Dynamics Of Electronic Excitations Research Articles

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

Dynamics of electronic excitations involved in laser-induced damage in HfO2 and SiO2 films.

The dynamics of electron excitations associated with the initiation of laser-induced damage in hafnia and silica monolayer films are investigated using time-resolved damage testing involving a pair of 0.7 ps pulses with adjustable delay and laser pulse fluences. Results in hafnia indicate that the relaxation profile depends on the pump-pulse fluence (initial excitation), and as a result, it exhibits an effective lifetime that is variable. Analogous experiments in silica form two different types of damage morphologies that are observed on different ranges of delay times.

Effect of logarithmic perturbations in ohmic like spectral densities in dynamics of electronic excitation using variational polaron transformation approach

Electronic excitation energy transfer is a ubiquitous process that has generated prime research interest since its discovery. Recently developed variational polaron transformation-based second-order master equation is capable of interpolating between Förster and Redfield limits with exceptional accuracy. Forms of spectral density functions studied so far through the variational approach provide theoretical support for various experiments. Recently introduced ohmic like spectral density function that can account for logarithmic perturbations provides generality and exposition to a unique and practical set of environments. In this paper, we exploit the energy transfer dynamics of a two-level system attached to an ohmic like spectral density function with logarithmic perturbations using a variational polaron transformed master equation. Our results demonstrate that even for a relatively large bath coupling strength, quantum coherence effects can be increased by introducing logarithmic perturbations of the order of one and two in super-ohmic environments. Moreover, for particular values of the ohmicity parameter, the effect of logarithmic perturbations is observed to be insignificant for the overall dynamics. In regard to ohmic environments, as logarithmic perturbations increase, damping characteristics of the coherent transient dynamics also increase in general. It is also shown that, having logarithmic perturbations of the order of one in an ohmic environment can result in a less efficient energy transfer for relatively larger system bath coupling strengths.

Ultrafast intraband Auger process in self-doped colloidal quantum dots

Summary Investigating the separate dynamics of electrons and holes has been challenging, although it is critical for the fundamental understanding of semiconducting nanomaterials. n-Type self-doped colloidal quantum dots (CQDs) with excess electrons occupying the low-lying state in the conduction band (CB) have attracted a great deal of attention because of not only their potential applications to infrared optoelectronics but also their intrinsic system that offers a platform for investigating electron dynamics without elusive contributions from holes in the valence band. Here, we show an unprecedented ultrafast intraband Auger process, electron relaxation between spin-orbit coupling states, and exciton-to-ligand vibrational energy transfer process that all occur exclusively in the CB of the self-doped β-HgS CQDs. The electron dynamics obtained by femtosecond mid-infrared spectroscopy will pave the way for further understanding of the blinking phenomenon, disproportionate charging in light-emitting diodes, and hot electron dynamics in higher quantum states coupled to surface states of CQDs.

Intraband dynamics and exciton trapping in the LH2 complex of Rhodopseudomonas acidophila.

Over the last several decades, the light-harvesting protein complexes of purple bacteria have been among the most popular model systems for energy transport in excitonic systems in the weak and intermediate intermolecular coupling regime. Despite this extensive body of scientific work, significant questions regarding the excitonic states and the photo-induced dynamics remain. Here, we address the low-temperature electronic structure and excitation dynamics in the light-harvesting complex 2 of Rhodopseudomonas acidophila by two-dimensional electronic spectroscopy. We find that, although at cryogenic temperature energy relaxation is very rapid, exciton mobility is limited over a significant range of excitation energies. This points to the presence of a sub-200 fs, spatially local energy-relaxation mechanism and suggests that local trapping might contribute substantially more in cryogenic experiments than under physiological conditions where the thermal energy is comparable to or larger than the static disorder.

Photoexcitation dynamics in hybrid associates of Ag2S quantum dots with methylene blue

The dynamics of electronic excitations in hybrid associates of Ag2S quantum dots with methylene blue (MB) molecules are studied by femtosecond transient absorption spectroscopy and nanosecond time-resolved spectroscopy. The situation is considered where the relative positions of the energy levels of the associate components allow photoinduced charge transfer. The luminescence quenching for MB+ cations and Ag2S quantum dots in the hybrid associates provides evidence for charge transfer, which is accompanied by a quick return of MB+ to the ground state (absorption is recovered). We propose that there is electron transfer from the MB LUMO level to the energy level of the luminescence center in the quantum dot. In this case, the bleaching band with a peak at 590 nm corresponding to the MBOH· form of the dye is formed at a subpicosecond time scale. The corresponding decay time is about 100 ps.

Local and Nonlocal Electron Dynamics of Au/Fe/MgO(001) Heterostructures Analyzed by Time-Resolved Two-Photon Photoemission Spectroscopy.

Employing femtosecond laser pulses in front and back side pumping of Au/Fe/MgO(001) combined with detection in two-photon photoelectron emission spectroscopy, we analyze local relaxation dynamics of excited electrons in buried Fe, injection into Au across the Fe-Au interface, and electron transport across the Au layer at 0.6 to 2.0eV above the Fermi energy. By analysis as a function of Au film thickness we obtain the electron lifetimes of bulk Au and Fe and distinguish the relaxation in the heterostructure's constituents. We also show that the excited electrons propagate through Au in a superdiffusive regime and conclude further that electron injection across the epitaxial interface proceeds ballistically by electron wave packet propagation.

Active control of ultrafast electron dynamics in plasmonic gaps using an applied bias

In this joint experimental and theoretical study we demonstrate coherent control of the optical eld emission and electron transport in plasmonic gaps subjected to intense single-cycle laser pulses. Our results show that a small applied dc bias or an external THz eld allows to modulate and direct the electron photocurrents in the gap of a connected nanoantenna operating as an ultra-fast nanoscale vacuum diode for lightwave electronics. Using Time-Dependent Density Functional Theory calculations we elucidate the main physical mechanisms behind the observed eects and show that an applied dc eld modies the optical eld emission and quiver motion of photoemitted electrons within the gap. The quantum many-body theory reproduces the measured net electron transport in the experimental device which allows us to establish a new paradigm for controlling nanocircuits at Petahertz frequencies. The interaction of intense short laser pulses with matter provides access to the dynamics of electronic excita-tions in a highly nonlinear regime characterised by emission of energetic electron bursts of sub-cycle duration and by generation of high harmonics used to track the evolution of the quantum systems at attosecond time scales [1 4]. For metal surfaces and metal nanoparticles, the coupling of light with collective electronic excitations (plas-mons) allows to engineer enhanced optical elds at the hot spots with characteristic sizes well below the dirac-tion limit [5, 6]. Thus, the optical eld emission regime can be reached for incident eld strengths signicantly smaller than those required for molecular and atomic species in the gas phase [4, 7, 8]. In contrast to electron photoemission via multiphoton absorption, optical eld emission can be seen as an electron tunneling at the metal vacuum interface in a situation where the potential barrier is strongly modied by the instantaneous optical eld. The in-depth studies performed for metallic nanotips and plasmonic nanoparticles revealed that optical eld electron emission can be manipulated at femtosecond time scales via the carrier-envelope phase (CEP) of the driving laser pulse [911]. Using THz elds or an applied bias along with the optical excitation oers additional possibilities [1217] of coherent control. In this context, among the plasmonic nanoobjects that can be applied for light wave electronics [1823], the dimer antenna with a nanoscale gap is very relevant. On the one hand, the coupling between electrons and photons in narrow gaps of dimer antennas leads to light emission originating from inelastic electron tunneling events [2427]. On the other hand, the highly nonlinear optical eld electron emission process [19, 22, 23, 28, 29] as well as optically assisted electron tunneling [3032] allow rectication at optical frequencies and CEP control of the electron transport across the junction [21, 33]. In this Letter, we demonstrate coherent control of the net electric current in a nanocircuit comprising a single bowtie nanoantenna with a 6 nm wide gap as presented in Fig. 1. It follows from our results that, along with the CEP of the incident pulse, the petaherz currents of the optically emitted electrons in the gap of the nanoantenna can be directed and controlled by applying a dc eld two orders of magnitude smaller than the optical eld in the junction. Notably, such a dc bias alone does not trigger any electron ow across the gap, i.e. electron tunneling is impossible for this width of the junction. Our study thus extends the possible application of the static or THz elds beyond the control of the electron (photo)emission from metallic tips [1217] and electron tunneling [3436]. Along with dielectric, semiconductor [3739], graphene-based [18, 40], and tunneling [33] structures, the analogue of the ultrafast rectifying vacuum diode demonstrated here (see also [29]) paves the way towards petaherz electronics [41]. The gold bowtie nanoantenna has been fabricated by electron beam lithography on a silica substrate. The two arms of the antenna are interfaced macroscopically (see Fig. 1b) with a transimpedance amplier which allows for a readout of the optically driven currents via a lock-in scheme. An additional DC bias can be applied across the antenna gap by means of a bias tee. The ultrafast currents are driven by the electric eld transients of single-cycle light pulses of a carrier wavelength of 1250 nm and a duration of 4.2 fs. These pulses are generated by a

Density matrix formalism for femtosecond x-ray absorption and its excitonic enhancement

A reduced density matrix (RDM) formalism to simulate the dynamics of core-valence electronic excitation in a metal induced by a femtosecond x-ray laser pulse is presented. The theory is based on the time-dependent unrestricted Hartree–Fock (TDUHF) equation for a one-electron RDM combined with an off-diagonal collision term in the Born approximation (BA), where molecular orbitals (MOs) of a model cluster are used as a basis set. These MOs are calculated with a newly developed semiempirical method in the neglect of diatomic differential overlap approximation that takes into account atomic core structures and valence-orbital hybridization. The off-diagonal component of core-valence RDM is decomposed into a rapidly oscillating factor synchronizing with the x-ray field and a slowly varying envelope; time evolution of the latter yields induced electric polarization and complex electric susceptibility. As a numerical test, K-shell absorption near-edge spectra of copper are computed for a single-shot, weak femtosecond x-ray pulse. It is thereby shown that the attractive interaction between an excited electron and a core hole, described by the exchange part of the self-energy matrix, gives rise to excitonic enhancement of x-ray near-edge absorption. The TDUHF approximation significantly overestimates the excitonic enhancement; additional consideration of the screening of electron-hole interaction by valence electrons in BA leads to a prediction of absorption coefficients comparable to the existing experimental values.

Modeling the conformational preference, spectroscopic properties, UV light harvesting efficiency, biological receptor inhibitory ability and other physico-chemical properties of five imidazole derivatives using quantum mechanical and molecular mechanics tools

Five imidazole derivatives, 1-boc-imidazole (B1), 1-(trifluoroacetyl)imidazole (B2), 1-(2-hydroxyethyl)imidazole (B3), 1-(aminopropyl)imidazole (B4) and 1-ethylimidazole (B5) were analyzed for the structural, nonlinear optical, electronic and biological properties. The functional nature of the compounds were analyzed using vibrational spectra and Raman spectra and was compared with the scaled, simulated spectra obtained using the density functional theory using appropriate functional and basis set with diffused orbitals and are found to be close agreement. Relaxed potential energy scan predicts the stable conformers. Analysis of the outermost molecular orbitals gave their energy difference, aiding in the forecasting stability and other electronic properties. Time dependent density functional theory was used to analyze the electron excitation and de-excitation dynamics of these molecules and to predict the use of these molecules as effective UV photo sensitizers. This work further discusses in detail, the natural bond orbital (NBO) studies for intra molecular stabilization factors, molecular electrostatic potential (MEP) and hyperpolarizability calculations. Further molecular docking studies were conducted for the compounds with Glucanendo-1,6-beta-glucosidase and Protein-glutamatemethylesterase to predict their utility as potential inhibitors.

Femto- to Microsecond Dynamics of Excited Electrons in a Quadruple Cation Perovskite

Quadruple cation mixed halide perovskite, GA0.015Cs0.046MA0.152FA0.787Pb(I0.815Br0.185)3, single crystals were grown for the first time using an inverse temperature crystallization process. Solar c...

Electronic relaxation dynamics in [Au25(SR)18]-1 (R = CH3, C2H5, C3H7, MPA, PET) thiolate-protected nanoclusters.

We investigate the excited electron dynamics in [Au25(SR)18]-1 (R = CH3, C2H5, C3H7, MPA, PET) [MPA = mercaptopropanoic acid, PET = phenylethylthiol] nanoparticles to understand how different ligands affect the excited state dynamics in this system. The population dynamics of the core and higher excited states lying in the energy range 0.00-2.20 eV are studied using a surface hopping method with decoherence correction in a real-time DFT approach. All of the ligated clusters follow a similar trend in decay for the core states (S1-S6). The observed time constants are on the picosecond time scale (2-19 ps), which agrees with the experimental time scale, and this study confirms that the time constants observed experimentally could originate from core-to-core transitions and not from core-to-semiring transitions. In the presence of higher excited states, R = H, CH3, C2H5, C3H7, and PET demonstrate similar relaxations trends whereas R = MPA shows slightly different relaxation of the core states due to a smaller gap between the LUMO+1 and LUMO+2 gap in its electronic structure. The S1 (HOMO → LUMO) state gives the slowest decay in all ligated clusters, while S7 has a relatively long decay. Furthermore, separate electron and hole relaxations were performed on the [Au25(SCH3)18]-1 nanocluster to understand how independent electron and hole relaxations contribute to the overall relaxation dynamics.

Femtosecond dynamics of photoexcitation in hybrid systems of CdS quantum dots with methylene blue

Abstract The dynamics of electronic excitation in hybrid associates of colloidal CdS quantum dots passivated by thioglycolic acid (CdS/TGA) with methylene blue cationic dye molecules (MB + ) was studied by femtosecond transient absorption spectroscopy and nanosecond time-resolved spectroscopy. From the luminescence decays it was found that non-radiative resonance energy transfer (FRET) from the recombination luminescence centers accompanied by acceptor fluorescence is observed in the CdS/TGA-MB + systems with MB + monomers. It was found that in the exciton decay dynamics of the hybrid systems based on CdS/TGA QDs and methylene blue, there is substantial competition between the resonance energy transfer in MB + and the energy transfer to MB • and MBOH • forms.

First-Principles Modeling of Electronic Stopping in Complex Matter under Ion Irradiation.

Electronic stopping refers to the dynamical energy-transfer process to electrons in matter from highly energetic charged particles such as high-velocity protons. We discuss recent progress in theoretical studies of electronic stopping in condensed matter under ion irradiation, focusing on modern electronic structure theory's role in enabling the study of electronic excitation dynamics that result from the energy transfer. In the last few decades, first-principles simulation approaches based on real-time time-dependent density functional theory have greatly advanced the field. While linear response theory is widely used to study electronic stopping processes, especially for simple solids, novel first-principles dynamics approaches now allow us to study chemically complex systems and also yield detailed descriptions of electronic excitations at the molecular scale. Outstanding challenges for further advancement of electronic stopping modeling are also discussed from the viewpoint of electronic structure theory.

Correction to "Electronic Excitation Dynamics in DNA under Proton and α-Particle Irradiation".

Correction to "Electronic Excitation Dynamics in DNA under Proton and α-Particle Irradiation".

Electron dynamics method using a locally projected group diabatic Fock matrix for molecules and aggregates

We propose a method using reduced size of Hilbert space to describe an electron dynamics in molecule and aggregate based on our previous theoretical scheme (Yonehara and Nakajima, 2017). The real-time time-dependent density functional theory is combined with newly introduced projected group diabatic Fock matrix. First, this projection method is applied to a test donor-acceptor dimer, namely, a naphthalene-tetracyanoethylene with and without initial local excitations and light fields. Secondly, we calculate an absorption spectrum of five-unit-polythiophene monomer. The importance of feedback of instantaneous density to Fock matrix is also clarified. In all cases, half of the orbitals were safely reduced without loss of accuracy in descriptions of properties. The present scheme provides one possible way to investigate and analyze a complex excited electron dynamics in molecular aggregates within a moderate computational cost.

Enhanced Radiative Recombination of Excitons and Free Charges Due to Local Deformations in the Band Structure of MAPbBr3 Perovskite Crystals

We have imaged the emissions from excitons and free charges in a methylammonium lead bromide perovskite (MAPbBr3) crystal. In a direct band gap semiconductor, dynamics of excited electrons and holes in hybrid lead-halide perovskites is rather complex because of the formation of excitons and the presence of traps and structural inhomogeneities. A recent report by Nah et al. (Nah, S.; Spokoyny, B.; Stoumpos, C.; Soe, C. M. M.; Kanatzidis, M.; Harel, E. Nat. Photonics 2017, 11, 285-288) has identified spatially segregated populations of excitons and free charges in perovskites. Understanding the cause of segregation and how such distributions contribute to the photoresponse has remained a challenge. Here, we have used phase-modulated two-photon photoluminescence microspectroscopy to separately quantify the emissions from excitons and free charges in a MAPbBr3 crystal. Our results show that while most of the emission at room temperature is from the recombination of free charges, there are also localized spots where the emissions from excitons dominate. By analyzing the enhancement of the emissions at low temperatures, we show that the trap-mediated nonradiative recombinations are suppressed by local deformations in the band structure. The deformations that are induced by local strains in the crystal prevent excitons and free charges from reaching the traps. Our results indicate that strain engineering through structural designs can improve the performance of perovskite light-emitting diodes. (Less)