Multielectron Dynamics Research Articles

Momentum transfers in correlation-assisted tunneling

We consider correlation-assisted tunnel ionization of a small molecule by an intense low-frequency laser pulse. In this mechanism, the departing electron excites the state of the ion via a Coulomb interaction. We show that the wave packets emerging from this process can have nontrivial spatial structure and give a measurable indicator of correlated multielectron dynamics during the tunneling step. We also show that the saddle-point approximation requires special attention in this geometric analysis.

Correlated multielectron dynamics in mid-infrared laser pulse interactions with neon atoms

The multielectron dynamics in nonsequential triple ionization (NSTI) of neon atoms driven by mid-infrared (MIR) laser pulses is investigated with the three-dimensional classical ensemble model. In consistent with the experimental result, our numerical result shows that in the MIR regime, the triply charged ion longitudinal momentum spectrum exhibits a pronounced double-hump structure at low laser intensity. Back analysis reveals that as the intensity increases, the responsible triple ionization channels transform from direct (e, 3e) channel to the various mixed channels. This transformation of the NSTI channels leads to the results that the shape of ion momentum spectra becomes narrow and the distinct maxima shift towards low momenta with the increase of the laser intensity. By tracing the triply ionized trajectories, the various ionization channels at different laser intensities are clearly identified and these results provide an insight into the complex dynamics of the correlated three electrons in NSTI.

Method of accurately calculating mean field operator in multi-configuration time-dependent Hartree—Fock frame

The accurate theoretical expressions of the mean field operator associated with the multi-configuration time-dependent Hartree—Fock (MCTDHF) method are presented in this paper. By using a theoretical formula, derived without approximation, we can study the multi-electron correlation dynamics accurately. Some illustrative calculations are carried out to check the accuracy of the expression of the mean field operator. The results of illustrative calculations indicate the reliability of the accurate expression of the mean field operator. This theoretical calculation method for the mean field operator may be of considerable help in future studies of the correlated dynamics of many-electron systems in strong laser fields.

Multidimensional high harmonic spectroscopy: a semi-classical perspective on measuring multielectron rearrangement upon ionization

High harmonic spectroscopy has the potential to combine attosecond temporal with sub-Angstrom spatial resolution of the early nuclear and multielectron dynamics in molecules. It involves strong-field ionization of the molecule by an infrared (IR) laser field followed by time-delayed recombination of the removed electron with the molecular ion. The time-delay is controlled on the attosecond time scale by the oscillation of the IR field and is mapped into the harmonic number, providing a movie of molecular dynamics between ionization and recombination. One of the challenges in the analysis of a high harmonic signal stems from the fact that the complex dynamics of both ionization and recombination with their multiple observables are entangled in the harmonic signal. Disentangling this information requires a multidimensional approach, capable of mapping ionization and recombination dynamics into different independent parameters. We suggest multidimensional high harmonic spectroscopy as a tool for characterizing ionization and recombination processes separately allowing for simultaneous detection of both the ionization delays and sub-cycle ionization rates. Our method extends the capability of the two-dimensional set-up suggested recently by Shafir et al on reconstructing ionization delays, while keeping the reconstruction procedure as simple as in the original proposal. The scheme is based on the optimization of the high harmonic signal in orthogonally polarized strong fundamental and relatively weak multicolour control fields.

Time-dependent complete-active-space self-consistent-field method for multielectron dynamics in intense laser fields

The time-dependent complete-active-space self-consistent-field (TD-CASSCF) method for the description of multielectron dynamics in intense laser fields is presented, and a comprehensive description of the method is given. It introduces the concept of frozen-core (to model tightly bound electrons with no response to the field), dynamical-core (to model electrons tightly bound but responding to the field), and active (fully correlated to describe ionizing electrons) orbital subspaces, allowing compact yet accurate representation of ionization dynamics in many-electron systems. The classification into the subspaces can be done flexibly, according to simulated physical situations and desired accuracy, and the multiconfiguration time-dependent Hartree-Fock (MCTDHF) approach is included as a special case. To assess its performance, we apply the TD-CASSCF method to the ionization dynamics of one-dimensional lithium hydride (LiH) and LiH dimer models, and confirm that the present method closely reproduces rigorous MCTDHF results if active orbital space is chosen large enough to include appreciably ionizing electrons. The TD-CASSCF method will open a way to the first-principle theoretical study of intense-field induced ultrafast phenomena in realistic atoms and molecules.

SOLVING THE SINGLE LAYER MULTICONFIGURATION TIME-DEPENDENT HARTREE–FOCK EQUATION IN SECOND QUANTIZATION REPRESENTATION BASED ON A DECOMPOSITION OF THE OVERALL FOCK SPACE

Very recently, the multilayer multiconfiguration time-dependent Hartree (MCTDH) method based on a decomposition of the overall Fock space in second quantization representation was proposed [Wang H, Thoss M, J Chem Phys131:024114, 2009], and they have presented demonstrative numerical example on vibrationally coupled electron transport. Followed by the thinking, the strategies of the implement of the theory to deal with multielectron dynamics with consideration coulomb interaction were discussed in detail. Some demonstrative calculations have been carried out by using the single layer MCTDHF method. The influences of the different choice way of initial condition, such as the species of basis function, the type of decomposition of the full Fock space, the choice of the spin orbitals, the number of the single particle function and so on, on the imaginary time propagation are mainly studied. The bridge between the generalized theory and the calculation application has been set up.

Multi-configuration time dependent Hartree Fock method in three different representations

As a powerful tool to deal with multi-electron dynamics in strong laser field, multi-configuration time-dependent Hartree Fock (MCTDHF) method has attracted great attention. In this paper, the strategy of the implement of the MCTDHF theory is illustrated in the paper in detail. We develop three types of Fortran source soft packages based on three different bases to study correlated dynamics of atoms and molecules in strong laser field. The calculated soft-packages based on the theory are introduced in detail. The calculations are performed by using the soft-package. The prospect of the MCTDHF approach to simulating the multi-electron dynamics in strong laser field are also presented.

Multi-electron Dynamics using Time Dependent R-Matrix theory

We demonstrate the versatility and capability of the Time Dependent R-Matrix (TDRM) Theory for describing multi-electron dynamics in general atomic targets by applying it to multi-photon ionization of Argon and Aluminium.

Strong‐field ionization of C60 sputtered neutral molecules using 1015 W/cm2 of fs IR radiation

Implementation of a new high energy, IR laser source for secondary neutral mass spectrometry, is presented. It consists of an 800 nm, 40 fs, 10 mJ, 1 kHz laser pumping an optical parametric amplifier generating 1.2–2.6 µm tunable pulses. The wavelength and intensity dependence of photoionized sputtered indium clusters and gas‐phase histamine are presented, exhibiting less nonadiabatic multielectron dynamics (decreased fragmentation) at longer wavelength. Future prospects for secondary neutral bioimaging experiments are explored with a model system consisting of a 400 mesh grid, vapor deposited with adenine, and fixed over indium foil. Copyright © 2012 John Wiley & Sons, Ltd.

Multi-Electron Wave Packet Dynamics in Applied Electric Field

We investigated multi-electron wave packet dynamics considering Coulomb interaction under applied electric field by solving the time-dependent Hartree–Fock equation. We confirm that the Coulomb interaction works to prolong the lifetime of the wave packets. Moreover, we find that the applied electric field also prolongs the wave packet lifetime. This indicates that the particle nature of electrons will be dominant in future nanodevices under high electric field.

SubfemtosecondK-Shell Excitation with a Few-Cycle Infrared Laser Field

Subfemtosecond bursts of extreme ultraviolet radiation, facilitated by a process known as high-order harmonic generation, are a key ingredient for attosecond metrology, providing a tool to precisely initiate and probe ultrafast dynamics in the microcosms of atoms, molecules, and solids. These ultrashort pulses are always, and as a by-product of the way they are generated, accompanied by laser-induced recollisions of electrons with their parent ions. By using a few-cycle infrared (λ(0)=2.1 μm) driving laser, we were able to directly excite high-energy (∼870 eV) inner-shell electrons through laser-induced electron recollision, opening the door to time-resolved studies of core-level and concomitant multielectron dynamics.

Attosecond tracing of correlated electron-emission in non-sequential double ionization

Despite their broad implications for phenomena such as molecular bonding or chemical reactions, our knowledge of multi-electron dynamics is limited and their theoretical modelling remains a most difficult task. From the experimental side, it is highly desirable to study the dynamical evolution and interaction of the electrons over the relevant timescales, which extend into the attosecond regime. Here we use near-single-cycle laser pulses with well-defined electric field evolution to confine the double ionization of argon atoms to a single laser cycle. The measured two-electron momentum spectra, which substantially differ from spectra recorded in all previous experiments using longer pulses, allow us to trace the correlated emission of the two electrons on sub-femtosecond timescales. The experimental results, which are discussed in terms of a semiclassical model, provide strong constraints for the development of theories and lead us to revise common assumptions about the mechanism that governs double ionization.

Electron Correlation in Real Time

Electron correlation, caused by the interaction among electrons in a multielectron system, manifests itself in all states of matter. A complete theoretical description of interacting electrons is challenging; different approximations have been developed to describe the fundamental aspects of the correlation that drives the evolution of simple (few-electron systems in atoms/molecules) as well as complex (multielectron wave functions in atoms, molecules, and solids) systems. Electron correlation plays a key role in the relaxation mechanisms that characterize excited states of neutral or ionized atoms and molecules populated by absorption of extreme ultraviolet (XUV) or X-ray radiation. The dynamics of these states can lead to different processes such as Fano resonance and Auger decay in atoms or interatomic Coulombic decay or charge migration in molecules and clusters. Many of these relaxation mechanisms are ubiquitous in nature and characterize the interaction of complex systems, such as biomolecules, adsorbates on surfaces, and hydrogen-bonded clusters, with XUV light. These mechanisms evolve typically on the femtosecond (1 fs=10(-15) s) or sub-femtosecond timescale. The experimental availability of few-femtosecond and attosecond (1 as=10(-18) s) XUV pulses achieved in the last 10 years offers, for the first time, the opportunity to excite and probe in time these dynamics giving the possibility to trace and control multielectron processes. The generation of ultrashort XUV radiation has triggered the development and application of spectroscopy techniques that can achieve time resolution well into the attosecond domain, thereby offering information on the correlated electronic motion and on the correlation between electron and nuclear motion. A deeper understanding of how electron correlation works could have a large impact in several research fields, such as biochemistry and biology, and trigger important developments in the design and optimization of electronic devices.

Time-and-energy-resolved measurement of Auger cascades following Kr 3d excitation by attosecond pulses

We show that attosecond metrology has evolved from proof-of-principle experiments to a level where complex processes can be resolved in time that cannot be accessed using any other existing technique. The cascaded Auger decay following ionization and excitation of the 3d-subshell in Kr with subfemtosecond 94 eV soft x-ray pulses has been energy- and time-resolved in an x-ray pump–infrared probe experiment. This Auger cascade reveals rich multi-electron dynamics, which despite the fact that there are many experimental and theoretical data available, is not yet fully understood. We present time-resolved data showing the sequence of the temporal dynamics in the cascaded Auger decay. The decay time of several groups of lines has been measured, including the lines at the low-energy part of the spectrum, which are predominantly produced by the second-step Auger transitions. Our experimental data reveal long lifetimes (up to 70 fs) of the subvalence excited ionic (intermediate) states in the cascaded resonant Auger decay. Extensive theoretical calculations within the multiconfiguration Dirac–Fock (MCDF) approach show that the observed long lifetime may be attributed to the second-step Auger decay of the resonantly excited 3d−1np states with n = 6,7. Furthermore, our experimental data show that the electrons with a kinetic energy around 25 eV (generally assigned as M4,5N1N1 1S0 normal Auger lines) have a component corresponding to the second-step Auger decay of the ion after resonant Auger transition 3d−1np → 4s2 4p3 4dnp → 4s2 4p4 with a lifetime of 26 ± 4 fs.

Quantum dynamics with fermion coupled coherent states: Theory and application to electron dynamics in laser fields

We present an alternate version of the coupled-coherent-state method, specifically adapted for solving the time-dependent Schr\odinger equation for multielectron dynamics in atoms and molecules. This theory takes explicit account of the exchange symmetry of fermion particles, and it uses fermion molecular dynamics to propagate trajectories. As a demonstration, calculations in the He atom are performed using the full Hamiltonian and accurate experimental parameters. Single- and double-ionization yields by 160-fs and 780-nm laser pulses are calculated as a function of field intensity in the range 10${}^{14}$--10${}^{16}$ W/cm${}^{2}$, and good agreement with experiments by Walker et al. is obtained. Since this method is trajectory based, mechanistic analysis of the dynamics is straightforward. We also calculate semiclassical momentum distributions for double ionization following 25-fs and 795-nm pulses at 1.5\ifmmode\times\else\texttimes\fi{}10${}^{15}$ W/cm${}^{2}$, in order to compare them with the detailed experiments by Rudenko et al. For this more challenging task, full convergence is not achieved. However, major effects such as the fingerlike structures in the momentum distribution are reproduced.

Probing collective multi-electron dynamics in xenon with high-harmonic spectroscopy

High-harmonic spectroscopy probes atomic structure by looking at the short-wavelength emission excited from atoms by ultrafast pulses of laser light. It is now shown that this technique can even detect signatures of electron–electron interactions.

Dynamics of photo-activated Coulomb complexes

Intense light with frequencies above typical atomic or molecular ionization potentials as provided by free-electron lasers couples many photons into extended targets such as clusters and biomolecules. This implies, in contrast to traditional multi-photon ionization, multiple single-photon absorption. Thereby many electrons are removed from their bound states and either released or trapped if the target charge has become sufficiently large. We develop a simple model for this photo-activation to study electron migration and interaction. It satisfies scaling relations that help to relate quite different scenarios. Understanding this multi-electron dynamics on very short time scales is vital for assessing the radiation damage inflicted and for paving the way to coherent diffraction imaging of single molecules.

Cluster nanoplasmas in strong FLASH pulses: formation, excitation and relaxation

Clusters exposed to intense laser radiation quickly turn into nanoplasmas: short-lived electron plasmas confined by the charged cluster ions on a nanometre scale. Although the cluster will eventually explode, the transient multi-electron dynamics during the pulse is of great interest and largely unexplored. It determines the mechanism of energy absorption and thus may help to understand measured electron and ion spectra, also in other samples, such as large molecules. Furthermore, an experimental setup with short pulses and access to observables, which becomes possible because of the sample's finite size, offers novel possibilities of investigating the non-equilibrium dynamics of plasmas. Here, the formation, excitation and relaxation of nanoplasmas in rare-gas clusters driven by strong pulses from free-electron lasers (FELs) with photon frequencies in the range of about 10 to 100 eV as currently available at FLASH are discussed. It is the unique combination of brilliant, tunable and short-pulse radiation in this machine and upcoming x-ray FELs which makes such studies feasible.

Fast electrons from multi-electron dynamics in xenon clusters induced by inner-shell ionization

Fast electrons emitted from xenon clusters in strong femtosecond 90 eV pulses have been measured at the Free-electron Laser in Hamburg (FLASH). Energy absorption occurs mainly through atomic inner-shell photo-ionization. Photo-electrons are trapped in the strong Coulomb potential of the cluster ions and form a non-equilibrium plasma with supra-atomic density. Its equilibration through multiple energy-exchanging collisions within the entire cluster volume produces electrons with energies well beyond the dominant emission line of atomic xenon. Here, in contrast to traditional low-frequency laser plasma heating, the plasma gains energy from electrons delivered through massive single-photon excitation from bound states. Electron emission induced by thermalization of a non-equilibrium plasma is expected to be a general phenomenon occurring for strong atomic x-ray absorption in extended systems.

Strong-field control and spectroscopy of attosecond electron-hole dynamics in molecules

Molecular structures, dynamics and chemical properties are determined by shared electrons in valence shells. We show how one can selectively remove a valence electron from either Pi vs. Sigma or bonding vs. nonbonding orbital by applying an intense infrared laser field to an ensemble of aligned molecules. In molecules, such ionization often induces multielectron dynamics on the attosecond time scale. Ionizing laser field also allows one to record and reconstruct these dynamics with attosecond temporal and sub-Angstrom spatial resolution. Reconstruction relies on monitoring and controlling high-frequency emission produced when the liberated electron recombines with the valence shell hole created by ionization.