Double Ionization Of Atoms Research Articles

Timing Correlated-Electron Emission from Strong Field Atomic Double Ionization with Polarization-Gated Attoclock.

Attoclock provides a powerful tool for probing the ultrafast electron dynamics in strong laser fields. However, this technique has remained restricted to single electron or sequential double ionized electron dynamics. Here, we propose a novel attoclock scheme with a polarization-gated few-cycle laser pulse and demonstrate its application in timing the correlated-electron emission in strong field double ionization of argon. Our experimental measurements reveal that the correlated-electron emission occurs mainly through two channels with time differences of 234±22 as and 1043±73 as, respectively. Classical model calculations well reproduce the experimental results and deepen our understanding of ultrafast electron correlation dynamics.

Compton double ionization of the helium atom: Can it be a method of dynamical spectroscopy of ground state electron correlation?

Recent experiments on single and double ionization of atoms by Compton scattering potentially open the door to establishing new methods of dynamical spectroscopy complementing the well-known methods like (e,2e) and (e,3e) spectroscopy. In this paper we explore the possibility to use double ionization of the helium atom by Compton scattering as a tool for direct spectroscopy of electron-electron correlations. For rather high photon energies of a few dozens of keV, a non-relativistic theoretical description of the process and the Kramers-Heisenberg-Waller approximation can be consistently used with a high computational efficiency.

Double-continuum R-matrix theory and codes for many-electron atoms: ionizing electron collisions and time-dependent laser-induced double-ionization

SynopsisAn extension of ab initio many-electron R-matrix theory is described which is relevant both to the time-dependent description of excitation and single and double ionization of atoms in laser pulses, and to time-independent calculation of intermediate energy electron collisions where ionized states of the target atom can be excited. Corresponding general many-electron ‘inner region’ code modules have been developed for use in combination with existing R-matrix method codes for diverse applications with up to two electrons explicitly described by a continuum orbital basis.

Electron correlation in channel-resolved strong-field molecular double ionization

Strong-field ionization plays a fundamental role in attosecond science as the source of electron wave packets which ultimately provide attosecond bursts of radiation as they are driven back to the atom or molecule of origin. Double ionization is an important probe of electron correlation (correlated electron dynamics), which lies at the core of attosecond science. While double ionization of atoms has been studied extensively, the double ionization of molecules is relatively unexplored---particularly for the case of polyatomic systems. We present coincidence (three and four particle) velocity map imaging measurements of molecular double ionization in 1,3-cyclohexadiene and 1,4-cyclohexadiene using few cycle ($\ensuremath{\sim}10$ fs) laser pulses. Our measurements allow us to distinguish between dissociative and nondissociative double ionization, and we find a difference in the correlation of electrons that come from the different channels.

Similarity between the angular distributions of the first- and second-step electrons in sequential two-photon atomic double ionization

Angular distributions of electrons produced in sequential two-photon atomic double ionization are considered theoretically by comparing the results obtained for the first and the second electron. It is shown analytically that the angular distributions of the two emitted electrons in many cases are similar for ionization from a closed-shell atom. Numerical examples of this counter-intuitive similarity are presented. In addition, examples are given demonstrating the difference between the angular distributions of the first photoelectron in sequential two-photon double ionization and in conventional one-photon single ionization.

Orbital Energies for Seniority-Zero Wave Functions.

A new single-pair operator for seniority-zero wave functions is introduced closely related to the single-particle Fock operator from conventional Hartree-Fock theory. This allows one to ascribe orbital energies in the context of model Hamiltonians, for which no single-particle operators exist. Several applications demonstrate the usefulness of these orbital energies. In analogy to Koopmans' theorem, atomic double ionization potentials are successfully predicted. A computationally efficient second-order perturbation scheme for seniority-zero wave functions scaling quadratically with system size is defined and applied to the dissociation of nitrogen and to strongly correlated two-dimensional Heisenberg lattices. An extension of the method for full seniority is presented leading to an intruder-free single reference perturbation theory with improved asymptotic convergence.

Single and Double Ionization of Atoms and Small Molecules by Short-Pulse Intense Laser Fields

Examples of recent calculations for the interaction of intense short-pulse laser fields with (quasi)one- and two-electron atomic (H, He, Li) and molecular (H+2, H2) targets are presented. In light of the often challenging parameter sets chosen in both experiment and other theoretical work, the need to carefully check both the numerical aspects and physical effects such as resonances is emphasized.

Ionization of atomic hydrogen by electrons: the role of the contributions of the pseudo-states in the second Born approximation

The second Born approximation is often used, particularly when we study double processes such as the ionization–excitation and the double ionization of atoms and molecules by charged particles. But when we apply this approximation, it needs the knowledge of all excited states of the target. In this study, we apply the second Born approximation by using 294 excited and pseudo-states for the ionization of atomic hydrogen by electrons. We compare the results of our model with those given by other models and to all experiments performed with an incident energy of 250 eV. We show that our new version of the second Born approximation gives better agreement than previous versions even for high values of the energy of the ejected electrons (50 eV).

Single-photon double ionization of He with generalized Sturmian basis

In this work we present the application of the generalized Sturmian basis to the single photon - double ionization of atoms.

Non-dipole effects in the angular distribution of photoelectrons in sequential two-photon atomic double ionization

Motivated by the recent achievements of experiments using x-ray free electron lasers, we have developed a theory for the angular distributions of photoelectrons in sequential two-photon double ionization (2PDI) of atoms beyond the dipole approximation. Expressions for the angular distributions are obtained taking into account the full multipole expansion of radiation in electric and magnetic moments. The formalism is further specified for the first-order corrections to the dipole approximation. As an illustrative example, the sequential 2PDI of the 2p shell in atomic neon is studied. The numerical calculations predict distinct non-dipole effects observable in experiments at present x-ray free electron lasers.

Theoretical estimate of fivefold differential cross sections and spin asymmetry forK-shell(e,3e)processes with transversely-spin-polarized relativistic electrons

Ab initio first-order Born calculations of the fivefold differential cross section (FDCS) and transverse spin asymmetry in FDCS for the $K$-shell double ionization of atoms by transversely polarized incident electron are reported. The FDCS and transverse spin asymmetry in FDCS are found to be sensitive to atomic number $Z$, ejection angle, and energy-sharing ratio of the ejected electrons. FDCS is decomposed into longitudinal and transverse contributions and their effects on the angular profile of FDCS such as shifting in the binary peak are reported. Hitherto, all $(e,3e)$ experiments have been performed in the nonrelativistic energy regime without consideration of spin polarization. We hope that the present calculation of FDCS and spin asymmetry in FDCS may induce quantum mechanically complete $(e,3e)$ experiments in the relativistic energy regime wherein spin-dependent interaction becomes prominent.

Two-photon double ionization of atoms in attosecond x-ray radiation fields

We consider two-photon double ionization of helium with 100, 200, and 400 eV excess energy for the two ejected electrons, corresponding to photon energies of 89.5, 139.5, and 239.5 eV, respectively. We focus on the case of ultrashort pulses (two oscillations of the field) and develop an approach to calculate the two-photon transition matrix elements within the lowest order of the time-dependent perturbation theory. One of the major difficulties in calculating such amplitudes is the infinite summation over a complete set of intermediate states. In the subfemtosecond regime, however, this summation can be performed accurately by means of the closure approximation. This results in a simple expression for the two-photon amplitude that contains a dipole term and a quadrupole term. The dipole term can be clearly associated to a process in which each electron absorbs a photon whereas the quadrupole term is associated to a process in which one electron absorbs two photons and ejects the second one by collision. We analyze in detail how the relative weight of both processes influences the behavior of the electron energy and angular distributions. In particular we study how the shape of these distributions changes with the amount of electron correlations taken into account in both initial and final states. For 100 eV excess energy, our results for the electron energy distribution are compared with those obtained by solving the time-dependent Schrodinger equation. All these results unveil the crucial role of electron correlations in this transient regime of ionization which is neither sequential nor direct.

Two-Electron Time-Delay Interference in Atomic Double Ionization by Attosecond Pulses

A two-color two-photon atomic double ionization experiment using subfemtosecond uv pulses can be designed such that the sequential two-color process dominates and one electron is ejected by each pulse. Nonetheless, ab initio calculations show that, for sufficiently short pulses, a prominent interference pattern in the joint energy distribution of the sequentially ejected electrons can be observed that is due to their indistinguishability and the exchange symmetry of the wave function.

Angular distributions and correlations in sequential two-photon atomic double ionization

Theory of the sequential two-photon double ionization of atoms in the XUV wavelength region is discussed in the context of recent experiments with the free-electron laser. Angular distributions and angular correlations of photoelectrons are analyzed within the statistical tensor formalism and examples are given for sequential two-photon double ionization of noble gases.

Effects of Elliptical Polarization on Strong-Field Short-Pulse Double Ionization

We predict new end-of-pulse behavior in high-field atomic double ionization. Calculations of atomic electron trajectories in short intense laser pulses confirm our analysis of elliptical polarization. We exhibit a four-band structure in ion momentum distributions under various ellipticities, and predict that sequential and nonsequential double ionization can be cleanly distinguished under elliptical polarization.

Angular distributions of electrons in sequential two-photon double ionization of atoms by free electron laser radiation

Calculated results for angular distribution and angular correlation of electrons produced in the process of two-photon double ionization of Xe are presented. In particular, a peculiar dependence of the angular distribution of the second step electrons on the alignment of the intermediate ionic state is predicted. The angular correlations of the electrons qualitatively depend on the term of the residual double charged ion. The results can be used in ongoing and future experiments with free-electron laser.

Strong Field Double Ionization: The Phase Space Perspective

We identify the phase-space structures that regulate atomic double ionization in strong ultrashort laser pulses. The emerging dynamical picture complements the recollision scenario by clarifying the distinct roles played by the recolliding and core electrons, and leads to verifiable predictions on the characteristic features of the "knee", a hallmark of the nonsequential process.

Theory of laser-driven double-ionization of atoms at Ti:sapphire laser wavelengths

Progress in the theoretical understanding of non-sequential double-ionization of atoms is reviewed from its beginnings with Kuchiev's work in the late 1980s and Corkum's work in the early 1990s to the present day. The crucial role of laboratory experiment as a persistent stimulus to theoretical endeavour is underlined but the predictive roles of simple, yet fundamental, theory and also of a full quantum mechanical description are not forgotten. A theoretical forward look is provided.

Double ionization of He and Li by ion impact: Final state correlation

The total cross-sections for atomic double ionization are evaluated with a two-step approximation, by considering that the projectile ionizes successively each electron. Single ionization probabilities are calculated within the continuum distorted-wave with eikonal initial state (CDW-EIS) formalism. The initial and final atomic wave functions are obtained by solving numerically the atomic wave equations with an optimized potential model (OPM). The electron–electron correlation in the final state is investigated following three different approaches: the usual Gamow factor, a modified Gamow factor with an effective charge and a mean value of the electron–electron repulsive Coulomb wave. The calculations are compared with experimental data and good agreement is found for double ionization of He and Li atoms by H +, He 2+and Li 3+ impact, at intermediate energies.

Recollision Dynamics and Time Delay in Strong-Field Double Ionization

Three-dimensional classical ensembles are employed to study recollision dynamics in double ionization of atoms by 780-nm intense lasers. After recollision one electron typically remains bound to the atom for a portion of a laser cycle, during which time the nucleus strongly influences its direction of motion. The electron then escapes over a suppressed barrier, with its final momentum depending critically on the laser phase at escape. The other electron remains unbound after collision, and typically drifts out in a momentum hemisphere opposite from its motion just after the collision. Several example trajectories at intensity 0.4 PW/cm(2) with various time delays between recollision and ionization are presented.