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.

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.

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.

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.

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.

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.

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.

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.

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 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.

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.

Transient Spectroscopic Properties of [60]Fullerene-Containing Cyclic Sulphoxide

The properties of the triplet excited state of [60]fullerene-containing cyclic sulphoxide have been investigated by time-resolved absorption spectroscopy. Transient absorption bands of [60]fullerene-containing cyclic sulphoxide showed two decay-components, which were attributed to triplet excited states of different spin multiplicity. The properties of photoexcited states of [60]fullerene-containing cyclic sulphoxide are also reported.

Influence of target temperature on femtosecond laser-ablated brass plasma spectroscopy

Spectral intensity,electron temperature and density of laser-induced plasma(LIP) are important parameters for affecting sensitivity of laser-induced breakdown spectroscopy(LIBS).Increasing target temperature is an easy and feasible method to improve the sensitivity.In this paper,a brass target in a temperature range from 25℃ to 200℃ was ablated to generate the LIP using femtosecond pulse.Time-resolved spectral emission of the femtosecond LIBS was measured under different target temperatures.The results showed that,compared with the experimental condition of 25℃,the spectral intensity of the femtosecond LIP was enhanced with more temperature target.In addition,the electron temperature and density were calculated by Boltzmann equation and Stark broadening,indicating that the changes in the electron temperature and density of femtosecond LIP with the increase of the target temperature were different from each other.By increasing the target temperature,the electron temperature increased while the electron density decreased.Therefore,in femtosecond LIBS,a hightemperature and low-density plasma with high emission can be generated by increasing the target temperature.The increase in the target temperature can improve the resolution and sensitivity of femtosecond LIBS.

Complexation of radionuclide ^152＋154Eu(Ⅲ) with alumina-bound fulvic acid studied by batch and time-resolved laser fluorescence spectroscopy

To contribute to the understanding of Eu(Ⅲ)interaction properties on hydrous alumina particles in the absence and presence of fulvic acid(FA),the complexation properties of Eu(Ⅲ)with hydrous alumina,FA and FA-alumina hybrids are studied by batch and time-resolved laser fluorescence spectroscopy(TRLFS)techniques.The continuous increase in the fluorescence lifetime of Eu-alumina and Eu-FA with increasing pH indicates that the complexation is accompanied by decreasing number of hydration water in the first coordination sphere of Eu(Ⅲ).Eu(Ⅲ)is adsorbed onto alumina particles as outer-sphere surface complexes of≡(Al-O)-Eu·(OH)·7H_2O and≡(Al-O)-Eu·6H_2O at low pH values,and as inner-sphere surface complexes as≡(Al-O)_2-Eu^+·4H_2O at high pH.In FA solution,Eu(Ⅲ)forms complexes with FA as(COO)_2Eu^+(H_2O)_x and the hydration water number in the first coordination sphere decreases with pH increasing.The formation of≡COO-Eu-(O-Al≡)·4H_2O is observed on FA-alumina hybrids,suggesting the formation of strong inner-sphere surface complexes in the presence of FA.The surface complexes are also characterized by their emission spectra[the ratio of emission intensities of^5D_0→~7F_1(λ=594nm)and^5D_0→~7F_2(λ=619nm)transitions]and their fluorescence lifetime.The findings is important to understand the contribution of FA in the complexation properties of Eu(Ⅲ)on FA-alumina hybrids that the clarification of the environmental behavior of humic substances is necessary to understand fully the behavior of Eu(Ⅲ),or its analogue trivalent lanthanide and actinide ions in natural environment.

Femtosecond time-resolved difference absorption spectroscopy of all-trans-β-Apo-8′-carotenal

The femtosecond time-resolved difference absorption spectra of all-trans-β-Apo-8′-carotenal have been recorded and analyzed by the singular-value decomposition (SVD) method followed by global fitting using a sequential model for the excited-state energy relaxation. With this model, we have obtained the excited-state absorption spectra and the lifetimes of the corresponding excited states both in nonpolar solvent n-hexane and polar solvent methanol. Three excited states, namely S3(170fs), S2(2.32ps) and S1(26ps) in n-hexane, and two excited states S2(190fs) and S1(9.4ps) in methanol have been observed. The excited-state absorption spectra of all-trans-β-Apo-8′-carotenal in methanol display a red shift and broadeness, while the lifetime of S1 state becomes shorter. It is proposed that these effects are related to the presence of a carbonyl functional group that leads to the solvent effect on the excited-state energy level. At the same time, it is shown that the SVD method is a useful tool in resolving the time-resolved absorption spectra.

Time-resolved terahertz spectroscopy of charge carrier dynamics in the chalcogenide glass As_(30)Se_(30)Te_(40)[Invited]

Broadband(1.6–18 THz) terahertz time-domain spectroscopy(THz-TDS) and time-resolved terahertz spectroscopy(TRTS) were performed on a 54 μm thick chalcogenide glass(As_(30)Se_(30)Te_(40)) sample with a two-color laser-induced air plasma THz system in transmission and reflection modes, respectively. Two absorption bands at 2–3 and 5–8 THz were observed. TRTS reveals an ultrafast relaxation process of the photoinduced carrier response, well described by a rate equation model with a finite concentration of mid-bandgap trap states for self-trapped excitons.The photoinduced conductivity can be well described by the Drude–Smith conductivity model with a carrier scattering time of 12–17 fs, and we observe significant carrier localization effects. A fast refractive index change was observed 100 fs before the conductivity reached its maximum, with 2 orders of magnitude larger amplitude than expected for the optically induced THz Kerr effect, indicating that free carriers are responsible for the transient index change.

Effects of aging on working memory performance and prefrontal cortex activity: A time-resolved spectroscopy study

Objective:This study aimed to employ time‐resolved spectroscopy(TRS)to explore age‐related differences in prefrontal cortex(PFC)activity while subjects performed a working memory task.Methods:We employed TRS to measure PFC activity in ten healthy younger and ten healthy older subjects while they performed a working memory(WM)task.All subjects performed the Sternberg test(ST)in which the memory‐set size varied between one and six digits.Using TRS,we recorded changes in cerebral blood oxygenation as a measure of changes in PFC activity during the task.In order to identify left/right asymmetry of PFC activity during the working memory task,we calculated the laterality score,i.e.,Δoxy‐Hb(rightΔoxy‐Hb—leftΔoxy‐Hb);positive values indicate greater activity in the right PFC,while negative values indicate greater activity in the left PFC.Results:During the ST,statistical analyses showed no significant differences between the younger and older groups in accuracy for low memory‐load and high memory‐load.In high memory‐load tasks,however,older subjects were slower than younger subjects(P<0.05).We found that the younger group showed right lateral responses with a stronger right than left activation in the frontal pole,whereas the older group showed bilateral responses(P<0.05).Conclusions:The present results are consistent with the hemispheric asymmetry reduction in older adults(HAROLD)model;working memory tasks cause asymmetrical PFC activation in younger adults,while older adults tend to show reduced hemispheric lateralization.

Communication Chain-foldability effect on coaggregation Evidence derived from time-resolved fluorescence spectroscopy

Very recently,we have proposed that all culprit molecules of arteriosclerosis mightpossess a common and measurable inherent property,namely,a coaggregating ten-dency.Extensive kinetic measurements of these tendencies in terms of △CAgC valueshave led to a new and surprising observation,i.e.,the cholesteryl ester with a longsaturated 18-carbon chain(CE-18)actually has a much smaller tendency towardcoaggregation than that of the ester with a“short”8-carbon chain(CE-8).Thisstate of affairs has been coined as the“chain-foldability effect”,i.e.,the 18-carbon

Development and Application of Ultrafast Circular Dichroism Spectroscopy Techniques

Chirality hold broad applications in life sciences,quantum devices,and various other areas.Traditionally,molecular chirality can be characterized by using steady-state circular dichroism spectroscopy.However,the techniques that can characterize excited state chirality are progressively capturing the public interest as it can provide the dynamic information for chirality generation and transfer.In this review,we focus on the theoretical background and the developmental history of femtosecond time-resolved circular dichroism spectroscopy(TRCD)techniques around the world.Additionally,we provide examples to showcase the utility of these techniques in the analysis of the dynamical molecular chemical structures,the investigation of molecular chirality generation,and the detection of electron spin dynamics in semiconductor quantum dots.

Size-dependent optical properties and carriers dynamics in CdSe/ZnS quantum dots

Size-dependence of optical properties and energy relaxation in CdSe/ZnS quantum dots （QDs） were investigated by two-colour femtosecond （fs） pump-probe （400/800 nm） and picosecond time-resolved photoluminescence （ps TRPL） experiments. Pump-probe measurement results show that there are two components for the excited carriers relaxation, the fast one with a time constant of several ps arises from the Auger-type recombination, which shows almost particle sizeindependence. The slow relaxation component with a time constant of several decades of ns can be clearly determined with ps TRPL spectroscopy in which the slow relaxation process shows strong particle size-dependence. The decay time constants increase from 21 to 34 ns with the decrease of particle size from 3.2 to 2.1 nm. The room-temperature decay lifetime is due to the thermal mixing of bright and dark excitons, and the size-dependence of slow relaxation process can be explained very well in terms of simple three-level model.