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.

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.

Time-resolved spectroscopy of collinear femtosecond and nanosecond dual-pulse laser-induced Cu plasmas

In this paper,we investigate the time-resolved spectroscopy of collinear femtosecond(fs)and nanosecond(ns)dual-pulse(DP)laser-induced plasmas.A copper target was used as an experimental sample,and the fs laser was considered as the time zero reference point.The interpulse delay between fs and ns laser beams was 3μs.First,we compared the time-resolved peak intensities of Cu(I)lines from Cu plasmas induced by fs+ns and ns+fs DP lasers with collinear configuration.The results showed that compared with the ns+fs DP,the fs+ns DP laser-induced Cu plasmas had stronger peak intensities and longer lifetimes.Second,we calculated time-resolved plasma temperatures using the Boltzmann plot with three spectral lines at Cu(I)510.55,515.32 and 521.82 nm.In addition,time-resolved electron densities were calculated based on Stark broadening with Cu(I)line at 521.82 nm.It was found that compared with ns+fs DP,the plasma temperatures and electron densities of the Cu plasmas induced by fs+ns DP laser were higher.Finally,we observed images of ablation craters under the two experimental conditions and found that the fs+ns DP laser-produced stronger ablation,which corresponded to stronger plasma emission.

Time-resolved spectroscopy for 5s^('4)D_(7/2) state transitions undergoing electron–ion recombination in femtosecond laser-produced copper plasma

In the femtosecond laser-produced Cu-plasma, the transient transition dynamics that the excited state 5s4D7/2 via electron-ion recombination transfers to 4p4F9/20 （465.11 nm, Λ1 line） and 4p4D7/20 （529.25 nm, Λ2 line） states are investigated by using the time-resolved spectroscopy. The occupation number and relevant lifetime of the excited state 5s4D7/2, the temporal evolutions of spectral intensities for Λ1 line and Λ2 line emissions are demonstrated to be in direct proportion to the employed laser intensity, which reveals the transient features of transition dynamics clearly differing from that resulted in the traditional collision excitation. Furthermore, some unique characteristics for Λ1 and Λ2 transitions stemming from electron-ion recombination are examined in detail.

Time-resolved spectroscopy of femtosecond laser-induced Cu plasma with spark discharge

The combination of spark discharge and laser-induced breakdown spectroscopy (LIBS) is called spark discharge assisted LIBS.It works under laser-plasma triggered spark discharge mode,and shows its ability to enhance spectral emission intensity.This work uses a femtosecond laser as the light souuce,since femtosecond laser has many advantages in laser-induced plasma compared with nanosecond laser,meanwhile,the study on femtosecond LIBS with spark discharge is rare.Time-resolved spectroscopy of spark discharge assisted femtosecond LIBS was investigated under different discharge voltages and laser energies.The results showed that the spectral intensity was significantly enhanced by using spark discharge compared with LIBS alone.And,the spectral emission intensity using spark discharge assisted LIBS increased with the increase in the laser energy.In addition,at low laser energy,there was an obvious delay on the discharge time compared with high laser energy,and the discharge time with positive voltage was different from that with negative voltage.

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.

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.

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.

Determination of Iron in Water Solution by Time-Resolved Femtosecond Laser-Induced Breakdown Spectroscopy

The influence of the energy of femtosecond laser pulses on the intensity of Fe I （371.99 nm） emission line and the continuous spectrum of the plasma generated on the surface of Fe^3＋ water solution by a Ti： sapphire laser radiation with pulse duration 〈45 fs and energies up to 7 mJ is determined. A calibration curve was obtained for Fe3＋ concentration range from 0.5 g/L to the limit of detection in water solution, and its saturation was detected for concentrations above 0.25 g/L, which is ascribed to self-absorption. The 3σ- limit of detection obtained for Fe in water solution is 2.6 mg/L in the case of 7 mJ laser pulse energy. It is found that an increase of laser pulse energy insignificantly affects on LOD in the time-resolved LIBS and leads to a slight improvement of the limit of detection.

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.

Detection of K in soil using time-resolved laser-induced breakdown spectroscopy based on convolutional neural networks

One of the technical bottlenecks of traditional laser-induced breakdown spectroscopy(LIBS) is the difficulty in quantitative detection caused by the matrix effect. To troubleshoot this problem,this paper investigated a combination of time-resolved LIBS and convolutional neural networks(CNNs) to improve K determination in soil. The time-resolved LIBS contained the information of both wavelength and time dimension. The spectra of wavelength dimension showed the characteristic emission lines of elements, and those of time dimension presented the plasma decay trend. The one-dimensional data of LIBS intensity from the emission line at 766.49 nm were extracted and correlated with the K concentration, showing a poor correlation of R_c^2?=?0.0967, which is caused by the matrix effect of heterogeneous soil. For the wavelength dimension, the two-dimensional data of traditional integrated LIBS were extracted and analyzed by an artificial neural network(ANN), showing R_v^2?=?0.6318 and the root mean square error of validation(RMSEV)?=?0.6234. For the time dimension, the two-dimensional data of time-decay LIBS were extracted and analyzed by ANN, showing R_v^2?=?0.7366 and RMSEV?=?0.7855.These higher determination coefficients reveal that both the non-K emission lines of wavelength dimension and the spectral decay of time dimension could assist in quantitative detection of K.However, due to limited calibration samples, the two-dimensional models presented over-fitting.The three-dimensional data of time-resolved LIBS were analyzed by CNNs, which extracted and integrated the information of both the wavelength and time dimension, showing the R_v^2?=?0.9968 and RMSEV?=?0.0785. CNN analysis of time-resolved LIBS is capable of improving the determination of K in soil.

Time-resolved infrared spectroscopic investigation of Ga_(2)O_(3) photocatalysts loaded with Cr_(2)O_(3)-Rh cocatalysts for photocatalytic water splitting

Investigation of the charge dynamics and roles of cocatalysts is crucial for understanding the reaction of photocatalytic water splitting on semiconductor photocatalysts.In this work,the dynamics of photogenerated electrons in Ga_(2)O_(3) loaded with Cr_(2)O_(3)-Rh cocatalysts was studied using time-resolved mid-infrared spectroscopy.The structure of these Cr_(2)O_(3)-Rh cocatalysts was identified with high-resolution transmission electron microscopy and CO adsorption Fourier-transform infrared spectroscopy,as Rh particles partly covered with Cr_(2)O_(3).The decay dynamics of photogenerated electrons reveals that only the electrons trapped by the Rh particles efficiently participate in the H2 evolution reaction.The loaded Cr_(2)O_(3) promotes electron transfer from Ga_(2)O_(3) to Rh,which accelerates the electron-consuming reaction for H2 evolution.Based on these observations,a photocatalytic water-splitting mechanism for Cr_(2)O_(3)-Rh/Ga_(2)O_(3) photocatalysts has been proposed.The elucidation of the roles of the Cr_(2)O_(3)-Rh cocatalysts aids in further understanding the reaction mechanisms of photocatalytic water splitting and guiding the development of improved photocatalysts.

Tracking dynamic evolution of catalytic active sites in photocatalytic CO2 reduction by in situ time-resolved spectroscopy

In situ time-resolved spectroscopy is an effective method to monitor the catalysis reaction in real time and reveal the catalytic mechanistic pathway.The dynamic evolution of coordination and electronic structures of catalytic active sites during the CO2 reduction reaction is still a "black box," impeding the design of high-efficiency catalysts.In a recent report published in J.Am.Chem.Soc.,by multiple in situ time-resolved spectroscopy.

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 Optical Emission Spectroscopy Diagnosis of CO_2 Laser-Produced SnO_2 Plasma

The spectral emission and plasma parameters of SnO2 plasmas have been investigated. A planar ceramic SnO2 target was irradiated by a CO2 laser with a full width at half maximmn of 80 ns. The temporal behavior of the specific emission lines from the SnO2 plasma was characterized. The intensities of Sn I and Sn Ⅱ lines first increased, and then decreased with the delay time. The results also showed a faster decay of Sn I atoms than that of Sn II ionic species. The temporal evolutions of the SnO2 plasma parameters （electron temperature and density） were deduced. The measured temperature and density of SnO2 plasma are 4.38 eV to 0.5 eV and 11.38×1017 cm 3 to 1.1×1017^ cm-3, for delay times between 0.1 μs and 2.2 #s. We also investigated the effect of the laser pulse energy on Sn02 plasma.

Time-Resolved Emission Spectroscopic Study of Laser-Induced Steel Plasmas

Laser-induced steel plasma is generated by focusing a Q-switched Nd：YAG visible laser（532 nm wavelength） with an irradiance of 1 x 109 W/cm2 on a steel sample in air at atmospheric pressure.An Echelle spectrograph coupled with a gateable intensified charge-coupled detector is used to record the plasma emissions.Using time-resolved spectroscopic measurements of the plasma emissions,the temperature and electron number density of the steel plasma are determined for many times of the detector delay.The validity of the assumption by the spectroscopic methods that the laser-induced plasma（LIP） is optically thin and is also in local thermodynamic equilibrium（LTE） has been evaluated for many delay times.From the temporal evolution of the intensity ratio of two Fe I lines and matching it with its theoretical value,the delay times where the plasma is optically thin and is also in LTE are found to be 800 ns,900 ns and 1000 ns.

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.

Establishment of Double-antigen Sandwich Time-resolved Fluorescence Immunoassay for Detection of Pest des Petits Ruminants Virus

[Objectives]This study was conducted to explore rapid and large-scale screening and detection of peste des petits ruminants(PPR),so as to provide important technical means for prevention,control and purification of PPR.[Methods]Soluble N protein and NH fusion protein were successfully obtained in an Escherichia coli expression system by optimizing E.coli codon and expression conditions.Furthermore,based on purified soluble N protein and NH fusion protein,a double-antigen sandwich time-resolved fluorescence immunoassay method for detection of peste des petits ruminants virus(PPRV)was established.[Results]The method has high sensitivity and specificity and can specifically detect the antibody against PPRV in sheep serum,and it has no cross reaction with other related diseases.The method was used to detect 292 clinical samples,and compared with French IDVET competition ELISA kit.The coincidence rates of positive samples and negative samples from the two kinds of test kits were 92.47%and 97.26%,respectively,and the overall coincidence rate was 94.86%.The intra-group and inter-group coefficients of variation in the repeatability test were less than 10%.[Conclusions]Compared with the traditional ELISA method,the double-antigen sandwich time-resolved fluorescence immunoassay for detection of PPRV has equivalent sensitivity and specificity,and simple and rapid operation,and thus high application and popularization value.

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.

Femtosecond Time-Resolved Spectroscopic Photoemission Electron Microscopy for Probing Ultrafast Carrier Dynamics in Heterojunctions

The fast developing semiconductor industry is pushing to shrink and speed up transistors. This trend requires us to understand carrier dynamics in semiconductor heterojunctions with both high spatial and temporal resolutions. Recently, we have successfully set up a timeresolved photoemission electron microscopy (TR-PEEM), which integrates the spectroscopic technique to measure electron densities at specific energy levels in space. This instrument provides us an unprecedented access to the evolution of electrons in terms of spatial location, time resolution, and energy, representing a new type of 4D spectro-microscopy. Here in this work, we present measurements of semiconductor performance with a time resolution of 184 fs, electron kinetic energy resolution of 150 meV, and spatial resolution of about 150 nm or better. We obtained time-resolved micro-area photoelectron spectra and energy-resolved TR-PEEM images on the Pb island on Si(111). These experimental results suggest that this instrument has the potential to be a powerful tool for investigating the carrier dynamics in various heterojunctions, which will deepen our understanding of semiconductor properties in the submicron/nanometer spatial scales and ultrafast time scales.