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Multi-configurational Ehrenfest simulations of ultrafast nonadiabatic dynamics in a charge-transfer complex
摘要： Multi-configurational Ehrenfest (MCE) approaches, which are intended to remedy the lack of correlations in the standard mean-field Ehrenfest method, have been proposed as coherent-state based ans?tze for quantum propagation [D. V. Shalashilin, J. Chem. Phys. 130, 244101 (2009)] and also as the classical limit of the variational Gaussian-based multiconfiguration time dependent Hartree (G-MCTDH) method [S. R?mer and I. Burghardt, Mol. Phys. 111, 3618 (2013)]. In the present paper, we establish the formal connection between these schemes and assess the performance of MCE for a coherent-state representation of the classical-limit subsystem. As a representative model system, we address the ultrafast, coherent charge transfer dynamics in an oligothiophene-fullerene donor-acceptor complex described by a two-state linear vibronic coupling model. MCE calculations are compared with reference calculations performed with the MCTDH method, for 10–40 vibrational modes. Beyond a dimensionality of 10 modes, it is shown that the correct representation of electronic coherence depends crucially on the sampling of initially unoccupied Gaussians.
关键词： nonadiabatic dynamics,Multi-configurational Ehrenfest,Gaussian wavepackets,MCTDH,ultrafast dynamics,charge-transfer complex
更新于2025-09-23 15:23:52
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Ultrafast Electron Cooling and Decay in Monolayer WS <sub/>2</sub> Revealed by Time- and Energy-Resolved Photoemission Electron Microscopy
摘要： A comprehensive understanding of the ultrafast electron dynamics in two-dimensional transition metal dichalcogenides (TMDs) is necessary for their applications in optoelectronic devices. In this work, we contribute a study of ultrafast electron cooling and decay dynamics in the supported and suspended monolayer WS2 by time- and energy-resolved photoemission electron microscopy (PEEM). Electron cooling in the Q valley of the conduction band is clearly resolved in energy and time, on a timescale of 0.3 ps. Electron decay is mainly via defect trapping process on a timescale of several picoseconds. We observed that the trap states can be produced and increased by laser illumination under ultra-high vacuum, and the higher local optical-field intensity led to the faster increase of trap states. The enhanced defect trapping could significantly modify the carrier dynamics and should be paid attention to in photoemission experiments for two-dimensional materials.
关键词： transition metal dichalcogenides,defect trapping,ultrafast dynamics,energy-resolved,electron cooling,photoemission electron microscopy
更新于2025-09-23 15:21:01
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Ultrafast Excited-State Dynamics of Hydrogen-Bonded Cytosine Microsolvated Clusters with Protic and Aprotic Polar Solvents
摘要： Microsolvation effects on the ultrafast excited-state deactivation dynamics of cytosine (Cy) were studied in hydrogen-bonded Cy clusters with protic and aprotic solvents using mass-resolved femtosecond pump-probe ionization spectroscopy. Two protic solvents, water (H2O) and methanol (MeOH), and one aprotic solvent, tetrahydrofuran (THF), were investigated, and transients of Cy·(H2O)1-6, Cy·(MeOH)1-3, and Cy·THF microsolvated clusters produced in supersonic expansions were measured. With the aid of electronic structure calculations, we assigned the observed dynamics to the low-energy isomers of various Cy clusters and discussed the microsolvation effect on the excited-state deactivation dynamics. With the protic solvents only the microsolvated clusters of Cy keto tautomer were observed. The observed decay time constants of Cy·(H2O)n are 0.5 ps for n=1 and ~0.2–0.25 ps for n=2–6. For Cy·(MeOH)n clusters, the decay time constant for n=1 cluster is similar to that of the Cy monohydrate, but for n=2 and 3 the decays are about a factor of two slower than the corresponding microhydrates. With the aprotic solvent, THF, hydrogen-bonded complexes of both keto and enol tautomers are present in the beam. The keto-Cy·THF shows a similar decay as the keto-Cy monomer, whereas the enol-Cy·THF exhibits a two-fold slower decay than the enol-Cy monomer, suggesting an increase in the barrier to excited-state deactivation upon binding of one THF molecule to the enol form of Cy.
关键词： Cytosine,Excited-state deactivation,Hydrogen-bonded clusters,Microsolvation,Ultrafast dynamics
更新于2025-09-23 15:21:01
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Ultrafast dynamics observation during femtosecond laser-material interaction
摘要： Femtosecond laser technology has attracted significant attention from the viewpoints of fundamental and application; especially femtosecond laser processing materials present the unique mechanism of laser-material interaction. Under the extreme nonequilibrium conditions imposed by femtosecond laser irradiation, many fundamental questions concerning the physical origin of the material removal process remain unanswered. In this review, cutting-edge ultrafast dynamic observation techniques for investigating the fundamental questions, including time-resolved pump-probe shadowgraphy, ultrafast continuous optical imaging, and four-dimensional ultrafast scanning electron microscopy, are comprehensively surveyed. Each technique is described in depth, beginning with its basic principle, followed by a description of its representative applications in laser-material interaction and its strengths and limitations. The consideration of temporal and spatial resolutions and panoramic measurement at different scales are two major challenges. Hence, the prospects for technical advancement in this field are discussed finally.
关键词： ultrafast dynamics,ultrafast continuous optical imaging,femtosecond laser manufacturing,4D ultrafast scanning electron microscopy,pump-probe shadowgraphy
更新于2025-09-23 15:19:57
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Photoelectron Spectroscopy of the Hexafluorobenzene Cluster Anions: (C<sub>6</sub>F<sub>6</sub>)<sub><i>n</i></sub><sup>?</sup> (<i>n</i> = 1 – 5) and I<sup>?</sup>(C<sub>6</sub>F<sub>6</sub>)
摘要： Frequency-resolved (2D) photoelectron (PE) spectra of the anionic clusters (C6F6)n?, for n = 1 – 5, and time-resolved PE spectra of I?C6F6 are presented using a newly built instrument and supported by electronic structure calculations. From the 2D PE spectra, the vertical detachment energy (VDE) of C6F6? was measured to be 1.60 ± 0.01 eV and the adiabatic detachment energy (ADE) ≤ 0.70 eV. The PE spectra also contain fingerprints of resonance dynamics over certain photon energy ranges, in agreement with the calculations. An action spectrum over the lowest resonance is also presented. The 2D spectra of (C6F6)n? show that the cluster can be described as C6F6?(C6F6)n?1. The VDE increases linearly (200 ± 20 meV n?1) due to the stabilising influence on the anion of the solvating C6F6 molecules. For I?C6F6, action spectra of the absorption just below both detachment channels are presented. Time-resolved PE spectra of I?C6F6 excited at 3.10 eV and probed at 1.55 eV reveal a short-lived non-valence state of C6F6? that coherently evolves into the valence ground state of the anion and induces vibrational motion along a specific buckling coordinate. Electronic structure calculations along the displacement of this mode show that at the extreme buckling angle, the probe can access an excited state of the anion that is bound at that geometry, but adiabatically unbound. Hence, slow electrons are emitted and show dynamics that probe predominantly the outer-turning point of the motion. A PE spectrum taken at t = 0 contains vibrational structure, assigned to a specific Raman and/or IR active mode of C6F6.
关键词： electron attachment,non-valence states,cluster anions,hexafluorobenzene,ultrafast dynamics,photoelectron spectroscopy
更新于2025-09-19 17:15:36
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RECENT ADVANCES IN ULTRAFAST TIME-RESOLVED SCANNING TUNNELING MICROSCOPY
摘要： Making smaller and faster functional devices has led to an increasing demand for a microscopic technique that allows the investigation of carrier and phonon dynamics with both high spatial and temporal resolutions. Traditional optical pump–probe methods can achieve femtosecond temporal resolution but fall short in the spatial resolution due to the diffraction limit. Scanning tunneling microscopy (STM), on the contrary, has realized atomic-scale spatial resolution relying on the high sensitivity of the tunneling current to the tip-sample distance. However, limited by the electronics bandwidth, STM can only push the temporal resolution to the microseconds scale, restricting its applications to probe various ultrafast dynamic processes. The combination of these two methods takes advantages of optical pump–probe techniques and highly localized tunneling currents of STM, providing one viable solution to track atomic-scale ultrafast dynamics in single molecules and low-dimensional materials. In this review, we will focus on several ultrafast time-resolved STM methods by coupling the tunneling junctions with pulsed electric waves, THz, near-infrared and visible laser. Their applications to probe the carrier dynamics, spin dynamics, and molecular motion will be highlighted. In the end, we will present an outlook on the challenges and new opportunities in this field.
关键词： terahertz technique,femtosecond laser,Scanning tunneling microscopy,pump-probe technique,ultrafast dynamics.
更新于2025-09-19 17:15:36
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Femtosecond ultrafast dynamics study on the photoemission performance of reflection-mode GaAlAs photocathode
摘要： The photoemission characteristic of re?ection-mode GaAlAs photocathode has been investigated, while a convenient and nondestructive method for evaluating the photocathode performances will be introduced. In this paper, two same re?ection-mode photocathode structures with di?erent research methods are adopted. One way is using multi-information measurement system to measure the photoemission characteristic curve of GaAlAs photocathode. Another method is the femtosecond transient re?ection spectroscopy. The ultrafast dynamics property of the GaAlAs carrier is studied with the femtosecond transient re?ectivity change, and the quantitative calculating is also shown. According to the results of two experimental methods, we analyze some photoemission performance parameters, such as carrier lifetime, di?usion length etc. The results show that the surface electron escape probability P and the drift length LDE are di?erent, while are the same of the back recombination velocity Sv by the two method. In a word, compared with multi-information measurement system, the femtosecond ultrafast dynamics method is more suitable to research on the photoemission characteristic of re?ection-mode GaAlAs photocathode which is no damage and accurate.
关键词： Femtosecond ultrafast dynamics,GaAlAs,Spectroscopy,Photocathode,Photoemission characteristic,Re?ection-mode
更新于2025-09-19 17:15:36
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[IEEE 2019 44th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) - Paris, France (2019.9.1-2019.9.6)] 2019 44th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) - Intraband Ultrafast Terahertz Conductivity Dynamics in Graphene
摘要： The cooling mechanism is crucial for nanoelectronics, which determines the response time limited for the devices. The cooling process after optical pump in graphene, which is a well-known material with ultrahigh carrier mobility, is therefore an urgent research topic. Ultrafast thermalization and relaxation process in a monolayer graphene film grown on a quartz substrate is investigated by femtosecond laser pump, terahertz probe induced negative technique. A typical photo found, and the electron-phonon-impurity supercollision process is determined to about 2.8 picosecond at different pump photon energy and pump fluence. The competition between the impurity supercollision and the interband-intraband scattering in graphene could give new insight into the design of the application of graphene photonic devices.
关键词： ultrafast dynamics,cooling mechanism,electron-phonon-impurity supercollision,terahertz conductivity,graphene
更新于2025-09-16 10:30:52
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Ultrafast Dynamics in Quantum Dot Doped Nanocomposites at Low Temperatures: Study by Means of Site-Selective Spectroscopy
摘要： Study of the dynamics and luminescence properties of nanocomposite materials based on semiconductor quantum dots is an important and urgent trend of modern physics and materials science. Semiconductor quantum dots have a number of unusual photophysical and spectral properties, which are determined by their structure, size and microscopic features of interaction with the local environment. Considerable efforts are aimed to search for and study new luminescence labels [1] and suitable matrices for functional quantum-dot-based materials, e.g., liquid crystals [2]. In this paper we discuss the new data on ultrafast dynamics in nanocomposites doped with semiconductor colloidal nanocrystals (quantum dots, QDs) revealed by two types of site-selective spectroscopy techniques. Optical dephasing in a thin film of double-coated CdSe/CdS/ZnS QDs was studied in a wide range of cryogenic temperatures using a method of the photon echo. Experiments were performed using unique incoherent photon echo spectrometer (see e.g. [3]). We measured the decay curves of 2-pulse incoherent photon echo signals in ensemble of QDs and obtained very short relaxation times (about hundreds of femtoseconds) in a temperature range from 4.5 K to 50 K [4]. These results are extremely different from the data obtained in the ensembles of impurity organic dye molecules for which the relaxation times are about nanoseconds at this very temperature. Possible reasons for such ultrafast optical dephasing can be associated: (1) with structural inhomogeneities of QDs themselves; (2) with the features of internal dynamics of the emitting core (e.g., blinking of QDs); (3) with the surface states in a QDs shells. Furthermore, the size variance and strongly inhomogeneous local environment (local fields in a solid matrix) can lead to fast relaxation in the QDs ensemble. In order to separate the contributions to the optical dephasing of the processes of interaction of quantum dots with the matrix from the processes inside the QDs themselves, we measured the temperature dependences of the luminescence spectra of nanocomposites. Generally, with the temperature decrease, the maxima of the exciton bands in a luminescence spectra shift to the UV spectral range. Such phenomenon can be described taking into account the electron-phonon (exciton-phonon) interaction. The temperature shift of the exciton luminescence spectrum can be described using the modified Varshni equation [5]. Theoretical model that takes into account the electron-phonon interaction made it possible to quantitatively describe the temperature dependences of the exciton luminescence spectra of QDs, as well as to determine the values of the Huang-Rhys factor and the average energy of phonons in nanocrystals. In our experiments no noticeable temperature changes in the exciton maxima widths have been observed, which is related, on the one hand, to the resolution of used experimental technique and, on the other hand, to the occurrence of significant inhomogeneous broadening of the spectra caused by a large dispersion of QD sizes. Describing the temperature behavior of the position and width of the luminescence spectra of QDs one should take into account an interaction of the electronic transition of the impurity with vibrational excitations, e.g. with local or quasi-local phonons in a solid matrix. In this connection, we have studied the influence of the matrix on luminescence properties of QD-doped nanocomposites. The combination of the photon echo and luminescence spectroscopy is a powerful method to study the ultrafast processes of interaction of the impurity ensemble of QDs with a solid matrix as well as the intrinsic dynamics of quantum dots themselves.
关键词： ultrafast dynamics,luminescence,quantum dots,photon echo,nanocomposites,site-selective spectroscopy
更新于2025-09-11 14:15:04
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Ultrafast Dynamics and Ultrasensitive Single-Particle Intermittency in Small-Sized Toxic Metal Free InP Based Core/Alloy-shell/Shell Quantum Dots: Excitation Wavelength Dependency Towards Variation of PLQY
摘要： Observation of strong deviation of photoluminescence excitation curve from absorption curve specially at lower wavelength range, below 450 nm in case of small sized toxic metal free InP based core-alloy-shell quantum dots hints towards interesting exciton dynamics. PL quantum yield (PLQY) has been observed to be dependent on the excitation wavelength. Monitoring the bleach dynamics employing femtosecond ultrafast pump-probe technique it could be shown that the rise-time increases with decrease in pump excitation wavelength from 100 fs for 550 nm excitation to 220 fs for 430 nm pump/excitation. Therefore exciton cooling takes longer time for lower wavelength excitation and thus the exciton becomes more prone to get trapped. About two fold enhancement in magnitude of normalized bleach signal at the band edge (~0.1 (for 430nm excitation) to ~0.2 (for 550nm excitation)) following exciton relaxation has been observed. Thus, in comparison to lower wavelength excitation, for near band-edge pump/excitation there is higher probability of radiative exciton recombination, therefore increasing PLQY. Hot exciton trapping dynamics has been noted to be occurring at a timescale ~750 fs. From ultra-sensitive single particle measurement, magnitude of power-law exponent for both ON and OFF events remain similar to each other and the magnitude remains unaltered for different excitation wavelengths, (405 nm to 568 nm). As long as 100 s ON event could be observed making this InP based non-toxic QD quite suitable for single particle tracking etc. Interestingly, ON event truncation time has been found to increase from 6s to 16s and OFF event truncation time has been found to decrease from 11s to 5.5s, thereby exciton detrapping rate / trapping rate increases from 0.5 to nearly 3 on going from 405 nm to 568 nm excitation. Thus, as the trapping gets suppressed and detrapping gets enhanced, PLQY gets enhanced. The extent of relative decrease of PLQY value with increase in excitation energy above band-edge has been observed to be much more pronounced in CdSe based CAS QD than InP based CAS QD.
关键词： ultrafast dynamics,photoluminescence quantum yield,excitation wavelength dependency,InP based quantum dots,single-particle intermittency
更新于2025-09-11 14:15:04