signal was observed for HCT standards,while a direct decay of CL signal was obtained for CPL standards.The mixed CL signal was monitored and recorded on the whole process of continuous-flow/stopped-flow,and the obtained data were processed by the chemometric approach of artificial neural network.The relative prediction error（RPE）of CPL and HCT was 5.9% and 8.7%,respectively.The recoveries of CPL and HCT in tablets were found to fall in the range between 95% and 106%.The proposed method was successfully applied to the simultaneous determination of CPL and HCT in a compound pharmaceutical formulation.

Time-resolved fluoroimmunoassay of zearalenone in cereals with a europium chelate as label

A competitive indirect time-resolved fluoroimmunoassay(TRFIA) was developed for detection of zearalenone(ZEN) in cereals,in which ZEN conjugated to bovine serum albumin(BSA) is used as solid-phase antigen.A competitive indirect TRFIA was conducted by simultaneously incubating ZEN in standard or extracted samples with anti-ZEN monoclonal antibody over ZEN-BSA coated plates,and then determining the bound ZEN monoclonal antibody with goat anti-mouse europium conjugate.Samples were extracted with methanol/water...

Investigation on Exciton Relaxation Kinetics of ZnCulnS/ZnSe/ZnS Quantum Dots by Time-Resolved Spectroscopy Techniques

The exciton relaxation kinetics of ZnCuInS/ZnSe/ZnS quantum dots （QDs） is investigated by time-resolved spectroscopy techniques in detail. Based on the rate distribution model, the wavelength-dependent emission dynamics shows that the intrinsic exciton, the exciton in the interface defect state and that in donor-acceptor pair state （DAPS） together participate in the photoluminescence process of QDs, and the whole emission process is mainly dependent on the DAPS emission. Transient absorption data show that the intrinsic exciton and the interface defect species maybe together appear after excitation and the intensity-dependent Auger recombination process also exists in QDs at high excitation intensity.

Analysis of Turbulence Structure in the Stirred Tank with a Deep Hollow Blade Disc Turbine by Time-resolved PIV

The turbulence structure in the stirred tank with a deep hollow blade(semi-ellispe) disc turbine(HEDT) was investigated by using time-resolved particle image velocimetry(TRPIV) and traditional PIV.In the stirred tank,the turbulence generated by blade passage includes the periodic components and the random turbulent ones.Traditional PIV with angle-resolved measurement and TRPIV with wavelet analysis were both used to obtain the random turbulent kinetic energy as a comparison.The wavelet analysis method was successfully used in this work to separate the random turbulent kinetic energy.The distributions of the periodic kinetic energy and the random turbulent kinetic energy were obtained.In the impeller region,the averaged random turbulent kinetic energy was about 2.6 times of the averaged periodic one.The kinetic energies at different wavelet scales from a6 to d1 were also calculated and compared.TRPIV was used to record the sequence of instantaneous velocity in the impeller stream.The evolution of the impeller stream was observed clearly and the sequence of the vorticity field was also obtained for the identification of vortices.The slope of the energy spectrum was approximately-5/3 in high frequency representing the existence of inertial subrange and some isotropic properties in stirred tank.From the power spectral density(PSD) ,one peak existed evidently,which was located at f0(blade passage frequency) generated by the blade passage.

Time-resolved shadowgraphs and morphology analyses of aluminum ablation with multiple femtosecond laser pulses

Aluminum ablation by multiple femtosecond laser pulses is investigated via time-resolved shadowgraphs and scanning electron microscope （SEM） images of the ablation spot. The spatial distribution of the ejected material and the radius of the shock wave generated during the ablation are found to vary with the increase in the number of pulses. In the initial two pulses, nearly concentric and semicircular stripes within the shock wave front are observed, unlike in subsequent pulses. Ablation by multiple femtosecond pulses exhibits different characteristics compared with the case induced by single femtosecond pulse because of the changes to the aluminum target surface induced by the preceding pulses.

Study on local topology model of low/high streak structures in wall-bounded turbulence by tomographic time-resolved particle image velocimetry

The relationship between the in the logarithmic law （log-law） region of bursting event and the low/high-speed streak a turbulent boundary layer is investigated. A tomographic time-resolved particle image velocimetry （TRPIV） system is used to measure the instantaneous three-dimensional-three-component （3D-3C） velocity field. The momentum thickness based Reynolds number is about 2 460. The topological information in the log-law region is obtained experimentally. It is found that the existence of the quadrupole topological structure implies a three-pair hairpin-like vortex packet, which is in connection with the low/high-speed streak. An idealized 3D topological model is then proposed to characterize the observed hairpin vortex packet and low/high-speed streak.

Simultaneous measurements of global vibrational spectra and dephasing times of molecular vibrational modes by broadband time-resolved coherent anti-Stokes Raman scattering spectrography

In broadband coherent anti-Stokes Raman scattering （CARS） spectroscopy with supercontinuum （SC）, the simultaneously detectable spectral coverage is limited by the spectral continuity and the simultaneity of various spectral components of SC in an enough bandwidth. By numerical simulations, the optimal experimental conditions for improving the SC are obtained. The broadband time-resolved CARS spectrography based on the SC with required temporal and spectral distributions is realised. The global molecular vibrational spectrum with well suppressed nonresonant background noise can be obtained in a single measurement. At the same time, the measurements of dephasing times of various molecular vibrational modes can be conveniently achieved from intensities of a sequence of time-resolved CARS signals. It will be more helpful to provide a complete picture of molecular vibrations, and to exhibit a potential to understand not only both the solvent dynamics and the solute-solvent interactions, but also the mechanisms of chemical reactions in the fields of biology, chemistry and material science.

Eu-Chelate Construct Ultrasensitive Time-Resolved Fluoroimmunoassay-Assay of Pepsinogen I

Eu-chelate were used to construct a two-site sandwich-type assay for pepsinogen Ⅰ （PGI） with time-resolved fluoroimmunoassay （TRFIA） as a detection technique. On the noncompetitive assay, captured monoclonal antibodies （McAbs） coated on wells were directed against a specific antigenic site on the PGI. Another McAbs, called as labeling McAbs, were prepared with the Eu-chelate of N-（p-isothiocyanatobenzyl）-diethylenetriamine-N, N, N, N-tetraacetic acid and directed against a different antigenic site on the PGI. The fluorescence counts of bound Eu^3＋ -McAbs were measured with the auto DELFIA1235 system. The PGI in sera from healthy volunteers were determined by PGI-TRFIA. The within-run and between-run CVs of the PGI-TRFIA were 1.9% and 4.7%, respectively, and the recovery rate was 102.65%. The assay had a detection limit of 0.05 μg· L^-1. The PGI-TRFIA provided a linear response from 3.5 to 328 μg· L^-1. The cross-reacting rate with pepsinogen Ⅱ was negligible. The linear correlation of PGI-TRFIA and radioimmunassay measurements resulted in a correlation coefficient of 0.977. The means of healthy volunteers were 154 ±43 μg·L^-1 for serum PGI. The availability of a highly sensitive, reliable, and convenient method for quantifying PGI will allow investigations into the possible diagnostic value of this analyte in various clinical conditions, including gastric carcinoma, duodenal ulcer, gastritis and severe atrophic gastritis.

A Highly Sensitive Femtosecond Time-Resolved Sum Frequency Generation Vibrational Spectroscopy System with Simultaneous Measurement of Multiple Polarization Combinations

Characterization of real-time and ultrafast motions of the complex molecules at surface and interface is critical to understand how interracial molecules function. It requires to develop surface-sensitive, fast-identification, and time-resolved techniques. In this study, we employ several key technical procedures and successfully develop a highly sensitive femtosecond time-resolved sum frequency generation vibrational spectroscopy （SFG-VS） system. This system is able to measure the spectra with two polarization combinations （ssp and ppp, or psp and ssp） simultaneously. It takes less than several seconds to collect one spectrum. To the best of our knowledge, it is the fastest speed of collecting SFG spectra reported by now. Using the time-resolved measurement, ultrafast vibrational dynamics of the N-H mode of α-helical peptide at water interface is determined. It is found that the membrane environment does not affect the N-H vibrational relaxation dynamics. It is expected that the time-resolved SFG system will play a vital role in the deep understanding of the dynamics and interaction of the complex molecules at surface and interface. Our method may also provide an important technical proposal for the people who plan to develop time-resolved SFG systems with simultaneous measurement of multiple polarization combinations.

Time-resolved measurement of atomic emission enhancement by fs-ns dual-pulsed laser-induced breakdown spectroscopy

Time-resolved measurement of atomic emission enhancement is performed by using a 500-fs KrF laser pulse incident upon a high density supersonic O2 gas jet, synchronized with an orthogonal ns frequency-doubled Nd：YAG laser pulse. The ultra-short pulse serves as an igniter of the gas jet, and the subsequent ns-laser pulse significantly enhances the atomic emission. Analysis shows that the contributions to the enhancement effect are made mainly by the bremsstrahlung radiation and cascade ionization.

Polarization Dependent Time-Resolved Infrared Spectroscopy and Its Applications

Polarization dependent time-resolved infrared （TRIR） spectroscopy has proven to be a useful technique to study the structural dynamics in a photochemical process. The angular information of transient species is obtainable in this measurement, which makes it a valuable technique for the investigation of electron distribution, molecular structure, and conformational dynamics. In this review, we briefly introduce the principles and applications of polarization dependent TRIR spectroscopy. We mainly focused on the following topics： （i） an overview of TRIR spectroscopy, （ii） principles of TRIR spectroscopy and its advantages compared to the other ultrafast techniques, （iii） examples that use polarization dependent TRIR spectroscopy to probe a variety of cheinical and dynamical phenomena including protein conformational dynamics, excited state electron localization, and photoisomerization, （iv） the limitations and prospects of TRIR spectroscopy.

Development of time-resolved immunofluorometric assays for the detection of house dust mite-allergic IgE in human sera

Dermatophagoides farinae and D. pteronyssinus are the prevalent house dust mites (HDM) in tropical countries and are associated with allergic diseases. This investigation developed a time- resolved immunofluorometric assay (TR-IFMA) for the first time to detect specific IgE antibody in patients with skin prick test positive to HDM but no detectable IgE by other means. Levels of IgE to natural and recombinant HDM allergens were measured by TR-IFMA in 50 HDM-allergic patients and 19 healthy participants compared to sandwich enzyme-linked immunosorbent assay (ELISA). A recombinant allergen, rDerf2, showed a 14 kDa band corresponding to broad range proteins of natural HDM.TR-IFMA showed sensitivity lower than 0.35 kUA/l. TR-IFMA employing three HDM antigens showed good correlations with sandwich ELISA at R2 0.93-0.96. TR-IFMA detected HDM IgE in 62, 62, 25 percent of allergic patient serum sample compared to 28, 32, and 22 percent detected by ELISA result using three HDM allergen. TR-IFMA also detected 26.3, 31.6, and 5.3 percent positive samples from 19 healthy participants while ELISA showed 0, 5.3, and 0 percents IgE positive samples. The use of rDerf2 as an HDM allergen for the assay was verified with no statistically different from other HDM allergens. TR-IFMA showed lower detection limit than ELISA and yielded higher sensitivity for serum of people with allergic symptoms with no detectable HDM IgE. It is anticipated that TR-IFMA for HDM-specific IgE detection will play an important role in future diagnosis of HDM allergy in clinical laboratories and for different research purposes.