Dynamics Of Electronic Excitations Research Articles

Femtosecond dynamics of momentum-dependent magnetic excitations from resonant inelastic X-ray scattering in CaCu2O3.

Taking spinon excitations in the quantum antiferromagnet CaCu2O3 as an example, we demonstrate that femtosecond dynamics of magnetic electronic excitations can be probed by direct resonant inelastic x-ray scattering (RIXS). To this end, we isolate the contributions of single and double spin-flip excitations in experimental RIXS spectra, identify the physical mechanisms that cause them, and determine their respective time scales. By comparing theory and experiment, we find that double spin flips need a finite amount of time to be generated, rendering them sensitive to the core-hole lifetime, whereas single spin flips are, to a very good approximation, independent of it. This shows that RIXS can grant access to time-domain dynamics of excitations and illustrates how RIXS experiments can distinguish between excitations in correlated electron systems based on their different time dependence.

Spin-dependent lifetimes and nondegenerate spin-orbit driven hybridization points in Heusler compounds

We present an ab initio calculation of the $\stackrel{P\vec}{k}$- and spin-resolved electronic lifetimes in the half-metallic Heusler compounds ${\mathrm{Co}}_{2}\mathrm{Mn}\mathrm{Si}$ and ${\mathrm{Co}}_{2}\mathrm{Fe}\mathrm{Si}$. We determine the spin-flip and spin-conserving contributions to the lifetimes and study in detail the behavior of the lifetimes around states that are strongly spin mixed by spin-orbit coupling. We find that for nondegenerate bands the spin mixing alone does not determine the energy dependence of the (spin-flip) lifetimes. Qualitatively, the lifetimes reflect the lineup of electron and hole bands. We predict that different excitation conditions lead to drastically different spin-flip dynamics of excited electrons and may even give rise to an enhancement of the nonequilibrium spin polarization.

Predicting Energy Conversion Efficiency of Dye Solar Cells from First Principles

In this work we target on accurately predicting energy conversion efficiency of dye-sensitized solar cells (DSC) using parameter-free first principles simulations. We present a set of algorithms, mostly based on solo first principles calculations within the framework of density functional theory, to accurately calculate key properties in energy conversion including sunlight absorption, electron injection, electron–hole recombination, open circuit voltages, and so on. We choose two series of donor-π-acceptor dyes with detailed experimental photovoltaic data as prototype examples to show how these algorithms work. Key parameters experimentally measured for DSC devices can be nicely reproduced by first-principles with as less empirical inputs as possible. For instance, short circuit current of model dyes can be well reproduced by precisely calculating their absorption spectra and charge separation/recombination rates. Open circuit voltages are evaluated through interface band offsets, namely, the difference between the Fermi level of electrons in TiO2 and the redox potential of the electrolyte, after modification with empirical formulas. In these procedures the critical photoelectron injection and recombination dynamics are calculated by real-time excited state electronic dynamics simulations. Estimated solar cell efficiency reproduces corresponding experimental values, with errors usually below 1–2%. Device characteristics such as light harvesting efficiency, incident photon-to-electron conversion efficiency, and the current–voltage characteristics can also be well reproduced and compared with experiment. Thus, we develop a systematic ab initio approach to predict solar cell efficiency and photovoltaic performance of DSC, which enables large-scale efficient dye screening and optimization through high-throughput first principles calculations with only a few parameters taken from experimental settings for electrode and electrolyte toward a renewable energy based society.

Femtosecond electron-lattice thermalization dynamics in a gold film probed by pulsed surface plasmon resonance

The dynamics of electronic excitations and their relaxation in a gold film is studied on the femtosecond time scale with a pump-probe technique. For the pump beam we use pulses with wavelengths centered at 800 nm, 400 nm or both. The surface plasmon resonance (SPR) in Kretschmann's configuration is used as a sensitive and fast-response probe of the dynamics of the dielectric properties of the gold film. The quantity that is monitored is the intensity of the reflected light at an incidence angle close to the SPR. With changes of the dielectric properties induced by the pump beam and during subsequent relaxation, the amount of the reflected light of the probe beam, sent with a variable delay, also changes, thus providing information on the temporal characteristics of the thermalization process. Special features of SPR probing with short pulses are also accounted for in this work. The thermalization of the electronic subsystem and energy transfer to the lattice are discussed in connection with the two-temperature relaxation model that takes into account temperature dependences of the electronic heat capacity and the electron-phonon coupling.

Tracking the relaxation pathway of photo-excited electrons in 1T-TiSe2

The ultrafast dynamics of excited electrons in 1T-TiSe2 after absorption of a 390 nm light pulse is probed by time- and angle-resolved photoemission spectroscopy using femtosecond XUV pulses. It is demonstrated that the experimental approach can provide a comprehensive view of hot carrier motion in momentum space during relaxation back to equilibrium. This capability opens a new avenue in the investigation of energy dissipation processes in solids after intense optical excitation.

Pulse‐Train Photoelectron Spectroscopy of Electronic and Nuclear Dynamics in Molecules

We theoretically study the application of a femtosecond pulse train to simultaneously probe the electronic excitation dynamics and nuclear vibrational motion in the excited-state LiH molecule by means of time-resolved photoelectron spectroscopy. The step-like population transfers caused by continual interaction with a sequence of pulses and refractory periods are shown to give rise to time evolution of the photoelectron kinetic energy distribution as the history of molecular electronic and vibrational states. Such a signal should lead to a novel and direct way to investigate electronic and nuclear simultaneous dynamics involving multiple excited states.

Excited electron dynamics at ferrocene-terminated self-assembled monolayers on Au(1 1 1): Lengthened lifetime of image potential state

Ultraviolet photoemission spectroscopy (UPS) and two-photon photoemission (2PPE) spectroscopy have been employed to investigate the electronic states and the excited electron dynamics of ferrocene (Fc)-terminated self-assembled monolayers (SAMs) on Au(111). For the 11-ferrocenyl-1-undecanethiol (FcC11-) SAM, an image potential state (IPS) located at EF +3.8eV was observed by time-resolved 2PPE. Its lifetime, 90fs, was found to be much shorter than that of decanethiol (C10-) SAM, 11ps. By forming the C10-SAM beforehand, the IPS lifetime for FcC11-&C10-SAM was lengthened to ∼9ps; this can enable improvement in molecular ordering and substantial formation of C10 domains.

Systematic investigation of the structure and photophysical properties of CdSe, CdSe/ZnS QDs and their hybrid with β-carotene

A systematic investigation was conducted to determine the excitation energy dependent emission characteristics of CdSe quantum dot–β-carotene and CdSe/ZnS core–shell quantum dot–β-carotene hybrid samples. The emission intensity was recorded at different excitation energies by continuously varying the excitation energy from the band edge value of the QDs to a maximum value. In both the hybrids, the emission intensity increased with an increase of the excitation energy and then a sudden quenching occurred, which is in contrast with the results from bare samples, where the emission intensity showed a decreasing trend at all excitation energies. The quantum yields corresponding to the maximum emission are 47.12% and 69%, for CdSe QD–βC and CdSe/ZnS core–shell QD–βC hybrids, respectively. At higher excitation energy, electrons were transferred to the molecule's LUMO level, leaving behind holes in the valance band of the QD, and thus the produced charge separated state became responsible for the PL quenching in the hybrid sample. We have confirmed this charge separated state by lifetime measurements. It is because of this PL quenching behaviour of the hybrids that the nature of the interface band structure was deduced as type I. Furthermore, because of the involvement of higher energy photons in the quenching process, the transferred electrons in the LUMO level of the molecule were known as hot electrons. The present paper discusses the excited state electron dynamics across the interface between the two hybrid materials in detail.

Designs for molecular circuits that use electronic coherence

The mounting evidence of recent years regarding long-lived coherent dynamics of electronic excitations in several light-harvesting antenna proteins suggests the possibility of realizing and exploiting light-initiated quantum dynamics in synthetic molecular devices based on electronic energy transfer. Inspired by the field of molecular logic, we focus this discussion on the prospect of using quantum coherence to control the direction of energy flow in a molecular circuit. As a prototype system we consider a circuit consisting of three chromophores that deliver energy to two trap chromophores. Our aim is to control to which trap the energy is more likely to be delivered. This is achieved by switching one of the circuit chromophores ON and OFF from the system, such that the direction of energy flow substantially changes from the ON and OFF states of the circuit. We find that quantum coherence can allow a significant ability to direct energy transfer in the circuit. However, when realistic levels of noise are added, quantum coherence only slightly improves the ability to direct electronic energy in comparison to a classical hopping mechanism.

X線及び電子線散乱を用いた分子内電子運動と電子励起ダイナミクスの研究

X線及び電子線散乱を用いた分子内電子運動と電子励起ダイナミクスの研究

Coherent Energy Transfer under Incoherent Light Conditions.

Recent two-dimensional electronic spectroscopy (2DES) experiments have reported evidence of coherent dynamics of electronic excitations in several light-harvesting antennae. However, 2DES uses ultrafast coherent laser pulses as an excitation source; therefore, there is a current debate on whether coherent excitation dynamics is present under natural sunlight - incoherent - illumination conditions. In this letter, we show that even if incoherent light excites an electronic state with no initial quantum superpositions among excitonic states, energy transfer can proceed quantum coherently if nonequilibrium dynamics of the phonon environment takes place. Such nonequilibrium behavior manifests itself in non-Markovian evolution of electronic excitations and is typical of many photosynthetic systems. We therefore argue that light-harvesting antennae have mechanisms that could support coherent evolution under incoherent illumination.

Quantum-coherent energy transfer: implications for biology and new energy technologies

The important thing in science is not so much to obtain new facts as to discover new ways of thinking about them. Sir William Henry Bragg Nobel Prize For Physics, 1915 There is a growing body of scientific evidence that light-initiated reactions in the molecular components of photosynthetic

Autoionization cross section of potassium atom excited by electron impact

The autoionization cross section of potassium atoms excited by electron impact is measured in the energy range from the first autoionization threshold at 18.72 eV to 202 eV. The data are obtained by deter-mining the total intensity of electron spectra resulting from the decay of the 3p5n1l1n2l2 autoionizing states. The cross section has two maxima, 1.8 × 10−16 and 2.2 × 10−16 cm2, at 21 and 32 eV, respectively. The excitation dynamics of autoionization states suggests that the first maximum is associated with the resonance character of the near-threshold excitation. The second maximum, as well as the behavior of the cross section at energies above 50 eV, reflects the dynamics of electron excitation of quartet and doublet autoionizing states. The measured autoionization cross section is compared with known data for the total single ionization cross section of potassium atom by electron impact. The relative contribution of the autoionization cross section to the total single ionization cross section is found to reach 30% at 32 eV.

Luminescence and electronic excitations in KBe2BO3F2 crystals

This paper reports on a study of the dynamics of electronic excitations in KBe2BO3F2 (KBBF) crystals by low-temperature luminescent vacuum ultraviolet spectroscopy with nanosecond time resolution under photoexcitation by synchrotron radiation. The first data have been obtained on the kinetics of photoluminescence (PL) decay, time-resolved PL spectra, time-resolved PL excitation spectra, and reflection spectra at 7 K; the estimation has been performed for the band gap E g = 10.6−11.0 eV; the predominantly excitonic mechanism for PL excitation at 3.88 eV has been identified; and defect luminescence bands at 3.03 and 4.30 eV have been revealed. The channels of generation and decay of electronic excitations in KBBF crystals have been discussed.

A time-resolved luminescence spectroscopy study of non-linear optical crystals K2Al2B2O7

We report the results of complex study of luminescence and dynamics of electronic excitations in K2Al2B2O7 (KABO) crystals obtained using low-temperature luminescence-optical vacuum ultraviolet spectroscopy with sub-nanosecond time resolution under selective photoexcitation with synchrotron radiation. The paper discusses the decay kinetics of photoluminescence (PL), the time-resolved PL emission spectra (1.2–6.2eV), the time-resolved PL excitation spectra and the reflection spectra (3.7–21eV) measured at 7K. On the basis of the obtained results three absorption peaks at 4.7, 5.8 and 6.5eV were detected and assigned to charge-transfer absorption from O2− to Fe3+ ions; the intrinsic PL band at 3.28eV was revealed and attributed to radiative annihilation of self-trapped excitons, the defect luminescence bands at 2.68 and 3.54eV were separated; the strong PL band at 1.72eV was revealed and attributed to a radiative transition in Fe3+ ion.

Luminescence and electronic excitations in crystals K2Al2B2O7 with defects

This paper reports on the results of the comprehensive study of the dynamics of electronic excitations in K2Al2B2O7 (KABO) crystals, obtained by low-temperature luminescence vacuum ultraviolet spectroscopy with nanosecond time resolution upon photoexcitation by synchrotron radiation. For the first time, the data have been obtained on the photoluminescence (PL) decay kinetics, PL spectra with time resolution, PL excitation spectra with time resolution, and reflection spectra at 7 K; the intrinsic nature of PL at 3.28 eV has been established; luminescence bands of defects have been separated in the visible and ultraviolet spectral regions; an intense long-wavelength PL band has been detected at 1.72 eV; channels of the formation and decay of electronic excitations in K2Al2B2O7 crystals have been discussed.

Electronic excitation dynamics in multichromophoric systems described via a polaron-representation master equation

We derive a many-site version of the non-Markovian time-convolutionless polaron master equation [Jang et al., J. Chem Phys. 129, 101104 (2008)] to describe electronic excitation dynamics in multichromophoric systems. By treating electronic and vibrational degrees of freedom in a combined frame (polaron frame), this theory is capable of interpolating between weak and strong exciton-phonon coupling and is able to account for initial non-equilibrium bath states and spatially correlated environments. Besides outlining a general expression for the expected value of any electronic system observable in the original frame, we also discuss implications of the Markovian and Secular approximations highlighting that they need not hold in the untransformed frame despite being strictly satisfied in the polaron frame. The key features of the theory are illustrated using as an example a four-site subsystem of the Fenna-Mathews-Olson light-harvesting complex. For a spectral density including a localised mode, we show that oscillations of site populations may only be observed when non-equilibrium bath effects are taken into account. Furthermore, we illustrate how this formalism allows us to identify the electronic and vibrational components of the oscillatory dynamics.

Measuring Electron and Hole Transfer in Core/Shell Nanoheterostructures

Using femtosecond transient absorption and time-resolved photoluminescence spectroscopy, we studied the electron versus hole dynamics in photoexcited quasi-type-II heterostructured nanocrystals with fixed CdTe core radii and varying CdSe shell coverage. By choosing the pump wavelength in resonance with the core or the shell states, respectively, we were able to measure the excited electron and hole dynamics selectively. Both, the core- and the shell-excited CdTe/CdSe nanocrystals showed the same spectral emission and photoluminescence lifetimes, indicating that ultrafast electron and hole transfer across the core/shell interface resulted in the identical long-lived charge transfer state. Both charge carriers have subpicosecond transfer rates through the interface, but the subsequent relaxation rates of the hole (τ(dec) ∼ 800 ps) and electron (τ(avg) ∼ 8 ps) are extremely different. On the basis of the presented transient absorption measurements and fitting of the steady-state spectra, we find that the electron transfer occurs in the Marcus inverted region and mixing between the CdTe exciton and charge transfer states takes place and therefore needs to be considered in the analysis.

Excited electron dynamics of bismuth film grown on Si(1 1 1) surface by interferometric time-resolved two-photon photoemission spectroscopy

We have carried out studies on the excited-electron dynamics in the single and poly crystalline Bi films by interferometric time-resolved two-photon photoemission spectroscopy (ITR-2PPE). The fast energy relaxation times of the excited electrons due to the electron–electron scattering were observed. The energy relaxation times were less than 15 fs and those of the single crystalline film are faster than those of the poly crystalline film. The faster energy relaxation times of the single crystalline Bi film are attributed to the higher scattering rate induced by the higher densities of states by appearance of the surface states in the vicinity of the Fermi level.

Ab initio approach to the excited electron dynamics in rutile and anataseTiO2

The relaxation of excited electrons in the conduction band of titanium dioxide with therutile and anatase structure is investigated by means of a first-principle method. Theevaluations are based on the pseudo-potential plane-wave approach to the electronicband structure calculations, the density-functional perturbation theory for thecalculations of phonons and electron–phonon interactions, and on the ‘Fermi goldenrule’ for evaluations of the electron relaxation time and the energy loss time. Wedemonstrate two regimes of the electron relaxation. For the excited electrons with energyless than 0.01 eV above the conduction band bottom the relaxation occurs in thepico-second timescale, whereas at higher excitation energies the electron relaxation timeis within a few femto-seconds and the energy loss time is within a few tens offemto-seconds.