On the electron impact ionization of multi-charged ions: a theoretical investigation of relativistic effects

The electron impact ionization of highly charged ions is investigated. Using a relativistic distorted wave Born approximation, we explore the possible effects that should be observable in a high-energy electron impact ionization (e, 2e) coincidence experiment involving multi-charged ions. We present calculations of triple-differential cross sections. We will focus on relativistic and distortion effects and consider geometries where these may be easily observed.

Single ionization of helium by antiproton impact

We study the ionization of helium (fie) in collision with antiprotons (p) in the energy range from 10 keV to 1000 keV. We adopt a semiclassical single center close coupling approach in which the wave function for the electron is expanded in a B-spline basis centered on the nucleus of the He atom, The calculations are performed using two different models: the independent particle (IP) model and the one-electron (OE) approximation. The interaction between the active electron and the rest of the atom, i.e. passive electron and nucleus, is represented by a model potential. The results obtained are compared with experimental data as well as with existing theoretical calculations. (c) 2005 Published by Elsevier B.V.

Ionization of atomic hydrogen and He+ by slow antiprotons

We study the ionization of H(1s), He+(1s) and He+(2s) by antiprotons in the energy range from 0.1 to 500 keV. We adopt a semiclassical single centre close-coupling approach in which the wavefunction for the electron is expanded in a B-spline basis centred on the nucleus of the atom/ion. Comparison is made with existing theoretical calculations and available experimental data. The results are encouraging.

Dissociative ionization of molecules in intense laser fields

Accurate and efficient grid based techniques for the solution of the time-dependent Schrodinger equation for few-electron diatomic molecules irradiated by intense, ultrashort laser pulses are described. These are based on hybrid finite-difference, Lagrange mesh techniques. The methods are applied in three scenarios, namely H-2(+) with fixed internuclear separation, H-2(+) with vibrating nuclei and H-2 with fixed internuclear separation and illustrative results presented.

Dynamic tunnelling ionization of H-+(2) in intense fields

Intense-field ionization of the hydrogen molecular ion by linearly polarized light is modelled by direct solution of the fixed-nuclei time-dependent Schrodinger equation and compared with recent experiments. Parallel transitions are calculated using algorithms which exploit massively parallel computers. We identify and calculate dynamic tunnelling ionization resonances that depend on laser wavelength and intensity, and molecular bond length. Results for lambda similar to 1064 nm are consistent with static tunnelling ionization. At shorter wavelengths lambda similar to 790 nm large dynamic corrections are observed. The results agree very well with recent experimental measurements of the ion spectra. Our results reproduce the single peak resonance and provide accurate ionization rate estimates at high intensities. At lower intensities our results confirm a double peak in the ionization rate as the bond length varies.

Multi-ionization of helium and lithium using the independent electron and independent event models with intrinsic CDW

Cross sections for the multi-ionization of He and Li are presented for impact energies in the range of 50 to 1000 keV/amu. These are calculated using the eikonal initial state approximation to represent the input and exit channels of the active electrons. The ionization process is simulated in a variety of ways, most notably an attempt to account for the effects of electron correlation via the inclusion of a continuum density of states (CDS) term. Inadequacies, of the CDW formulation at small impact parameters, and of the models themselves, are discussed and conclusions are drawn on what repercussions this has for the cross sections calculated.

Electron-impact ionization of atomic hydrogen close to threshold

A systematic study of the ionization of atomic hydrogen by electron impact from 0.3 eV to a few eV above the ionization threshold has been carried out using a semiclassical-quantal calculation. Differential and integrated cross sections are presented at 0.3 eV above the energy threshold. Triple- differential cross sections (TDCS) are presented at constant theta(12) geometry where theta(12)=180degrees and 150degrees. Good agreement is achieved with the measurement [Roder, Phys. Rev. Lett. 79, 1666 (1997)] and calculations based on exterior complex scaling at 2 eV and 4 eV above threshold. Results of triple-differential cross sections are also presented at 0.3, 0.5, and 1.0 eV above threshold at both theta(12)=180degrees and 150degrees. At theta(12)=180degrees the small local maximum in the TDCS around theta(1)=90degrees reported by Pan and Starace [Phys. Rev. A 45, 4588 (1992)] at 0.5 eV above threshold is not observed in our calculation at energies down to 0.3 eV above threshold. The shape of our double differential cross sections seems to disagree qualitatively with the available calculations as we found two local maxima around 15degrees and 165degrees in our calculation. Single differential cross sections in our formulation appear naturally as a function of total excess energy E and, therefore, constant for all combinations of individual electron energies E-1 and E- 2 with E=E-1+E-2. Total ionization cross sections are also compared with measurement and available theoretical calculations and found to be in reasonably good agreement up to 10 eV above ionization threshold.

Semiclassical-quantal approach to the near-threshold ionization of hydrogen

A recent improved version of the semiclassical-quantal approach has been applied to the e(-)-H near-threshold ionization for theta (12) = 180 degrees geometry. It is found, that unlike other sophisticated theoretical methods such as distorted wave theory or convergent close-coupling calculation, the present relatively simpler approach produces correct behavior and numerical values for the triple-differential cross sections. We compare our results with recent absolute measurements and accurate numerical calculations at 2 eV and 4 eV above the threshold at constant theta (12) geometry.

Electron impact ionization of He above the threshold

Near-threshold ionization of He has been studied by using a uniform semiclassical wavefunction for the two outgoing electrons in the final channel. The quantum mechanical transition amplitude for the direct and exchange scattering derived earlier by using the Kohn variational principle has been used to calculate the triple differential cross sections. Contributions from singlets and triplets are critically examined near the threshold for coplanar asymmetric geometry with equal energy sharing by the two outgoing electrons. It is found that in general the tripler contribution is much smaller compared to its singlet counterpart. However, at unequal scattering angles such as theta (1) = 60 degrees, theta (2) = 120 degrees the smaller peaks in the triplet contribution enhance both primary and secondary TDCS peaks. Significant improvements of the primary peak in the TDCS are obtained for the singlet results both in symmetric and asymmetric geometry indicating the need to treat the classical action variables without any approximation. Convergence of these cross sections are also achieved against the higher partial waves. Present results are compared with absolute and relative measurements of Rosel et al (1992 Phys. Rev. A 46 2539) and Selles et al (1987 J. Phys. B. At. Mel. Phys. 20 5195) respectively.

Electron impact ionization of hydrogen-like molybdenum ions

The electron impact ionization cross sections of hydrogen-like molybdenum ions were measured with an electron beam ion trap at the electron energies of 49.4, 64.4 and 79.6 keV The results are 2.82(22) x 10(-23), 3.13(29) x 10(-23) and 3.23(51) x 10(-23) cm(2), respectively. These results are compared with the experimental results measured previously. The agreement with the results obtained with, scaling formulae is also discussed.

Electron-impact ionization of hydrogen-like iron ions

Electron-impact ionization cross sections have been determined for hydrogen like iron ions at selected electron energies between 1.45 and 4.3 times the threshold energy. The cross sections were obtained by measuring the equilibrium ionization balance in an electron beam ion trap. This ionization balance is obtained from x-ray measurements of radiative recombination into the K-shell of hydrogen-like and bare iron ions. The measured cross sections are compared with distorted-wave calculations and several semiempirical formulations.

Suppression of multiple ionization of atomic ions in intense ultrafast laser pulses

The interaction of an intense laser field with a beam of atomic ions has been investigated experimentally for the first time. The ionization dynamics of Ar+ ions and Ar neutrals in a 60 fs, 790 nm laser pulse have been compared and contrasted at intensities up to 10(16) W cm (-2). Our results show that nonsequential ionization from an Ar+ target is strongly suppressed compared with that from the corresponding neutral target. We have also observed for the first time the strong field ionization of high lying target metastable levels in the Ar+ beam.

Dissociative charge exchange and ionization of O-2 by fast H+ and O+ ions: Energetic ion interactions in Europa's oxygen atmosphere and neutral torus

Measurements of electron capture and ionization of O-2 molecules in collisions with H+ and O+ ions have been made over an energy range 10 - 100 keV. Cross sections for dissociative and nondissociative interactions have been separately determined using coincidence techniques. Nondissociative channels leading to O-2(+) product formation are shown to be dominant for both the H+ and the O+ projectiles in the capture collisions and only for the H+ projectiles in the ionization collisions. Dissociative channels are dominant for ionizing collisions involving O+ projectiles. The energy distributions of the O+ fragment products from collisions involving H+ and O+ have also been measured for the first time using time-of-flight methods, and the results are compared with those from other related studies. These measurements have been used to describe the interaction of the energetic ions trapped in Jupiter's magnetosphere with the very thin oxygen atmosphere of the icy satellite Europa. It is shown that the ionization of oxygen molecules is dominated by charge exchange plus ion impact ionization processes rather than photoionization. In addition, dissociation is predominately induced through excitation of electrons into high-lying repulsive energy states ( electronically) rather than arising from momentum transfer from knock-on collisions between colliding nuclei, which are the only processes included in current models. Future modeling will need to include both these processes.

Molecular effects in the ionization of N-2, O-2, and F-2 by intense laser fields

In this paper we study the response in time of N2, O2, and F2 to laser pulses having a wavelength of 390 nm. We find single-ionization suppression in O2 and its absence in F2, in accordance with experimental results at lambda= 800 nm. Within our framework of time-dependent density functional theory we are able to explain deviations from the predictions of intense-field many-body S-matrix theory (IMST). We confirm the connection of ionization suppression with destructive interference of outgoing electron waves from the ionized electron orbital. However, the prediction of ionization suppression, justified within the IMST approach through the symmetry of the highest occupied molecular orbital (HOMO), is not reliable since it turns out that—e.g., in the case of F2—the electronic response to the laser pulse is rather complicated and does not lead to dominant depletion of the HOMO. Therefore, the symmetry of the HOMO is not sufficient to predict ionization suppression. However, at least for F2, the symmetry of the dominantly ionized orbital is consistent with the nonsuppression of ionization.

Single-ionization of helium at Ti:sapphire wavelengths: rates and scaling laws

We present a numerical and theoretical study of intense-field single-electron ionization of helium at 390 nm and 780 nm. Accurate ionization rates (over an intensity range of (0.175-34) X10^14 W/ cm^2 at 390 nm, and (0.275 - 14.4) X 10^14 W /cm^2 at 780 nm) are obtained from full-dimensionality integrations of the time-dependent helium-laser Schroedinger equation. We show that the power law of lowest order perturbation theory, modified with a ponderomotive-shifted ionization potential, is capable of modelling the ionization rates over an intensity range that extends up to two orders of magnitude higher than that applicable to perturbation theory alone. Writing the modified perturbation theory in terms of scaled wavelength and intensity variables, we obtain to first approximation a single ionization law for both the 390 nm and 780 nm cases. To model the data in the high intensity limit as well as in the low, a new function is introduced for the rate. This function has, in part, a resemblance to that derived from tunnelling theory but, importantly, retains the correct frequency-dependence and scaling behaviour derived from the perturbative-like models at lower intensities. Comparison with the predictions of classical ADK tunnelling theory confirms that ADK performs poorly in the frequency and intensity domain treated here.