Nonsequential Multiple Ionization Research Articles

Comparison study for multiple ionization of carbonyl sulfide by linearly and circularly polarized intense femtosecond laser fields using Coulomb explosion imaging

We studied the relative yields and dissociation dynamics for two- and three-body Coulomb explosion (CE) channels from highly charged carbonyl sulfide molecules in intense laser fields using the CE imaging technique. The electron recollision contributions are evaluated by comparing the relative yields for the multiple ionization process in linearly polarized and circularly polarized (LP and CP) laser fields. The nonsequential multiple ionization is only confirmed for the charge states of 2 to 4 because the energy for further ionization from the inner orbital is much larger than the maximum recollision energy, 3.2Up. The novel deviations of kinetic energy releases distributions between LP and CP pulses are observed for the charge states higher than 4. It can be attributed to the stronger molecular bending in highly charged states before three-body CE with CP light, in which the bending wave packet is initialed by the triple or quartic ionization and spread along their potential curves. Compared to LP light, CP light ionizes a larger fraction of bending molecules in the polarization plane.

Highly ionized xenon and volumetric weighting in restricted focal geometries

The ionization of xenon atoms subjected to 42 fs, 800 nm pulses of radiation from a Ti:Sapphire laser was investigated. In our experiments, a maximum laser intensity of ∼2×1015W/cm2 was used. Xenon ions were measured using a time-of-flight ion mass spectrometer having an entrance slit with dimensions of 12μm×400μm. The observed yields Xen+(n=1−7) were partially free of spatial averaging. The ion yields showed sequential and nonsequential multiple ionization and dip structures following saturation. To investigate the dip structures and to perform a comparison between experimental and simulated data, with the goal of clarifying the effects of residual spatial averaging, we derived a hybrid analytical-numerical solution for the integration kernel in restricted focal geometries. We simulated xenon ionization using Ammosov-Delone-Krainov and Perelomov-Popov-Terent'ev theories and obtained agreement with the results of observations. Since a large number of experiments suffer from spatial averaging, the results presented are important to correctly interpret experimental data by taking into account spatial averaging.

Attosecond electron thermalization in laser-induced nonsequential multiple ionization: hard versus glancing collisions

A recollision-based largely classical statistical model of laser-induced nonsequential multiple (N-fold) ionization of atoms is further explored. Upon its return to the ionic core, the first-ionized electron interacts with the other N- 1 bound electrons either through a contact or a Coulomb interaction. The returning electron may leave either immediately after this interaction or join the other electrons to form a thermalized complex which leaves the ion after the delay Δt, which is the sum of a thermalization time and a possible additional dwell time. Good agreement with the available triple and quadruple ionization data in neon and argon is obtained with the contact scenario and delays of Δt=0.17 T and 0.265 T, respectively, with T the laser period.

Speed-up and slow-down collisions in laser-induced non-sequential multiple ionization

We investigate laser-induced non-sequential multiple ionization using a simple statistical model in which an electron recollides inelastically with its parent ion. In this collision, it thermalizes with the remaining N − 1 bound electrons within a time interval Δt. Subsequently, all the N electrons leave. We address the question of how the above time delays influence the individual contributions from the orbits in which the first electron, upon return, is accelerated or decelerated by the field, respectively, to the ion momentum distributions. In both cases, the time delays modify the drift momenta obtained by the N electrons when they reach the continuum at t+Δt, by moving such times towards or away from a crossing of the electric field. The contributions from both types of collisions are influenced in distinct ways, and the interplay between such trends determines the widths and the peak momenta of the distributions. Specifically in the few-cycle pulse case, we also show that such time delays do not affect the shapes of the momentum distributions in a radical fashion. Hence, even with a thermalization time Δt, non-sequential multiple ionization could in principle be used for absolute-phase diagnosis.

Pathways to non-sequential multiple ionization in strong laser fields

The outgoing electrons in non-sequential multiple ionization in intense laser fields arestrongly correlated. The correlations can be explained within a classical model forinteracting electrons in the presence of the external field. Here we extend the previousanalysis for two and three electrons to cases with up to eight electrons and identify thesaddle configurations that guard the channels for non-sequential multiple ionization. Forfour and fewer electrons, the electrons in the dominant configuration are equivalent; for sixand more electrons this is no longer the case. The case of five electrons is marginal, withtwo almost degenerate transition configurations. The total number of configurationsincreases rapidly, from two configurations for three electrons up to 26 configurations foreight electrons.

Introduction

In the interaction of intense laser fields with atoms, even subtle details of single ionization and high-harmonic generation can be well described in the so-called single-active-electron approximation where just one electron reacts to the laser field. Therefore, the surprisingly high efficiency of laser-induced nonsequential multiple ionization (where two or more electrons are removed by the field in one coherent process) as opposed to sequential ionization (where they are dislodged one at a time) has raised great interest. Indeed, this premium for cooperation, that is, the ratio of nonsequential over sequential ionization, has been found to be as high as six orders of magnitude in the case of double ionization of helium at near-infrared wavelengths. Ever since these observations were made, the question of the physical mechanism behind this cooperation has been intensely debated, but no final conclusions have been reached.

Strong-field multiple ionization of krypton

We have investigated the multiple ion yields of Kr, up to ${\mathrm{Kr}}^{4+}$ as a function of the field intensity, that are measured using a linearly polarized short-pulsed Ti:sapphire laser of $\ensuremath{\lambda}=800\mathrm{nm}$ in the range of up to mid-${10}^{15}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2},$ within an S-matrix model for nonsequential multiple ionization. Experimental data and theoretical predictions are in good agreement and the influence of different nonsequential channels on the measured ion yields are analyzed.

Collective multielectron tunneling ionization in strong fields

We investigate the quantum mechanical process of two-electron tunneling in strong external electric fields. Numerical solution of a two-electron s-wave model reveals the existence of collective tunneling ionization in a mode where both electrons stay at equal distance from the nucleus. Otherwise the lagging electron is immediately recaptured. The corresponding double ionization rate fails to explain nonsequential multiple ionization in strong-field laser experiments. However, an empirically modified version of the analytical one-electron tunneling rate of Ammosov, Delone, and Krainov agrees with the experiments to a surprising accuracy. The reason for this agreement is presently unknown.

Momentum distributions of ne(n+) ions created by an intense ultrashort laser pulse

Vector momentum distributions of Ne(n+) (n = 1,2,3) ions created by 30 fs, approximately 1 PW/cm(2) laser pulses at 795 nm have been measured using recoil-ion momentum spectroscopy. Distinct maxima along the light polarization axis are observed at 4.0 and 7.5 a.u. for Ne2+ and Ne3+ production, respectively. Hence, mechanisms based on an instantaneous release of two (or more) electrons can be ruled out as a dominant contribution to nonsequential strong-field multiple ionization. The positions of the maxima are in accord with kinematical constraints set by the classical "rescattering model."

Production of high-charge states of Xe in a femtosecond laser pulse

In a recent experiment for ionization of the neutral Xe atom with a femtosecond Ti:sapphire laser pulse, large yields of high charge states up to sixfold ionizations have been observed. Here, we present a dynamical model of laser-induced nonsequential multiple ionization of a complex atom. The calculated results for the ion yields are found to agree remarkably well for the six charge states measured. Analyses show a significant presence of nonsequential (or direct) escapes of up to four electrons in the experimental data. For future experimental references, predictions for the sevenfold and eightfold ionizations are also made.