Electron Dynamics Research Articles

Assembly of a Heterobimetallic Fe/Mn Cofactor in the para-Aminobenzoate Synthase Chlamydia Protein Associating with Death Domains (CADD) Initiates Long-Range Radical Hole-Hopping.

Chlamydia protein associating with death domains (CtCADD) is involved in the biosynthesis of p-aminobenzoic acid (pABA) for integration into folate, a critical cofactor that is required for pathogenic survival. CADD activates dioxygen and utilizes its own tyrosine and lysine as synthons to furnish the carboxylate, carbon backbone, and amine group of pABA in a complex multistep mechanism. Unlike other members of the heme oxygenase-like dimetal oxidase (HDO) superfamily that typically house an Fe2 cofactor, previous activity studies have shown that CtCADD likely uses a heterobimetallic Fe/Mn center. The structure of the Fe2+/Mn2+ cofactor and how the conserved HDO scaffold mediates metal selectivity have remained enigmatic. Adopting an in crystallo metalation approach, CtCADD was solved in the apo, Fe2+2, Mn2+2, and catalytically active Fe2+/Mn2+ forms to identify the probable site for Mn binding. The analysis of CtCADD active-site variants further reinforces the importance of the secondary coordination sphere on cofactor preference for competent pABA formation. Rapid kinetic optical and electron paramagnetic resonance (EPR) studies show that the heterobimetallic cofactor selectively reacts with dioxygen and likely initiates pABA assembly through the formation of a transient tyrosine radical intermediate and a resultant heterobimetallic Mn3+/Fe3+ cluster.

Electronic interactions in Dirac fluids visualized by nano-terahertz spacetime interference of electron-photon quasiparticles.

Ultraclean graphene at charge neutrality hosts a quantum critical Dirac fluid of interacting electrons and holes. Interactions profoundly affect the charge dynamics of graphene, which is encoded in the properties of its electron-photon collective modes: surface plasmon polaritons (SPPs). Here, we show that polaritonic interference patterns are particularly well suited to unveil the interactions in Dirac fluids by tracking polaritonic interference in time at temporal scales commensurate with the electronic scattering. Spacetime SPP interference patterns recorded in terahertz (THz) frequency range provided unobstructed readouts of the group velocity and lifetime of polariton that can be directly mapped onto the electronic spectral weight and the relaxation rate. Our data uncovered prominent departures of the electron dynamics from the predictions of the conventional Fermi-liquid theory. The deviations are particularly strong when the densities of electrons and holes are approximately equal. The proposed spacetime imaging methodology can be broadly applied to probe the electrodynamics of quantum materials.

Effects of Pt doping on surface properties and quenching of band edge emission in ZnO

Pt doped ZnO thin films were synthesized on soda-lime glass substrates using a cost-effective sol-gel method, followed by spin coating and thermal annealing at various temperatures. Several characterization techniques such as UV–Vis absorption spectroscopy, photoluminescence (PL), field emission scanning electron microscopy (FESEM), time-of-flight secondary ion mass spectroscopy (ToF-SIMS), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) are exploited to investigate the Pt doping effects on surface and optical properties of ZnO thin films. UV–Vis analysis demonstrated a slight decrease in the optical band gap from 3.3 eV to 3.2 eV with increased annealing, indicating enhanced suitability for optoelectronic applications. FESEM imaging revealed distinct worm-like structures and small Pt metal nanoparticles in unannealed samples, while thermal treatment supported the formation of spherical Pt nanoparticles and improved conductive pathways in the films. The Wagner diagram, coupled with Auger line transitions from XPS, quantified the oxidation states and chemical environments of Zn and Pt, respectively. Notably, the observed changes in the binding energy of the Zn-2p junction and the kinetic energy of the Zn-LMM Auger junction were significantly influenced by the Pt doping and the electronic properties of the substrate. The Wagner diagram provided a comprehensive visual representation of the parameters, facilitating a deeper understanding of electronic interactions and structural dynamics at the atomic level. XPS results confirmed the presence of ZnO, Zn(OH)2, Pt0 and PtO2 phases, indicating stability and constutient's interactions. ToF-SIMS also validated the uniform distribution of Pt nanoparticles within the ZnO thin film, confirming the effectiveness of the sol-gel method. As a result of Pt doping, PL studies revealed a large quenching effect on band edge emission in the UV and visible emission region of ZnO thin film, which is due to Pt acting as a recombination center for photogenerated carriers. Overall, our results highlight the influence of annealing and Pt incorporation on the optical and structural properties of ZnO thin films and highlight their potential applications for photonics and catalysis.

Electronic structure, lattice dynamics and superconductivity in Li2CuX and LiCu2X (X = Si, Ge and Sn) Heusler compounds

First-principles computational methods have been employed to explore the electronic, lattice dynamics, and superconducting properties of the intermetallic Heusler compounds Li2CuX and LiCu2X (X = Si, Ge, or Sn). The electronic structure calculations suggest that all of these compounds exhibit metallic properties. The dynamic stability of the compounds has been examined by computing the phonon dispersion spectra and phonon density of states. Six out of twelve phases are found to be dynamically stable. Additionally, the superconductivity of stable and metastable compounds has been investigated using the electron–phonon-mediated mechanism. In calculations, it turns out that, regular cubic full Heusler Fm-3m LiCu2Ge exhibited the highest superconducting critical temperature (Tc) of 2.0 K, with a calculated electron–phonon coupling constant (λ) of 0.58.

XFEL SASE pulses can enhance time-dependent observables

Abstract X-ray free electron lasers (XFELs) have emerged as powerful sources of short and intense X-ray pulses. We propose a simple and robust procedure which takes advantage of the inherent stochasticity of self-amplified stimulated emission (SASE) pulses to enhance the time-resolution and signal strength in the recorded data. Notably, the proposed method is able to enhance the average signal without knowledge of the signal strength of individual shots. Simple metrics for the probe pulses are introduced, such as an effective pulse duration applicable to SASE pulses characterised in the time domain using {\it{e.g.}}\ an X-band transverse cavity (XTCAV). The approach is evaluated using simulated and real pulse data in the context of ultrafast electron dynamics in a molecule. Utilising H$_2$ as a model system, we demonstrate the efficacy of the method theoretically, successfully enhancing the predicted nonresonant ultrafast X-ray scattering signal associated with electron dynamics. The method presented is broadly applicable and offers a general strategy for enhancing time-dependent observables at XFELs.

Direct Evidence of Breakdown of Spin Statistics in Ion-Atom Charge Exchange Collisions.

Recent experimental studies have questioned the validity of spin statistics assumptions, particularly in charge exchange processes occurring in atomic MeV collisions. Here, we study spin-resolved single electron capture processes in collisions between C^{3+} ions and helium within an energy range of 1.25-400 keV/u. Using high resolution reaction microscope and multielectronic theoretical approaches, we directly measure and calculate the true population information of the C^{2+}(1s^{2}2s2p ^{1,3}P) states at the time of electron capture, overcoming the previous experimental and theoretical difficulties. At the level of integral and scattering angle differential cross sections, our results demonstrate the breakdown of pure spin statistics arguments, especially at high impact energies where they are traditionally expected to be valid. These novel findings and conclusions raise intriguing questions both in the understanding of the electronic dynamics during such fast collisional processes and in exploring quantum manipulation of atomic and molecular reactivity.

Significant role of secondary electrons in the formation of a multi-body chemical species spur produced by water radiolysis.

Scientific insights into water photolysis and radiolysis are essential for estimating the direct and indirect effects of deoxyribonucleic acid (DNA) damage. Secondary electrons from radiolysis intricately associated with both effects. In our previous paper, we simulated the femtosecond (1 × 10- 15s) dynamics of secondary electrons ejected by energy depositions of 11-19eV into water via high-energy electron transport using a time-dependent simulation code. The results contribute to the understanding of simple "intra-spur" chemical reactions of tree-body chemical species (hydrated electrons, hydronium ion and OH radical) in subsequent chemical processes. Herein, we simulate the dynamics of the electrons ejected by energy depositions of 20-30eV. The present results contribute to the understanding of complex "inter-spur" chemical reactions of the multi-body chemical species as well as for the formation of complex DNA damage with redox site and strand break on DNA. The simulation results present the earliest formation mechanism of an unclear multi-body chemical species spur when secondary electrons induce further ionisations or electronic excitations. The formation involves electron-water collisions, i.e. ionisation, electronic excitation, molecular excitation and elastic scattering. Our simulation results indicate that (1) most secondary electrons delocalise to ~ 12nm, and multiple collisions are sometimes induced in a water molecule at 22eV deposition energy. (2) The secondary electrons begin to induce diffuse band excitation of water around a few nm from the initial energy deposition site and delocalise to ~ 8nm at deposition energies ~ 25eV. (3) The secondary electron can cause one additional ionisation or electronic excitation at deposition energies > 30eV, forming a multi-body chemical species spur. Thus, we propose that the type and density of chemical species produced by water radiolysis strongly depend on the deposition energy. From our results, we discuss formation of complex DNA damage.

Deciphering the Electronic Coupling Dynamics of Laser-induced Ru/Cu Electrocatalyst for Dual-Side Hydrogen Production and Formic Acid Co-synthesis via DFT Analysis.

Herein, a straightforward approach using pulsed laser technology to synthesize selective hexagonal-close-packed (hcp) Ru nanoparticles attached to Cu nanospheres (Ru/Cu) as bifunctional electrocatalyst for catalyzing the hydrogen evolution reaction (HER) and formaldehyde oxidation reaction (FOR) are reported. Initially, Ru-doped CuO flakes are synthesized using a coprecipitation method followed by transformation into Ru/Cu composites through a strategy involving pulsed laser irradiation in liquid. Specifically, the optimized Ru/Cu-4 composite not only demonstrates a low overpotential of 182mV at 10 mA·cm-2 for the HER but also an ultralow working potential of 0.078V (versusreversible hydrogen electrode) for the FOR at the same current density. Remarkably, the FOR∥HER-coupled electrolyzer employing the Ru/Cu-4∥Ru/Cu-4 system achieves H2 production at both electrodes with a cell voltage of 0.42 V at 10 mA·cm-2 while co-synthesizing formic acid. Furthermore, density functional theory analyses elucidate that the superior activity of the Ru/Cu composite originates from optimized adsorption energies of reactive species on the catalyst surfaces during the HER and FOR, facilitated by the synergistic coupling between Ru and Cu. This study presents an alternative strategy for synthesizing highly effective electrocatalytic materials for use in energy-efficient H2 production with the cosynthesis of value-added chemicals suitable for practical applications.

Time Delays as Attosecond Probe of Interelectronic Coherence and Entanglement.

Attosecond chronoscopy enables the exploration of correlated electron dynamics in real time. One key observable of attosecond physics is the determination of "time zero" of photoionization, the time delay with which the wave packet of the ionized electron departs from the ionic core. This observable has become accessible by experimental advances in attosecond streaking and reconstruction of attosecond beating by interference of two-photon transitions (RABBIT) techniques. In this Letter, we explore photoionization time delays by strong extreme ultraviolet fields beyond the linear-response limit. We identify novel signatures in time delays signifying strong coupling between atoms and light fields and the light-field dressing of the ion. As a prototypical case, we study the interelectronic coherence and entanglement in helium driven by a strong extreme ultraviolet field. By the numerical solution of the time-dependent Schrödinger equation in its full dimensionality, we show that the time delay of the photoionized electron allows one to monitor the ultrafast variations of coherence dynamics and entanglement in real time.

Spectral broadening and vibronic dynamics of the S2 state of canthaxanthin in the orange carotenoid protein

We have performed a series of broadband multidimensional electronic spectroscopy experiments to probe the electronic and vibrational dynamics of the canthaxanthin chromophore of the Orange Carotenoid Protein (OCP) from Synechocystis sp. PCC 6803 in its photoactivated red state, OCPR. Cross-peaks observed below the diagonal of the two-dimensional electronic spectrum indicate that absorption transitions prepare the bright S2 state of the ketocarotenoid canthaxanthin near to a sequence of conical intersections, allowing passage to the dark S1 state via the Sx intermediate in <50 fs. Rapid damping of excited-state coherent wavepacket motions suggests that the branching coordinates of the conical intersections include out-of-plane deformation and C=C stretching coordinates of the π-conjugated isoprenoid backbone. The unusual proximity of the Franck–Condon S2 state structure to the conical intersections with Sx and S1 suggests that the protein surroundings of canthaxanthin prepare it to function as an excitation energy trap in the OCPR–phycobilisome complex. Numerical simulations using the multimode Brownian oscillator model demonstrate that the ground-state absorption spectrum of OCPR overlaps with the fluorescence emission spectrum of allophycocyanin due to spectral broadening derived especially from the intramolecular motions of the canthaxanthin chromophore in its binding site.

Attosecond formation of charge-transfer-to-solvent states of aqueous ions probed using the core-hole-clock technique

Charge transfer between molecules lies at the heart of many chemical processes. Here, we focus on the ultrafast electron dynamics associated with the formation of charge-transfer-to-solvent (CTTS) states following X-ray absorption in aqueous solutions of Na+, Mg2+, and Al3+ ions. To explore the formation of such states in the aqueous phase, liquid-jet photoemission spectroscopy is employed. Using the core-hole-clock method, based on Auger–Meitner (AM) decay upon 1s excitation or ionization of the respective ions, upper limits are estimated for the metal-atom electron delocalization times to the neighboring water molecules. These delocalization processes represent the first steps in the formation of hydrated electrons, which are determined to take place on a timescale ranging from several hundred attoseconds (as) below the 1s ionization threshold to only 20 as far above the 1s ionization threshold. The decrease in the delocalization times as a function of the photon energy is continuous. This indicates that the excited electrons remain in the vicinity of the studied ions even above the ionization threshold, i.e., metal-ion electronic resonances associated with the CTTS state manifolds are formed. The three studied isoelectronic ions exhibit quantitative differences in their electron energetics and delocalization times, which are linked to the character of the respective excited states.

Magnetic Hole and Its Resultant Electron Pitch‐Angle Distribution at Jupiter

AbstractMagnetic hole (MH), sometimes referred to as magnetic bottle, exhibits strong magnetic fields at its neck but weak magnetic fields at its belly. Such structure has been widely reported in the solar wind and the Earth's magnetosphere, but has not been reported at Jupiter. Here, for the first time, we report two MHs in the Jupiter's magnetosphere by utilizing measurements of the Juno mission, with one existing in the dawn side and the other existing in the dusk side. We find that the electron pitch‐angle distribution inside the MHs can be either cigar‐type or pancake‐type. The cigar‐type distribution probably appears at the belly of the MH, whereas the pancake‐type distribution probably appears at the neck. Our analyses of the depression of magnetic fields inside the MHs support such a conjunction. These results have advanced our understanding of the transient structure and its related electron dynamics in the giant planets' magnetosphere.

Abnormal Response of Indirect Excitons to Non‐Uniform Elastic Strain in MoS2 Flakes

AbstractRegulating the electronic structures and carrier dynamics in 2D materials via elastic strain is a fascinating avenue for tailoring their optoelectronic properties at atomic scale. Here, the abnormal response of indirect excitons to non‐uniform elastic strain in MoS2 flakes is reported. The non‐uniform elastic strain in the MoS2 flakes is introduced by transferring them to pre‐prepared convex cross structures on a SiO2/Si substrate, where the topography and strain distribution are determined by atomic force microscopy and Raman spectroscopy, respectively. It is observed that the emission energy of the indirect excitons shows an unexpected blue‐shift followed by a red‐shift with increasing the local tensile strain, which is in sharp contrast to the linear red‐shift of the direct excitons. Density functional theory calculations reveal that the abnormal energy shift of the indirect excitons in the MoS2 flakes arises from the strain‐induced competition between two indirect bandgap transitions and the spatial exciton funnel effect in the non‐uniform strain field. This work provides new insights into the elastic strain effect on the indirect excitons in 2D semiconductors, which possesses potential applications in flexible optoelectronic devices.

Constrained Hamiltonian dynamics for electrons in magnetic field and additional forces besides the Lorentz force acting on electrons

Abstract We consider the forces acting on electrons in magnetic field including the constraints and a condition arising from quantum mechanics. The force is calculated as the electron mass, m e , multiplied by the total time-derivative of the velocity field evaluated using the quantum mechanical many-electron wave function. The velocity field includes a term of the Berry connection from the many-body wave function; thereby, quantum mechanical effects are included. It is shown that additional important forces besides the Lorentz force exist; they include the gradient of the electron velocity field kinetic energy, the gradient of the chemical potential, and the ‘force’ for producing topologically protected loop currents. These additional forces are shown to be important in superconductivity, electric current in metallic wires, and charging of capacitors.

Abnormal Relaxation Behavior of Excited Electrons in the Flat Band of Kagome Compound Nb3Cl8.

Carrier dynamics is crucial in semiconductors, and it determines their conductivity, response time, and overall functionality. In flat bands (FBs), carriers with high effective masses are predicted to host unconventional transport properties. The FBs usually overlap with other trivial energy bands, however, making it difficult to accurately distinguish their carrier dynamics. In this paper, we have investigated the flat-band carrier dynamics of excited electrons in Nb3Cl8, which hosts ideal nonoverlapping FBs near the Fermi level. The optical transition between Hubbard bands is abnormally weakened, exhibiting weak interband absorption and its related slow photoresponse with a time constant of ∼120 s, which are associated with flat-band Mottness-induced large electron effective mass and parity-forbidden transitions. Besides, the localized states created by chlorine vacancies also act as trapping centers for carriers with a time constant of ∼600 s, which are similar to those of the compact localized states of the FB, making the relaxation behavior even more extraordinary. The presence and impacts of atomic defects are confirmed experimentally and theoretically. This work has revealed the abnormal flat-band carrier dynamics of Nb3Cl8, which is essential for understanding the optical, electrical, and thermal transport properties of flat-band materials.

Conformational and Solvent Effects on the Photoinduced Electron Transfer Dynamics of a Zinc Phthalocyanine-Benzoperylenetriimide Conjugate: A Nonadiabatic Dynamics Simulation.

Herein, we employed a combination of static electronic structure calculations and nonadiabatic dynamics simulations at linear-response time dependent density functional theory (LR-TDDFT) level with the optimally tuned range-separated hybrid (OT-RSH) functional to explore the ultrafast photoinduced dynamics of a zinc phthalocyanine-benzoperylenetriimide (ZnPc-BPTI) conjugate. Due to the flexibility of the linker, we identified two major conformations: the stacked conformation (ZnPc-BPTI-1) and the extended conformation (ZnPc-BPTI-2). Since the charge transfer states are much lower than the lowest local excitation in ZnPc-BPTI-1, which is contrary to ZnPc-BPTI-2, the ultrafast electron transfer (~3.6 ps) is only observed in the nonadiabatic simulations of ZnPc-BPTI-1 upon local excitation around the absorption maximum of ZnPc. However, when considering the solvent effects in benzonitrile: the lowest S1 states are both charge transfer states from ZnPc to BPTI for different conformers. Subsequent nonadiabatic dynamics simulations indicate that both conformers experience ultrafast electron transfer in benzonitrile with two time constants of 90 [100] fs and 1.40 [1.43] ps. Our present work not only agrees well with previous experimental study, but also points out the important role of conformational changes and solvent effects in regulating the photodynamics of organic donor-acceptor conjugates.

Atomic-scale imaging of laser-driven electron dynamics in solids

Resolving laser-driven electron dynamics on their natural time and length scales is essential for understanding and controlling light-induced phenomena. Capabilities to reveal these dynamics are limited by challenges in interpreting wave mixing of a driving and a probe pulse, low energy resolution at ultrashort time scales and a lack of atomic-scale resolution by standard spectroscopic techniques. Here, we demonstrate how ultrafast x-ray diffraction can access fundamental information on laser-driven electronic motion in solids. We propose a method based on subcycle-resolved x-ray-optical wave mixing that allows for a straightforward reconstruction of key properties of strong-field-induced electron dynamics with atomic spatial resolution. Namely, this technique provides both phases and amplitudes of the spatial Fourier transform of optically-induced charge distributions, their temporal behavior, and the direction of the instantaneous microscopic optically-induced electron current flow. It captures the rich microscopic structures and symmetry features of laser-driven electronic charge and current density distributions.

Dynamical channel coupling in strong-field ionization of CO2

We present a theoretical study employing the time-dependent density functional theory (TDDFT) to explore the effects of angle-resolved channel coupling in strong field ionization of carbon dioxide (CO2) molecules. Our results reveal significant angular sensitivity of both the channel-resolved ionization probabilities and the effects of laser-induced channel couplings. By applying a linearly polarized two-color field scheme, we demonstrate the ability to significantly modify the strength of the laser-induced coupling, evidenced by the changes in the population distributions among the ionic states induced by the strong-field ionization. Importantly, the two-color field optimally modulates the coupling strength at the alignment angle where ionization of the highest occupied molecular orbital (HOMO) electrons is most efficient. This optimization is attributed to the reduction of the electron shielding effect. Our research provides valuable insights into the coherent manipulation of electron distribution within the cation, paving the way for the precise control of ultrafast electron dynamics during strong-field ionization processes.

Local and nonlocal conditions for electron runaway in a gas gap with a conical cathode with a variable opening angle

The conditions for the generation of runaway electrons in an air gap are compared at different degrees of inhomogeneity of the electric field distribution provided by varying the opening angle of the conical cathode: in the range 40°–120° in experiments and 0°–180° in calculations. It is demonstrated that, in a weakly inhomogeneous electric field (according to the proposed classification, this corresponds to cones with angles greater than the Taylor angle of 98.6°), the runaway condition has a local character. The transition of free electrons into the runaway mode is determined by the local distribution of the electric field near their starting point—the tip of the cone. The local electric field strength must exceed a threshold value comparable to the strength critical for the runaway of electrons in a uniform field. In a strongly inhomogeneous field (cones with angles less than 98.6°), this condition is not sufficient for electrons to run away throughout the gap. Electrons accelerating in the near-cathode region may begin to slow down in a weak field at a distance from the cathode. In this case, the runaway condition becomes nonlocal. It is determined by the dynamics of electrons in the entire gap, primarily in the near-anode region, and reduces to the requirement that the potential difference applied to the gap exceeds a certain threshold value.

Spontaneous asymmetry effect induced by uniform magnetic fields in capacitively coupled plasmas under perfectly symmetric conditions

Abstract Introducing asymmetry in capacitively coupled plasmas (CCPs) is a common strategy for achieving independent control of ion mean energy and flux. Our 1d3v particle-in-cell/Monte Carlo collision simulations reveal that a uniform magnetic field within a specific range can induce spatial asymmetry in low-pressure CCPs, even under perfectly symmetric conditions. This asymmetry, characterized by a shift in the plasma density distribution and significant differences in electron kinetics between the two sides of the plasma, leads to strong ionization and most electron losses on the low-density side, while the high-density side experiences weak ionization and minimal electron losses. The underlying mechanism triggering this spontaneous asymmetry is the differential influence of the magnetic field on low-energy (local) and high-energy (relatively nonlocal) electrons. Under conditions of low pressure and an appropriate magnetic field, this disparity in electron kinetic behavior leads to a spontaneous amplification of the asymmetry induced by random fluctuations until a steady state is reached, culminating in a spontaneous asymmetric effect.