Double Ionization Of Helium Atom Research Articles

Laser-assisted two-color two-photon double ionization of helium atoms

Correlated momentum and kinetic energy distributions of two photoelectrons in laser-assisted two-color two-photon double ionization of helium are investigated by numerically solving a one-plus-one dimensional time-dependent Schrödinger equation (TDSE). We find that the weak assisting laser field can act as an energy transferring field, resulting in burst of double ionization triggered by the intense extreme ultraviolet (XUV) pulse. More importantly, the participation of the laser photon into the double ionization reshapes the correlation patterns in the momentum and kinetic energy distributions. The laser photon can be absorbed by any one of the two electrons, providing two channels that induces destructive interference in the correlated momentum and kinetic energy distributions, which is never found in previous work.

Laser-assisted XUV double ionization of helium atoms: Intensity dependence of joint angular distributions**Project supported by the National Natural Science Foundation of China (Grant Nos. 11774131 and 91850114).

We investigate the intensity effect of ultrashort assisting infrared laser pulse on the single-XUV-photon double ionization of helium atoms by solving full six-dimensional time-dependent Schrödinger equation with implement of finite element discrete variable representation. The studies of joint energy distributions and joint angular distributions of the two photoelectrons reveal the competition for ionized probabilities between the photoelectrons with odd parity and photoelectrons with even parity in single-XUV-photon double ionization process in the presence of weak infrared laser field, and such a competition can be modulated by changing the intensity of the weak assisting-IR laser pulses. The emission angles of the two photoelectrons can be adjusted by changing the laser parameters as well. We depict how the assisting-IR laser field enhances and/or enables the back-to-back and side-by-side emission of photoelectrons created in double ionization process.

Calculation of fully differential cross sections for the near threshold double ionization of helium atoms

Fully differential cross sectional (FDCS) results are reported for the electron-impact double ionization of helium atoms at 5 and 27 eV excess energy. The present attempt to calculate the FDCS in the second Born approximation and treating the postcollision interaction is helpful to analyze the measurements of Ren et al (2008 Phys. Rev. Lett. 101 093201) and Durr et al (2007 Phys. Rev. Lett. 98 193201). The second-order processes and postcollision interaction have been found to be significant in describing the trends of the FDCS. More theoretical effort is required to describe the collision dynamics of electron-impact double ionization of helium atoms at near threshold.

Criterion for Distinguishing Sequential from Nonsequential Contributions to the Double Ionization of Helium in Ultrashort Extreme-Ultraviolet Pulses.

We quantify sequential and nonsequential contributions in two-photon double ionization of helium atoms by intense ultrashort extreme-ultraviolet pulses with central photon energies ℏω_{ctr} near the sequential double-ionization threshold. If the spectrum of such pulses overlaps both the sequential (ℏω>54.4 eV) and nonsequential (ℏω<54.4 eV) double-ionization regimes, the sequential and nonsequential double-ionization mechanisms are difficult to distinguish. By tracking the double-ionization asymmetry in joint photoelectron angular distributions, we introduce the two-electron forward-backward-emission asymmetry as a measure that allows the distinction of sequential and nonsequential contributions. Specifically, for ℏω_{ctr}=50 eV pulses with a sine-squared temporal profile, we find that the sequential double-ionization contribution is the largest at a pulse length of 650 as, due to competing temporal and spectral constraints. In addition, we validate a simple heuristic expression for the sequential double-ionization contribution in comparison with ab initio calculations.

Multiphoton Rabi oscillations of correlated electrons in strong-field nonsequential double ionization

With quantum calculations, we have investigated the multiphoton nonsequential double ionization of helium atoms in intense laser fields at ultraviolet wavelengths. Very surprisingly, we found a so-far unobserved double-circle structure in the correlated electron momentum spectra. The double-circle structure essentially reveals multiphoton Rabi oscillations of two electrons, which are strongly supported by the oscillating population of a certain doubly excited state and by the oscillating double ionization signals. This two-electron multiphoton Rabi effect provides a profound understanding of electronic correlations and complicated multiphoton phenomena and is expected to be a new tool for broad applications, such as quantum coherent control.

Energy correlation in above-threshold nonsequential double ionization at 800 nm

We have investigated the energy correlation of the two electrons from nonsequential double ionization of helium atom in 800 nm laser fields at intensities below the recollision threshold by quantum calculations. The circular arcs structure of the correlated electron momentum spectra reveals a resonant double ionization process in which the two electrons transit from doubly excited states into continuum states by simultaneously absorbing and sharing excess energy in integer units of the photon energy. Coulomb repulsion between the two electrons in continuum states is responsible for the dominant back-to-back electron emission and two intensity-independent cutoffs in the two-electron energy spectra.

On the double ionization of helium by very slow antiproton impact

Here we present experimental information on the cross section for double ionization of helium atoms by slow antiproton impact. It is used to discern between many advanced theoretical calculations. Earlier measurements of the ratio R between the double and single ionization cross sections for antiproton impact on helium show a persistent increase for the projectile energy decreasing from 10MeV to 10keV. The present data show that below 10keV this increase stops and we give an upper limit to R.

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.

Ionization of atomic vapors and helium by collision with relativistic heavy ions

We have studied collisions of atomic vapors and helium with relativistic, highly charged ions having Z ∼ v ∼ c. Formulas for the cross-sections of ionization and excitation of discrete levels of the atomic vapor and the cross-section of single and double ionization of helium atoms have been obtained. Comparisons with experimental data of the single and double ionization of helium are presented.

The single and double ionization of helium atoms by fast nuclei and multicharged ions

The cross sections for the single and double ionization of helium by nuclei with charges Z = 1−100 at energies E = 0.05−4.5 MeV/amu have been calculated in the independent electron approximation with the use of the unitarized one-electron transition probabilities. It is shown that at E ⪆ 0.2 MeV/amu the single ionization cross sections σ i1 BD calculated in the Born version of the decay model agree, within 10–15%, with the available experimental cross sections for nuclei and multicharged hydrogen- and helium-like ions with charges Z≤ 8, and the calculated double ionization cross sections σ i2 BD at Z/ v > 0.2 are 1.5–2 times greater than the experimental σ i2, but their dependences on charge Z and ion velocity ν are close to the experimental ones. It is found that the ratios of the calculated and experimental cross sections σ i1 and σ i2 to the total Born ionization cross sections σ B i at Z/ υ ⪅ 1.5 depend, with the accuracy of 15%, only on the parameter Z/ ν and at any Z-value these ratios should be described by the functions of Z/ ν 1.25 with ±15% accuracy. Taking into correct account the nondipole transitions, we have shown that for ions with Z ⪆ 30 at √2 Z ⪅ ν ⪅ Z/3 the single ionization cross sections σ i1 should be a factor of 1.5 smaller than those for ions with Z = 4−8 at the same E/ Z. It is found that at Z > 3ν ⪆ 10 the ratio of double to single ionization cross section R i = σ i2 / σ i1 much less sensitive to Z and ν then in the region of Z = (0.3−1) ν.

Cross sections for double ionization of helium atoms by protons

The cross section for double ionization of a helium atom by protons has been calculated on the assumption that each electron is removed independently.

Equivalence of the Sudden Approximation to the High-Energy Limit of the First Born Approximation

The ratio of total cross sections for two different inelastic events in a given target as predicted by the sudden approximation is shown to be equal to the high-energy limit of the corresponding first Born approximation to the ratio. This relationship between the two theories is established for the special cases of the ratio of single to double ionization of helium atoms by electrons and by hydrogen atoms. The electron-helium-atom ratio is found to be the same as the hydrogen-atom-helium-atom ratio. The proof supposes that the same approximate wave functions were used in both theories and that the dominant term in the high-energy expansion of the first Born total cross section is independent of energy-conservation requirements. This latter point is established for a general ionization event.