Coherent Electron Motion Research Articles

Capturing electron-driven chiral dynamics in UV-excited molecules

Chiral molecules, used in applications such as enantioselective photocatalysis1, circularly polarized light detection2 and emission3 and molecular switches4,5, exist in two geometrical configurations that are non-superimposable mirror images of each other. These so-called (R) and (S) enantiomers exhibit different physical and chemical properties when interacting with other chiral entities. Attosecond technology might enable influence over such interactions, given that it can probe and even direct electron motion within molecules on the intrinsic electronic timescale6 and thereby control reactivity7–9. Electron currents in photoexcited chiral molecules have indeed been predicted to enable enantiosensitive molecular orientation10, but electron-driven chiral dynamics in neutral molecules have not yet been demonstrated owing to the lack of ultrashort, non-ionizing and perturbative light pulses. Here we use time-resolved photoelectron circular dichroism (TR-PECD)11–15 with an unprecedented temporal resolution of 2.9 fs to map the coherent electronic motion initiated by ultraviolet (UV) excitation of neutral chiral molecules. We find that electronic beatings between Rydberg states lead to periodic modulations of the chiroptical response on the few-femtosecond timescale, showing a sign inversion in less than 10 fs. Calculations validate this and also confirm that the combination of the photoinduced chiral current with a circularly polarized probe pulse realizes an enantioselective filter of molecular orientations following photoionization. We anticipate that our approach will enable further investigations of ultrafast electron dynamics in chiral systems and reveal a route towards enantiosensitive charge-directed reactivity.

Optimization of Coherent Dynamics of Localized Surface Plasmons in Gold and Silver Nanospheres; Large Size Effects

Noble metal nanoparticles have attracted attention in recent years due to a number of their exciting applications in plasmonic applications, e.g., in sensing, high-gain antennas, structural colour printing, solar energy management, nanoscale lasing, and biomedicines. The report embraces the electromagnetic description of inherent properties of spherical nanoparticles, which enable resonant excitation of Localized Surface Plasmons (defined as collective excitations of free electrons), and the complementary model in which plasmonic nanoparticles are treated as quantum quasi-particles with discrete electronic energy levels. A quantum picture including plasmon damping processes due to the irreversible coupling to the environment enables us to distinguish between the dephasing of coherent electron motion and the decay of populations of electronic states. Using the link between classical EM and the quantum picture, the explicit dependence of the population and coherence damping rates as a function of NP size is given. Contrary to the usual expectations, such dependence for Au and Ag NPs is not a monotonically growing function, which provides a new perspective for tailoring plasmonic properties in larger-sized nanoparticles, which are still hardly available experimentally. The practical tools for comparing the plasmonic performance of gold and silver nanoparticles of the same radii in an extensive range of sizes are also given.

Investigation of optical properties of zinc oxide nanocrystalline materials

The optical properties of Zinc Oxide (ZnO) nanowires are studied using two parallel channels of photon conduction. First is the coherent motion of electrons within the ZnO nanowire termed as Drude carriers and second is the incoherent hopping of electrons from one ZnO nanowire to another nanowire. The model has the relaxation rate as one of the material dependent free physical parameters. The frequency-dependent relaxation rates are presented in terms of memory functions. A peak in optical conductivity obtained near zero-frequency, which occurs due to Drude carriers, i.e., the incoherent motion of electrons whereas, frequency becomes saturated at very-low values at higher frequencies near the far-infrared region. A peak in optical conductivity is observed around mid-infrared frequencies due to the hopping of carriers from one ZnO nanoparticle to another. This is evidenced that both the Drude and hopping carriers contribute to optical conductivity in ZnO nanowire. The modal based on these two contribution schemes successfully explains the optical conductivity phenomena in ZnO nanowires.

Attosecond coherent electron motion in Auger-Meitner decay.

In quantum systems, coherent superpositions of electronic states evolve on ultrafast time scales (few femtoseconds to attoseconds; 1 attosecond = 0.001 femtoseconds = 10-18 seconds), leading to a time-dependent charge density. Here we performed time-resolved measurements using attosecond soft x-ray pulses produced by a free-electron laser, to track the evolution of a coherent core-hole excitation in nitric oxide. Using an additional circularly polarized infrared laser pulse, we created a clock to time-resolve the electron dynamics and demonstrated control of the coherent electron motion by tuning the photon energy of the x-ray pulse. Core-excited states offer a fundamental test bed for studying coherent electron dynamics in highly excited and strongly correlated matter.

Spectroscopic observation and ultrafast coherent vibrational dynamics of the aqueous phenylalanine radical.

The phenylalanine radical (Phe˙) has been proposed to mediate biological electron transport (ET) and exhibit long-lived electronic coherences following attosecond photoionization. However, the coupling of ultrafast structural reorganization to the oxidation/ionization of biomolecules such as phenylalanine remains unexplored. Moreover, studies of ET involving Phe˙ are hindered by its hitherto unobserved electronic spectrum. Here, we report the spectroscopic observation and coherent vibrational dynamics of aqueous Phe˙, prepared by sub-6 fs photodetachment of phenylalaninate anions. Sub-picosecond transient absorption spectroscopy reveals the ultraviolet absorption signature of Phe˙. Ultrafast structural reorganization drives coherent vibrational motion involving nine fundamental frequencies and one overtone. DFT calculations rationalize the absence of the decarboxylation reaction, a photodegradation pathway previously identified for Phe˙. Our findings guide the interpretation of future attosecond experiments aimed at elucidating coherent electron motion in photoionized aqueous biomolecules and pave way for the spectroscopic identification of Phe˙ in studies of biological ET.

Size effect on optical conductivity of Bi2Te3 nanoparticles due to softening of phonon frequencies

The frequency-dependent optical conductivity σ(w) strongly depends on the particle size in nanostructures. Phonon frequencies are believed to be decrease on a decrease in particle size due to the softening of phonon frequencies around grain boundaries in nanomaterials. The optical conductivity of Bi2Te3 nanoparticles is studied using the two components model: First is the coherent motion of Drude electrons within the Bi2Te3 nanoparticles, and second is the incoherent hopping of charge carriers from one Bi2Te3 nanoparticle to another. The model has the phonon frequency and relaxation rates as the independent free parameters, which depend on nanoparticle size. The relaxation rates that are a function of frequency are incorporated in optical conductivity through memory functions. The coherent motion of Drude carriers exhibited a peak in intensity near zero frequency regime and decreased to a minimum value in the far-infrared region. The incoherent hopping of electrons from one Bi2Te3 nanoparticle to another produces a broad peak in the frequency-dependent optical conductivity at around the mid-infrared region. On decreases in nanoparticle size, softening of phonon frequencies occurs at interfaces due to an increase in interface volume ratio; as a result, the peak in optical conductivity at the mid-infrared region shifts towards lower frequencies.

Picosecond coherent electron motion in a silicon single-electron source.

An advanced understanding of ultrafast coherent electron dynamics is necessary for the application of submicrometre devices under a non-equilibrium drive to quantum technology, including on-demand single-electron sources1, electron quantum optics2-4, qubit control5-7, quantum sensing8,9 and quantum metrology10. Although electron dynamics along an extended channel has been studied extensively2-4,11, it is hard to capture the electron motion inside submicrometre devices. The frequency of the internal, coherent dynamics is typically higher than 100 GHz, beyond the state-of-the-art experimental bandwidth of less than 10 GHz (refs. 6,12,13). Although the dynamics can be detected by means of a surface-acoustic-wave quantum dot14, this method does not allow for a time-resolved detection. Here we theoretically and experimentally demonstrate how we can observe the internal dynamics in a silicon single-electron source that comprises a dynamic quantum dot in an effective time-resolved fashion with picosecond resolution using a resonant level as a detector. The experimental observations and the simulations with realistic parameters show that a non-adiabatically excited electron wave packet15 spatially oscillates quantum coherently at ~250 GHz inside the source at 4.2 K. The developed technique may, in future, enable the detection of fast dynamics in cavities, the control of non-adiabatic excitations15 or a single-electron source that emits engineered wave packets16. With such achievements, high-fidelity initialization of flying qubits5, high-resolution and high-speed electromagnetic-field sensing8 and high-accuracy current sources17 may become possible.

Visualising the role of non-perturbative environment dynamics in the dissipative generation of coherent electronic motion

Targeted exciton transport is crucial for efficient light-harvesting, but its microscopic description in biological systems is complicated by strong environmental coupling, highly structured vibrational environments and non-Markovian open system dynamics. In this article we employ the non-perturbative hierarchical equations of motion (HEOM) technique to explore how structured environments and tuned electronic properties can lead to the generation of coherent motion across a directed transport network, i.e. one containing an energy gradient. By further exploiting the information contained in the auxiliary HEOM matrices, we also visualize the complete displacement distributions of the main reaction coordinate during the ultrafast relaxation, and show that highly non-Gaussian profiles emerge when the electronic dynamics become quasi-reversible and involve bath-induced delocalized states. These coherent dynamics are spontaneously generated by earlier incoherent relaxation events, and we also demonstrate the correlation between the environmental coordinates and a quantitative volume-based measure of non-Markovianity.

X-ray Raman scattering: a building block for nonlinear spectroscopy.

Ultraintense X-ray free-electron laser pulses of attosecond duration can enable new nonlinear X-ray spectroscopic techniques to observe coherent electronic motion. The simplest nonlinear X-ray spectroscopic concept is based on stimulated electronic X-ray Raman scattering. We present a snapshot of recent experimental achievements, paving the way towards the goal of realizing nonlinear X-ray spectroscopy. In particular, we review the first proof-of-principle experiments, demonstrating stimulated X-ray emission and scattering in atomic gases in the soft X-ray regime and first results of stimulated hard X-ray emission spectroscopy on transition metal complexes. We critically asses the challenges that have to be overcome for future successful implementation of nonlinear coherent X-ray Raman spectroscopy. This article is part of the theme issue 'Measurement of ultrafast electronic and structural dynamics with X-rays'.

Dynamics of Surface Plasmon Polaritons in Metal Nanowires

Metal nanostructures have found extensive use in a variety of applications in chemistry, including as substrates for molecular sensing and surface enhanced spectroscopy and as nanoscale heaters for photothermal therapy. These applications depend on the strong absorption and field enhancements associated with localized surface plasmon resonance (LSPR). This has led to a number of studies of how the LSPR line width, which measures energy losses for the coherent electron motion, depends on the size and shape of different types of metal nanoparticles, and the environment around the particle. Extended metal nanostructures, such as nanowires and nanoplates, display propagating surface plasmon modes, termed surface plasmon polaritons (SPPs), in addition to LSPRs. These modes are important for applications where metal nanostructures are used as waveguides. However, less is known about the damping of the propagating SPPs compared to the LSPRs. The energy losses for the propagating SPP modes can be investigated by ...

Quantum Nuclear Dynamics Pumped and Probed by Ultrafast Polarization Controlled Steering of a Coherent Electronic State in LiH.

The quantum wave packet dynamics following a coherent electronic excitation of LiH by an ultrashort, polarized, strong one-cycle infrared optical pulse is computed on several electronic states using a grid method. The coupling to the strong field of the pump and the probe pulses is included in the Hamiltonian used to solve the time-dependent Schrodinger equation. The polarization of the pump pulse allows us to control the localization in time and in space of the nonequilibrium coherent electronic motion and the subsequent nuclear dynamics. We show that transient absorption, resulting from the interaction of the total molecular dipole with the electric fields of the pump and the probe, is a very versatile probe of the different time scales of the vibronic dynamics. It allows probing both the ultrashort, femtosecond time scale of the electronic coherences as well as the longer dozens of femtoseconds time scales of the nuclear motion on the excited electronic states. The ultrafast beatings of the electronic coherences in space and in time are shown to be modulated by the different periods of the nuclear motion.

Ultrafast strong-field photoelectron emission from biased metal surfaces: exact solution to time-dependent Schrödinger Equation.

Laser-driven ultrafast electron emission offers the possibility of manipulation and control of coherent electron motion in ultrashort spatiotemporal scales. Here, an analytical solution is constructed for the highly nonlinear electron emission from a dc biased metal surface illuminated by a single frequency laser, by solving the time-dependent Schrödinger equation exactly. The solution is valid for arbitrary combinations of dc electric field, laser electric field, laser frequency, metal work function and Fermi level. Various emission mechanisms, such as multiphoton absorption or emission, optical or dc field emission, are all included in this single formulation. The transition between different emission processes is analyzed in detail. The time-dependent emission current reveals that intense current modulation may be possible even with a low intensity laser, by merely increasing the applied dc bias. The results provide insights into the electron pulse generation and manipulation for many novel applications based on ultrafast laser-induced electron emission.

Protocol for observing molecular dipole excitations by attosecond self-streaking

We propose a protocol to probe the ultrafast evolution and dephasing of coherent electronic excitation in molecules in the time domain by the intrinsic streaking field generated by the molecule itself. Coherent electronic motion in the endohedral fullerene \Necsixty~is initiated by a moderately intense femtosecond UV-VIS pulse leading to coherent oscillations of the molecular dipole moment that persist after the end of the laser pulse. The resulting time-dependent molecular near-field is probed through the momentum modulation of photoemission from the central neon atom by a time-delayed attosecond XUV pulse. Our ab-initio time-dependent density functional theory and classical trajectory simulations predict that this self-streaking signal accurately traces the molecular dipole oscillations in real time. We discuss the underlying processes and give an analytical model that captures the essence of our ab-initio simulations.

Exposing the role of electron correlation in strong-field double ionization: X-ray transient absorption of orbital alignment in Xe+ and Xe2+.

Orbital alignment measurements and theory are used to examine the role of electron correlation during atomic strong-field double ionization (795 nm, (1-5) × 10(14) W cm(-2)). High-order harmonic, transient absorption spectroscopy is used to measure the angular distributions of singly and doubly tunnel-ionized xenon atomic states via 4d core to 5p valence shell transitions between 55 and 60 eV. The experimental MJ alignment distributions are compared to results of a rate-equation model based on sequential ionization, previously developed for coherent electron motion, and now applied to account for the alignment prepared by tunneling ionization. The hole generated in the (2)P3/2 state of Xe(+) is measured to be entirely composed of |MJ| = 1/2, in agreement with theory. The result is a higher degree of alignment than previously reported. Because the model neglects effects of electron-ion recollision, the theory predicts a high degree of alignment in both spin-parallel (triplet) and antiparallel (singlet) terms of Xe(2+). However, the alignment generated with linearly polarized light is observed to be spin-state dependent. The measured alignments for triplet spin states ((3)P2 has |MJ| = [0 : 1 : 2] of [27±6 : 45±11 : 29±0] and (3)P1 has |MJ| = [0 : 1] of [56±2 : 44±2]) are in good agreement with the expectations of theory, which are [33 : 53 : 14] and [66 : 33], respectively. The results validate the rate equation model for sequential tunnel ionization. However, the alignment extracted for a singlet state is greatly diminished: (1)D2 is measured to be [18±1 : 39±2 : 43 ± 2] compared to theoretical expectation of [60 : 39 : 1] for |MJ| = [0 : 1 : 2]. The poor agreement with the sequential ionization model suggests that the alignment of (1)D2 is strongly influenced by the high propensity for the liberated first electron to return to and recollide with its parent atomic orbital. Therefore, although the influence of electron recollision appears minor in the triplet states and suggests sequential ionization, electron correlation between the ionic core and the first ionized electron cannot be ignored in the singlet state. Singlet states are likely to be generated through nonsequential double ionization over the intensity range where the experiments are performed.

Theory of time-resolved nonresonant x-ray scattering for imaging ultrafast coherent electron motion

Future ultrafast x-ray light sources might image ultrafast coherent electron motion in real-space and in real-time. For a rigorous understanding of such an imaging experiment, we extend the theory of non-resonant x-ray scattering to the time-domain. The role of energy resolution of the scattering detector is investigated in detail. We show that time-resolved non-resonant x-ray scattering with no energy resolution offers an opportunity to study time-dependent electronic correlations in non- equilibrium quantum systems. Furthermore, our theory presents a unified description of ultrafast x-ray scattering from electronic wave packets and the dynamical imaging of ultrafast dynamics using inelastic x-ray scattering by Abbamonte and co-workers. We examine closely the relation of the scattering signal and the linear density response of electronic wave packets. Finally, we demonstrate that time-resolved x-ray scattering from a crystal consisting of identical electronic wave packets recovers the instantaneous electron density.

Imaging coherent electronic motion in atoms by ultrafast electron diffraction

Imaging coherent electronic motion in atoms by ultrafast electron diffraction

Carrier-envelope phase effects on the strong-field photoemission of electrons from metallic nanostructures

Sharp metallic nanotapers irradiated with few-cycle laser pulses are emerging as a source of highly confined coherent electron wavepackets with attosecond duration and strong directivity. The possibility to steer, control or switch such electron wavepackets by light is expected to pave the way towards direct visualization of nanoplasmonic field dynamics and real-time probing of electron motion in solid state nanostructures. Such pulses can be generated by strong-field induced tunneling and acceleration of electrons in the near-field of sharp gold tapers within one half-cycle of the driving laser field. Here, we show the effect of the carrier-envelope phase of the laser field on the generation and motion of strong-field emitted electrons from such tips. This is a step forward towards controlling the coherent electron motion in and around metallic nanostructures on ultrashort length and time scales.

Real time tracing of valence-shell electronic coherences with attosecond transient absorption spectroscopy

The chemical properties of atoms, molecules and of more complex systems such as clusters, nanoparticles or condensed matter systems are determined by valence electrons. Real-time control of these properties requires the capability of tracing as well as of driving valence electrons on their native temporal scale of motion, that is, within tens to thousands of attoseconds. Here we detail the technique of attosecond transient absorption spectroscopy. It combines the extreme sensitivity of core-level spectroscopy with the unprecedented temporal resolution offered by the tools of attosecond technology. We use the technique to demonstrate real-time tracing and complete characterization of coherent electron motion triggered by single, double or multiple ionization of atoms exposed to intense, few-cycle pulses. Our work opens the door to high fidelity, time-domain studies and control of electron dynamics in the microcosm.

Interplay between Correlated Electrons and Quantum Phonons in Charge-Ordered and Mott-Insulating Organic Compounds

At an early stage of the photoinduced transition from an insulator to a metal in quasi-two-dimensional organic conductors, a coherent motion of electrons is observed in a charge-ordered insulator, but not so far in a Mott insulator. The mechanisms of these different photoinduced charge dynamics are theoretically studied by numerical solutions to the time-dependent Schrodinger equation for exact many-electron–phonon wave functions on small clusters of model systems. We use two-dimensional three-quarter-filled extended Holstein–Hubbard models on anisotropic triangular lattices. For a charge-ordered insulator on a lattice simplified from the structure of α-(BEDT-TTF)2I3, we indeed find a low-energy collective electronic motion coupled with quantum phonons even if the energy of photoexcitation is away from this energy. For a Mott insulator on a lattice simplified from the structure of κ-(BEDT-TTF)2X, however, such a collective motion does not appear, and quantum phonons are hardly excited.

Resonance in Coherent Electron Motion under Magnetic and Electronic Fields

Resonance in Coherent Electron Motion under Magnetic and Electronic Fields