Dynamics Of Electronic Excitations Research Articles

Optical Properties of H-Bonded Heterotriangulene Supramolecular Polymers: Charge-Transfer Excitations Matter.

H-bonded N-heterotriangulene (NHT) supramolecular polymers offer a nice playground to explore the nature and dynamics of electronic excitations in low-dimensional organic nanostructures. Here, we report on a comprehensive molecular modeling of the excited-state electronic structure and optical properties of model NHT stacks, highlighting the important role of intermolecular charge-transfer (CT) excitations in shaping their optical absorption and emission lineshapes. Most importantly, we show that the coupling between the local and CT excitations, modulated by the electric fields induced by the presence of polar amide groups forming H-bonded arrays along the stacks, significantly increases the resulting hybrid exciton bandwidth. We discuss these findings in the context of the efficient transport of singlet excitons over the μm length scale reported experimentally on individual self-assembled nanofibers with molecular-scale diameter.

Unveiling large charge transfer character of PSII in an iron-deficient cyanobacterial membrane: A Stark fluorescence spectroscopy study.

In this work, we applied Stark fluorescence spectroscopy to an iron-stressed cyanobacterial membrane to reveal key insights about the electronic structures and excited state dynamics of the two important pigment-protein complexes, IsiA and PSII, both of which prevail simultaneously within the membrane during iron deficiency and whose fluorescence spectra are highly overlapped and hence often hardly resolved by conventional fluorescence spectroscopy. Thanks to the ability of Stark fluorescence spectroscopy, the fluorescence signatures of the two complexes could be plausibly recognized and disentangled. The systematic analysis of the SF spectra, carried out by employing standard Liptay formalism with a realistic spectral deconvolution protocol, revealed that the IsiA in an intact membrane retains almost identical excited state electronic structures and dynamics as compared to the isolated IsiA we reported in our earlier study. Moreover, the analysis uncovered that the excited state of the PSII subunit of the intact membrane possesses a significantly large CT character. The observed notably large magnitude of the excited state CT character may signify the supplementary role of PSII in regulative energy dissipation during iron deficiency.

Luminescent Radicals.

Organic radicals are attracting increasing interest as a new class of molecular emitters. They demonstrate electronic excitation and relaxation dynamics based on their doublet or higher multiplet spin states, which are different from those based on singlet-triplet manifolds of conventional closed-shell molecules. Recent studies have disclosed luminescence properties and excited state dynamics unique to radicals, such as highly efficient electron-photon conversion in OLEDs, NIR emission, magnetoluminescence, an absence of heavy atom effect, and spin-dependent and spin-selective dynamics. These are difficult or sometimes impossible to achieve with closed-shell luminophores. This review focuses on luminescent organic radicals as an emerging photofunctional molecular system, and introduces the material developments, fundamental properties including luminescence, and photofunctions. Materials covered in this review range from monoradicals, radical oligomers, and radical polymers to metal complexes with radical ligands demonstrating radical-involved emission. In addition to stable radicals, transiently formed radicals generated in situ by external stimuli are introduced. This review shows that luminescent organic radicals have great potential to expand the chemical and spin spaces of luminescent molecular materials and thus broaden their applicability to photofunctional systems.

Dissecting the nature and dynamics of electronic excitations in a solid-state aggregate of a representative non-fullerene acceptor

Understanding electronic excitations and their dynamics in non-fullerene acceptors is crucial for enhancing opto-electronic properties. Using a Frenkel-exciton Hamiltonian and non-adiabatic dynamics, we reveal design strategies to achieve this goal.

First-Principles Approach for Coupled Quantum Dynamics of Electrons and Protons in Heterogeneous Systems.

The coupled quantum dynamics of electrons and protons is ubiquitous in many dynamical processes involving light-matter interaction, such as solar energy conversion in chemical systems and photosynthesis. A first-principles description of such nuclear-electronic quantum dynamics requires not only the time-dependent treatment of nonequilibrium electron dynamics but also that of quantum protons. Quantum mechanical correlation between electrons and protons adds further complexity to such coupled dynamics. Here we extend real-time nuclear-electronic orbital time-dependent density functional theory (RT-NEO-TDDFT) to periodic systems and perform first-principles simulations of coupled quantum dynamics of electrons and protons in complex heterogeneous systems. The process studied is an electronically excited-state intramolecular proton transfer of o-hydroxybenzaldehyde in water and at a silicon (111) semiconductor-molecule interface. These simulations illustrate how environments such as hydrogen-bonding water molecules and an extended material surface impact the dynamical process on the atomistic level. Depending on how the molecule is chemisorbed on the surface, excited-state electron transfer from the molecule to the semiconductor surface can inhibit ultrafast proton transfer within the molecule. This Letter elucidates how heterogeneous environments influence the balance between the quantum mechanical proton transfer and excited electron dynamics. The periodic RT-NEO-TDDFT approach is applicable to a wide range of other photoinduced heterogeneous processes.

Imaging surfaces at the space–time limit: New perspectives of time-resolved scanning tunneling microscopy for ultrafast surface science

Many fundamental processes in nature occur on ultrashort time scales within picoseconds to attoseconds, and on intrinsic length scales from nanometers to picometers. The structure of crystalline solids is dictated by long range order and the periodic arrangement of atoms, but the elementary excitations that define its interaction with the environment may vary locally at the atomic scale. Multiple domains and phases can coexist on length scales down to a few nanometer, and impurities and defects can influence the collective many-body response of solids at the single-atom level. Ultrafast pump–probe techniques provide valuable information about fundamental many-body interactions in solids and at surfaces, but spatially average over macroscopic spot sizes such that the influence of local order or disorder at angstrom scales is not directly accessible. Therefore, real-space observation of ultrafast dynamics with atomic spatial resolution is highly desirable, and motivates the development of time-resolved ultrafast scanning tunneling microscopy (USTM) since the early 1990’s. Tremendous progress has been made in this field in the past decade, and a number of breakthrough achievements have significantly advanced our possibilities to add ultrafast time resolution to the angstrom spatial resolution of STM. This article reviews new technical approaches and developments in the field of USTM. A particular focus will be the classification of light-matter interaction in tunnel junctions, based on the criteria for adiabatic tunneling from Keldysh's theory of strong-field ionization and a tunneling time as defined by Büttiker and Landauer, and on Tucker's definition of quantum detection in a tunnel junction mixer. Moreover, various mechanisms to generate an ultrafast tunneling current in USTM are discussed and are to some extent related to those from other techniques such as optical spectroscopy or photoemission spectroscopy. The resulting new possibilities for imaging the ultrafast dynamics of electronic and vibrational excitations at surfaces with USTM will be highlighted. Finally, the article outlines possible future directions of USTM for studying ultrafast processes and light-induced phenomena at surfaces and in quantum materials.

Excited state electronic structure and dynamics in diblock π-conjugated oligomers

The excited state electronic structure and ultrafast energy transfer in two diblock oligomers F3-TBT and F3-mP-TBT was explored (F3 = tri-(9,9-dioctylfluorene), TBT = 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole, mP = meta-phenylene). These diblock oligomers feature strongly coupled, π-conjugated F3 and TBT segments that have different bandgaps. F3-TBT is a fully π-conjugated chromophore in which the two conjugated segments are directly linked. By contrast, in F3-mP-TBT the chromophores are separated by a meta-linked phenylene unit. The objective of this study was to discern whether the Franck-Condon excited state produced by selective excitation of the F3 moiety in fully conjugated F3-TBT gives rise to a localized or fully delocalized excited state. The oligomers’ excited state dynamics were probed by steady-state UV–visible absorption, fluorescence and excitation wavelength dependent ultrafast time resolved transient absorption (TA) spectroscopy. Density functional theory (DFT) calculations were applied to provide insight concerning the electronic structure of the oligomers. The diblock oligomers feature distinct absorption bands due to transitions mainly localized on the F3 (high energy, 3.5 eV) and TBT (low energy, 2.75 eV) segments. Ultrafast TA spectroscopy with excitation corresponding to the low energy TBT unit reveals that the initial excitation is localized on TBT and does not vary with time. By contrast, in both oligomers excitation corresponding to high energy F3 produces an excited state that evolves rapidly (200–300 fs) into the lower energy state that is localized on the TBT chromophore. The ultrafast process is attributed to relaxation of an excited state that is predominantly localized on F3 into a lower energy state that is localized on TBT. The observation of an F3-localized excited state is unexpected for F3-TBT, given that the molecule is fully π-conjugated.

Ultrafast Laser Control of Antiferromagnetic-Ferrimagnetic Switching in Two-Dimensional Ferromagnetic Semiconductor Heterostructures.

Realizing ultrafast control of magnetization switching is of crucial importance for information processing and recording technology. Here, we explore the laser-induced spin electron excitation and relaxation dynamics processes of CrCl3/CrBr3 heterostructures with antiparallel (AP) and parallel (P) systems. Although an ultrafast demagnetization of CrCl3 and CrBr3 layers occurs in both AP and P systems, the overall magnetic order of the heterostructure remains unchanged due to the laser-induced equivalent interlayer spin electron excitation. More crucially, the interlayer magnetic order switches from antiferromagnetic (AFM) to ferrimagnetic (FiM) in the AP system once the laser pulse disappears. The microscopic mechanism underpinning this magnetization switching is dominated by the asymmetrical interlayer charge transfer combined with a spin-flip, which breaks the interlayer AFM symmetry and ultimately results in an inequivalent shift in the moment between two FM layers. Our study opens up a new idea for ultrafast laser control of magnetization switching in two-dimensional opto-spintronic devices.

Photoluminescence in Cerium-Doped Fluoride Borate Crystals

Antizeolite fluoride borates LiBa12(BO3)7F4 (LBBF) doped by Ce3+ ions demonstrate photoluminescence (PL) and hold promise for use as phosphors in white light-emitting diodes. In this study, we present the results of modeling the electronic and optical properties of Ce-containing LBBF structures where doping atoms are located at different sites of the host lattice. We also consider the presence of F-centers, which exhibit trap states located within the band gap. We further investigate the nonadiabatic excited state electronic dynamics to elucidate electron-relaxation pathways. The nonadiabatic couplings (NACs) calculations provide transition probabilities facilitated by the nuclear movement. The relaxation rates of electrons and holes are calculated using Redfield’s theory of the formalism of the reduced density matrix (RDM). The PL spectra are calculated using molecular dynamics (MD) sampling and time-integrated methods along the excited state trajectory based on NACs. Mechanisms of PL in LBBF:Ce3+ crystals are interpreted using both computational and experimental observations. This work illustrates the dependence of transition energies, intensities, and relaxation rates of Ce-containing LBBF crystals on a selection of doping sites. In addition to analysis of emission band contributed by cerium, this work allows to identify and reproduce spectral lines hypothetically corresponding to the interband and intraband transitions in F-centers of borate crystals, available in the absence of a metal center.

Application of artificial neural networks for modeling of electronic excitation dynamics in 2D lattice: Direct and inverse problems

Machine learning (ML) approaches are attracting wide interest in the chemical physics community since a trained ML system can predict numerical properties of various molecular systems with a small computational cost. In this work, we analyze the applicability of deep, sequential, and fully connected neural networks (NNs) to predict the excitation decay kinetics of a simple two-dimensional lattice model, which can be adapted to describe numerous real-life systems, such as aggregates of photosynthetic molecular complexes. After choosing a suitable loss function for NN training, we have achieved excellent accuracy for a direct problem—predictions of lattice excitation decay kinetics from the model parameter values. For an inverse problem—prediction of the model parameter values from the kinetics—we found that even though the kinetics obtained from estimated values differ from the actual ones, the values themselves are predicted with a reasonable accuracy. Finally, we discuss possibilities for applications of NNs for solving global optimization problems that are related to the need to fit experimental data using similar models.

Metalated porphyrin-napthalimide based donor-acceptor systems with long-lived triplet states and effective three-photon absorption

• Excited state electron dynamics of novel Metalated porphyrins-napthalimide molecules was investigated by femtosecond transient absorption spectroscopy (fs-TAS). • The rate constants of excited states are estimated using the global analysis of the obtained fs-TAS data. • Long-lived triplet state (lifetime ∼10 μ s for PN-Zn) formed via intersystem crossing. • Open aperture z-scan at non-resonant excitation of 800 nm revealed strong three-photon absorption by these molecules. • The experimental second hyperpolarizability was estimated to be ∼ 10 -31 esu and verified by DFT calculations. Porphyrin applications are primarily related to electronic transitions across a broad spectrum. In this context, understanding the excited state electron dynamics is necessary. Herein, we report the ultrafast photophysical characterization of one free base and three transition metallated porphyrin - napthalimide based donor - acceptor systems ( PN-Fb , PN-Ni , PN-Cu , and PN-Zn ) in dichloromethane (DCM) solution using transient absorption spectroscopy (TAS) in the spectral range of 430-780 nm by the Soret band excitation at 400 nm. The different rate constants were extracted using a target model analysis of the collected transient absorption spectra and established a consistent photophysical model of molecular excited state population relaxations. The photophysical model consisted of the following various processes: (a) internal conversion in the range of 215-400 fs, (b) vibrational relaxation in the range of 1.35-62 ps, and (c) singlet state relaxation times in the range of 0.17-2.06 ns, and finally (d) 0.03-10 μs was attributed to the triplet state lifetimes. We have also measured the nonlinear optical properties of these molecules by the open and closed aperture Z-scan studies, which was carried out by 800 nm, 70 fs photoexcitation. The theoretical fittings to the experimental data resulted in an effective three-photon absorption coefficient and positive nonlinear refractive index. Further, from the Z-scan data, the second-order hyperpolarizability of the molecules was calculated, and the values were 10 - 31 esu. Additionally, complete optimization of individual structures and construction of these PN systems was achieved using density functional theory (DFT) calculations and the obtained results were found to be consistent with the experimental observations.

Atomic structures and carrier dynamics of defects in a ZnGeP2 crystal

ZnGeP2 (ZGP) crystals have attracted tremendous attention for their applications as frequency conversion devices. Nevertheless, the existence of native point defects, including at the surface and in the bulk, lowers their laser-induced damage threshold by increasing their absorption and forming starting points of the damage, limiting their applications. Here, native point defects in a ZGP crystal are fully studied by the combination of high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and optical measurements. The atomic structures of the native point defects of the Zn vacancy, P vacancy, and Ge-Zn antisite were directly obtained through an HAADF-STEM, and proved by photoluminescence (PL) spectra at 77 K. The carrier dynamics of these defects are further studied by ultrafast pump-probe spectroscopy, and the decay lifetimes of 180.49, 346.73, and 322.82 ps are attributed to the donor Vp+ → valence band maximum (VBM) recombination, donor GeZn+ → VBM recombination, and donor–acceptor pair recombination of Vp+ → VZn-, respectively, which further confirms the assignment of the electron transitions. The diagrams for the energy bands and excited electron dynamics are established based on these ultrahigh spatial and temporal results. Our work is helpful for understanding the interaction mechanism between a ZGP crystal and ultrafast laser, doing good to the ZGP crystal growth and device fabrication.

Diagnostics of a nanosecond atmospheric plasma jet. Electron and ro-vibrational excitation dynamics

Atmospheric repetitive He discharge with 10 ns current peak width and V s−1 voltage front rise working in jet geometry is studied. The first part of the study is devoted to electrical and optical discharge characterization including voltage-current behavior, emission dynamics, as well as ro-vibrational dynamics of N2, N and OH molecules. It is found that He atoms get excited at the very early stage, as a result of ionization wave formation. This process follows by excitation of O, N2 and N. It is also shown that a rather small (0.1%–1%) air admixtures facilitate gas breakdown, as revealed by shortening of the discharge current risetime. The rotational excitation of N always overtakes He excitation by about 5 ns, with rotational temperature peaking at about 650 K and decaying afterwards, whereas rotational temperature of N2 always remains constant equal to about 300 K. This value is associated with gas kinetic temperature since the electron-rotational (e-R) excitation process is too slow, so it cannot be activated during the plasma phase and no additional change in rotational excitation of N2 happens. Nitrogen molecules remain vibrationally excited in the discharge, post-discharge and jet areas after the plasma pulse which is likely a result of ionization wave propagation. Upon water vapor injection the apparent OH rotational distributions reveal double Boltzmann slope representing thermalized and non-thermalized OH groups. Rotational temperature of the thermalized group correlates with the one of N showing, however, much longer relaxation time, whereas for the non-thermalized group it remains above 1 eV at all conditions. The obtained results confirm that the studied ns- discharge is a good candidate for temperature-sensitive applications, such as the bio-sample treatment. Further analysis related to the ionization waves, electron density and electric field behavior is undertaken in the second part.

Ultrafast transport and energy relaxation of hot electrons in Au/Fe/MgO(001) heterostructures analyzed by linear time-resolved photoelectron spectroscopy

In condensed matter, scattering processes determine the transport of charge carriers. In case of heterostructures, interfaces determine many dynamic properties like charge transfer and transport and spin current dynamics. Here we discuss optically excited electron dynamics and their propagation across a lattice-matched, metal-metal interface of single crystal quality. Using femtosecond time-resolved linear photoelectron spectroscopy upon optically pumping different constituents of the heterostructure we establish a technique which probes the electron propagation and its energy relaxation simultaneously. In our approach a near-infrared pump pulse excites electrons directly either in the Au layer or in the Fe layer of epitaxial Au/Fe/MgO(001) heterostructures while the transient photoemission spectrum is measured by an ultraviolet probe pulse on the Au surface. Upon femtosecond laser excitation, we analyze the relative changes in the electron distribution close to the Fermi energy and assign characteristic features of the time-dependent electron distribution to transport of hot and non-thermalized electrons from the Fe layer to the Au surface and vice versa. From the measured transient electron distribution we determine the excess energy which we compare with a calculation based on the two-temperature model and takes diffusive electron transport into account. On this basis we identify a transition from a super-diffusive to a diffusive transport regime to occur for a Au layer thickness of 20-30~nm.

Exploring far-from-equilibrium ultrafast polarization control in ferroelectric oxides with excited-state neural network quantum molecular dynamics.

Ferroelectric materials exhibit a rich range of complex polar topologies, but their study under far-from-equilibrium optical excitation has been largely unexplored because of the difficulty in modeling the multiple spatiotemporal scales involved quantum-mechanically. To study optical excitation at spatiotemporal scales where these topologies emerge, we have performed multiscale excited-state neural network quantum molecular dynamics simulations that integrate quantum-mechanical description of electronic excitation and billion-atom machine learning molecular dynamics to describe ultrafast polarization control in an archetypal ferroelectric oxide, lead titanate. Far-from-equilibrium quantum simulations reveal a marked photo-induced change in the electronic energy landscape and resulting cross-over from ferroelectric to octahedral tilting topological dynamics within picoseconds. The coupling and frustration of these dynamics, in turn, create topological defects in the form of polar strings. The demonstrated nexus of multiscale quantum simulation and machine learning will boost not only the emerging field of ferroelectric topotronics but also broader optoelectronic applications.

Shallow and deep trap states of solvated electrons in methanol and their formation, electronic excitation, and relaxation dynamics.

We present condensed-phase first-principles molecular dynamics simulations to elucidate the presence of different electron trapping sites in liquid methanol and their roles in the formation, electronic transitions, and relaxation of solvated electrons (emet−) in methanol. Excess electrons injected into liquid methanol are most likely trapped by methyl groups, but rapidly diffuse to more stable trapping sites with dangling OH bonds. After localization at the sites with one free OH bond (1OH trapping sites), reorientation of other methanol molecules increases the OH coordination number and the trap depth, and ultimately four OH bonds become coordinated with the excess electrons under thermal conditions. The simulation identified four distinct trapping states with different OH coordination numbers. The simulation results also revealed that electronic transitions of emet− are primarily due to charge transfer between electron trapping sites (cavities) formed by OH and methyl groups, and that these transitions differ from hydrogenic electronic transitions involving aqueous solvated electrons (eaq−). Such charge transfer also explains the alkyl-chain-length dependence of the photoabsorption peak wavelength and the excited-state lifetime of solvated electrons in primary alcohols.

Visualizing coherent vibrational motion in the molecular iodine B3Π0+u state using ultrafast XUV transient-absorption spectroscopy

Attosecond probing of core-level electronic transitions in molecules provides a sensitive tool for real-time observation of chemical dynamics. Here, we employ ultrafast extreme-ultraviolet (XUV) transient-absorption spectroscopy to investigate the excited state electronic and nuclear dynamics in a prototype molecule, ${\mathrm{I}}_{2}$. A few-femtosecond visible pump pulse is employed to excite the ${\mathrm{I}}_{2}$ molecule and an attosecond XUV pulse is used to probe the dynamics through iodine-$4d$ core-to-valence transitions. A highly extended vibrational wave packet $({\ensuremath{\nu}}^{\ensuremath{'}}=10--50,\phantom{\rule{0.28em}{0ex}}{\ensuremath{\nu}}_{max}^{\ensuremath{'}}=25)$ is prepared by one-photon absorption in the valence excited $B\phantom{\rule{0.28em}{0ex}}{}^{3}{\mathrm{\ensuremath{\Pi}}}_{{{0}^{+}}_{\mathrm{u}}}$ state of ${\mathrm{I}}_{2}$ and its motion is directly mapped due to the strong shift of the XUV core-level transition with internuclear separation. Through the imaging of this vibrational motion, we directly reconstruct the transition energy between the valence and the core-excited states as a function of internuclear distance. Besides single-photon dynamics, distinct direct dissociation pathways arising from two-photon pump absorption are also revealed.

Genesis of Nanogratings in Silica Bulk via Multipulse Interplay of Ultrafast Photo‐Excitation and Hydrodynamics

AbstractStructuring below diffraction limit is key to developing new laser processing technologies as well as to understanding light‐induced processes on mesoscopic scales, notably self‐organization. Here, an advanced numerical perspective on the generation of embedded self‐arranged sub‐wavelength periodic patterns is developed, describing multipulse ultrafast laser interaction with bulk silica glass. Combining light and material dynamics, the approach couples self‐consistently nonlinear propagation, electronic excitation, and fluid dynamics resulting in irreversible phase transitions and localized damage. With increasing the number of applied pulses, the modification changes from localized nanovoids and elongated random nanopatterns toward regular void nanogratings dominantly covering the spot of the focused laser beam. Driven by local and collective scattering events, the order imposed by electric field patterns is then amplified and stabilized by the material response. The model predicts the gradual evolution of the optical properties considering the complex interplay between material arrangement and the electromagnetic field distribution. It allows thus to define light transport optical functions optimizing losses and anisotropic effects.

Linear and nonlinear excitation induced ultrafast absorption dynamics in laser ablated and chemically synthesized gold nanoparticle colloids

Abstract Study of excited state electron dynamics on gold nanoparticles produced by femtosecond laser ablation and chemical synthesis routes are conducted through the pump-probe technique under nonlinear (800 nm, two-photon region) and linear (350 nm, one-photon region) excitations. The temporally delayed broadband probe in the plasmonic region signifies the occurrence of dominant ground state bleach and photo-induced absorption. The excited state electrons, specifically those corresponding to below-resonant excitation, undergo relaxation as a brisk tapering curve due to electron-electron coupling followed by a gradual rise attributed to electron-phonon coupling events. The gold colloids show longer duration of electron-phonon process at 350 nm excitation than at 800 nm with an almost linear dependence of decay time with increasing excitation energy. The resonant excitation regime of chemically synthesized sample shows a visible red-shift compared to their laser ablated counterparts. This study proves efficient in better understanding energy dependent electron dynamics at different excitation wavelengths.

Excited electron dynamics in the interface of 2H-1T hetero-phases of monolayer MoS2: time-dependent density functional theory study

Excited electron dynamics in the interface of 2H-1T hetero-phases of monolayer MoS2: time-dependent density functional theory study