Double Ionization Of Atoms Research Articles

Variable Time Lag and Backward Ejection in Full-Dimensional Analysis of Strong-Field Double Ionization

Ensembles of 400,000 two-electron trajectories in three space dimensions are used with Newtonian equations of motion to track atomic double ionization under very strong laser fields. We report a variable time lag between e-e collision and double ionization, and find that the time lag plays a key role in the emergence directions of the electrons. These are precursors to production of electron momentum distributions showing substantial new agreement with experimental data.

Binary encounter approximation for the electron-impact double ionization cross section of atoms and ions

An binary encounter approximation is formulated for the direct (without any excitation of autoionization levels) double ionization of atoms and ions by electron impacts. Analytical expressions are derived for the cross section, which qualitatively represent the main dependences on the incident electron energy and atomic parameters.

Double ionization of atoms by ion impact: two-step models

Total cross sections for the double ionization of He and Li atoms by the impact of H+, He2+ and Li3+ are calculated at intermediate and high energies within two-step models. The double ionization of He by the impact of other bare projectiles at a fixed energy is obtained as well. Single ionization probabilities are calculated within the continuum distorted wave –eikonal-initial-state (CDW–EIS) approximation. The required atomic bound and continuum wave functions are evaluated by numerically solving the atomic wave equation with an optimized potential model (OPM). Correlation between events is introduced by considering ion relaxation. The final state electronic correlation is considered by means of the so-called Gamow factor. We compare the transition probabilities resulting from our approach with those resulting from the use of a Rootham–Hartree–Fock initial state and a Coulomb continuum state with an effective charge. We find that the use of OPM waves gives a better agreement with the experimental results than with Coulomb waves.

Comparison of asymptotic forms of the wave function for double ionization

We investigate and compare two asymptotic wave functions for atomic double ionization in the region where the distance between the escaping electrons is small when compared to the distance from the nucleus. The first of the forms was derived by Alt and Mukhamedzhanov and solves the three body Schrodinger equation to an order of 1/r2. The second wave function was derived by Engelns, Klar and Malcherek for use in all asymptotic regimes. These forms will be compared numerically and analytically.

Charge and energy-dependence of the Gaussian description of the triply differential cross sections for equal-energy sharing photo-double-ionization of two-electrons ions

We evaluate triply differential cross sections (TDCSs) for the photo-double-ionization (PDI) of He-like ions, and equal electron energy sharing, by using the SC3 model for the three-body final state. These cross sections are fitted with the usual dipolar Gaussian form which is found able to describe the theoretical TDCS, and could be applied for the interpretation of experimental data even at intermediate photon energies. We determine the dependence of the correlation factor on the excess energy $({E}_{f})$ and target nuclear charge $(Z)$. We find that its width has an ${E}_{f}^{1∕4}$ dependence near the atomic double-ionization threshold but departs from this law and attains a plateau as the excess energy increases. We compare our results with the predictions of classical and semiclassical Wannier approaches.

Nonsequential Double Ionization as a Completely Classical Photoelectric Effect

We introduce a unified and simplified theory of atomic double ionization. Our results show that at high laser intensities (I>/=10(14) W/cm(2)) purely classical correlation is strong enough to account for all of the main features observed in experiments to date.

Origin of correlated electron emission in double ionization of atoms

We investigate the origin of coordinated two-electron ionization of atoms under irradiation by short, intense laser pulses. We introduce a wave-function masking technique that separates the wave function into multiple components, based on the location of the population at some preselected time. Propagating these components separately allows a semiclassical tracking of populations and recollision events, with the full wave function being obtainable at any time as the coherent sum of the masked components. This method reveals that for our model system the most significant pathway to coordinated double ionization involves a correlated recollision event in which the returning electron comes back to the core in opposition to the laser force.

Erratum: Physics of correlated double ionization of atoms in intense laser fields: Quasistatic tunneling limit [Phys. Rev. A63, 033404 (2001)

Erratum: Physics of correlated double ionization of atoms in intense laser fields: Quasistatic tunneling limit [Phys. Rev. A<b>63</b>, 033404 (2001)

Time ordering in fast double ionization

We consider the role of time ordering in the production of multiply charged ions by examining the role of time ordering in two-electron transitions such as double ionization of atoms in fast ion–atom and photon–atom collisions. If two or more electrons are uncorrelated in space, then transitions of these electrons evolve independently in time. If the electrons are correlated, then the transition of one electron can affect the time evolution of the other electron. In the interaction picture correlation in time is carried by the part of the time-ordering operator T which is antisymmetric in time. It is this part of T that gives nonequal weight to the time ordering of the interactions causing the electron transitions. The antisymmetric part of T is nonzero only if electron correlation is present. Thus, correlation in time between transitions of different electrons is connected to spatial electron correlation due to the electron–electron interaction. We also note an invariance in the product of the time and charge symmetry of the projectile in fast ion–atom collisions. Consequently, effects of the antisymmetric part of T may be found in regions where there are substantial contributions antisymmetric in the projectile charge, Z (e.g. Z 3 contributions) to transition probabilities and cross sections, as evident in double ionization of atoms and molecules.

Double ionization of atoms by multiply charged ions and a strong electromagnetic field: Effects of correlation in the continuum

A nonstationary theory of double ionization of two-electron atoms in collisions with multiply charged ions or by an intense electromagnetic field is developed. An approach that permits investigating both problems by a single method is formulated. A two-electron continuum wave function that takes into account the interaction of the electrons with the atomic nucleus and the external ionizer as well as with one another is obtained as a product of Coulomb waves with modified Sommerfeld parameters. The computational results obtained for the double ionization of helium atoms by multiply charged ions are in good quantitative agreement with the existing experimental data.

Photodouble ionization of atoms and molecules near threshold

New experimental techniques have been applied to the study of single-photon double ionization of atoms and molecules. A coincidence technique is described in which the spectra of nearly zero energy photoelectrons are recorded in two identical photoelectron energy analysers. The results are discussed in terms of the available theoretical calculations. New techniques for the measurement of double differential cross sections (DDCS) and triple differential cross sections (TDCS) are discussed. New results using these techniques are presented, in which DDCS and TDCS are measured as continuous functions of photon energy.

Single and double ionization of atoms by Compton scattering

Recent developments in the theoretical treatment of Compton scattering from bound electrons are examined. These include the investigation of electron-electron effects in scattering from helium and the application of relativistic S matrix techniques to the process. The possibility of extending the use of Compton scattering to the investigation of correlations in other systems is discussed. The contributions of resonant terms, not usually considered in calculations of integrated cross sections, are examined.

Ionization of atoms by fully stripped ions in the glauber approximation

We report a method of evaluation of cross sections for single and double ionization of atoms by fully stripped ions. The method is based on the Glauber approximation. In the case of a hydrogenic target the probability amplitude turns out to be a simple two-dimensional integral which can be evaluated numerically with convenience.