Double Ionization Probability Research Articles

Dissociative double-photoionization of butadiene in the 25-45 eV energy range using 3-D multi-coincidence ion momentum imaging spectrometry.

Dissociative double-photoionization of butadiene in the 25-45 eV energy range has been studied with tunable synchrotron radiation using full three-dimensional ion momentum imaging. Using ab initio calculations, the electronic states of the molecular dication below 33 eV are identified. The results of the measurement and calculation show that double ionization from π orbitals selectively triggers twisting about the terminal or central C-C bonds. We show that this conformational rearrangement depends upon the dication electronic state, which effectively acts as a gateway for the dissociation reaction pathway. For photon energies above 33 eV, three-body dissociation channels where neutral H-atom evaporation precedes C-C charge-separation in the dication species appear in the correlation map. The fragment angular distributions support a model where the dication species is initially aligned with the molecular backbone parallel to the polarization vector of the light, indicating a high probability for double-ionization to the "gateway states" for molecules with this orientation.

Double ionization of molecule H2 in intense ultrashort laser fields

By solving numerically the time-dependent Schrödinger equation (TDSE), we have calculated the double ionization probability when a vibrating hydrogen molecule interacts with intense ultrashort laser pulses. The results show that in the case of vibrating nuclei the double ionization probability is higher than that of the fixed nuclei. Additionally, the double ionization probability is larger if the molecule is vibrating in a higher level. This is due to the decreasing of ionization potential when the inter-nuclei separation increases.

Distinction between sequential and direct ionization in two-photon double ionization of helium

This paper aims to shed some light on the role of the direct, or nonsequential, ionization channel in the regime in which the sequential channel is open in two-photon double ionization (TPDI) of helium. In this regime the sequential channel dominates any direct contribution unless the laser pulse is of very short duration, in which case their distinction is hard to draw. Based on both a simple model and full solutions of the time-dependent Schr\"odinger equation, we aim to provide evidence of direct double ionization by identifying a term proportional to the pulse duration in the double ionization yield. Indeed, such a term is identified in the energy-differential yield. When it comes to the total double ionization probability, however, it turns out that the net first-order contribution is negative. The nature of the negative first-order contribution is discussed, and we argue that it is of correlated origin.

Sensitivity of strong-field double ionization to the initial ensembles in circularly polarized laser fields

Using the classical ensemble model, we investigate the sensitivity of double ionization in circularly polarized laser fields to the initial ensembles that are generated for an Ar atom with a definite total angular momentum. The numerical results show that the double-ionization probability in the counter-rotating case, where the electrons initially rotate against the laser field, can be much higher than that in the corotating case, where the electrons rotate with the laser field. By analyzing the energy distributions of double-ionized electrons versus time, we find that, for the counter-rotating case, the ionization of the second electron occurs much earlier than in the corotating case, and the first ionization can leave the $\mathrm{Ar}{}^{+}$ ion in a higher energy state, which can increase the second ionization probability. These results indicate that nonadiabatic effects can play an important role in double-ionization dynamics.

Auger decay and the direct double ionization probability of a 2p inner-shell hole in a singly charged Ar+ ion

Single and double Auger decay processes of Ar+ 2p −1 hole levels belonging to the configurations of 2p 53s 23p 5 and 2p 53s3p 6 are investigated in the framework of perturbation theory implemented by the distorted wave approximation. The single Auger decay channels and rates are determined and the predicted total rates can differ by more than an order of magnitude for the levels of 2p 53s 23p 5, yet they are very close for levels belonging to 2p 53s3p 6. By combining the double photoionization cross sections of the neutral species, our theoretical Auger spectra of the single Auger decay process nicely interpreted a recent experiment [S.M. Huttula et al., Phys. Rev. Lett. 110, 113002 (2013)]. A configuration averaged branching ratio of 6.5% and 7.3% are predicted for the direct double Auger decay to the total probability for 2p 53s 23p 5 and 2p 53s3p 6, respectively, which is smaller than that of Ar 2p −1 hole states.

Double ionization of He by time-delayed attosecond pulses

As He is exposed to double attosecond pulses with a time delay, double ionization can proceed in several different pathways. Through ab initio simulations by the fully dimensional time-dependent Schrödinger equation, we find that interferences among different pathways in the sequential two-photon double ionization can lead to interesting grid-like patterns in the joint energy spectra of the two electrons. We show that not only these interference patterns, but also the total double ionization probability critically depends on the relative carrier–envelope phase (CEP) between the two attosecond pulses. Our findings are successfully explained by a model based on second-order time-dependent perturbation theory. The present study demonstrates the feasibility of CEP control of double ionization, and provides an alternative way to characterize the relative CEP of attosecond pulses.

Resonance effects in two-photon double ionization of H2by femtosecond XUV laser pulses

We investigate the effect of the pulse length on the two-photon double ionization (DI) of H${}_{2}$ in the direct domain, for a femtosecond (fs) laser with a polarization vector oriented along the molecular axis. In the fixed-nuclei approximation, we find that the doubly excited ${Q}_{1}\phantom{\rule{0.28em}{0ex}}{}^{1}{\ensuremath{\Sigma}}_{u}^{+}$ states manifest themselves as resonances in the angle-integrated cross sections if the laser interaction lasts longer than about 3 fs. Decay into single-ionization channels does not significantly affect the shape of the angular distribution. A sharp rise in the probability for DI, due to virtual sequential ionization, occurs when the photon energy approaches the threshold for sequential DI.

Single-Photon Laser-Enabled Auger Spectroscopy for Measuring Attosecond Electron-Hole Dynamics

We propose and simulate a new type of attosecond time-resolved spectroscopy of electron-hole dynamics, applicable particularly to ultrafast hole migration. Attosecond ionization in the inner-valence region is followed by a vacuum ultraviolet probe inducing single-photon laser-enabled Auger decay, a one-photon-two-electron transition filling the inner-valence vacancy. The double ionization probability as a function of the attosecond pump-vacuum ultraviolet probe delay captures efficiently the ultrafast inner-valence hole dynamics. Detailed ab initio calculations are presented for inner-valence hole migration in glycine.

Numerical simulations of single-photon double ionization of the helium dimer

We study the energy exchange via electron correlation upon photon absorption over large distances in double photoionization of the helium dimer. Results of numerical simulations of the interaction of a planar helium dimer model with a short light pulse are found to be in good agreement with recent experimental data for the angular distribution of the emitted electron. The double ionization probability is closely related to that of the photoemission of an electron from one of the helium atoms along the internuclear axis. Together with an analysis of the temporal evolution of the two-electron probability distribution this provides direct evidence for the knockoff mechanism by which the photon energy is shared between the electrons over distances of several Angstroms in the dimer.

Effect of the laser magnetic field on nonsequential double ionization of He, Li+, and Be2+

The role of the magnetic field of the laser pulse and the importance of relativistic corrections to the equations of motion in the recollision double ionization of He, Li${}^{+}$, and Be${}^{2+}$ are investigated within a classical trajectory model. It is found that the inclusion of the magnetic field is important even at intensities as low as ${10}^{15}$ W/cm${}^{2}$, but that the relativistic corrections proportional to $1/{c}^{2}$ have no impact on either total probabilities or final momentum spectra. Two field configurations with counterpropagating pulses, previously proposed to circumvent the detrimental effect of the magnetic field on the double-ionization probability, are compared with the single-pulse case.

Double ionization of helium studied with the multi-configuration time-dependent Hartree–Fock method

The intensity dependence of double ionization probability of helium in a strong laser field is investigated by using the multi-configuration time-dependent Hartree–Fock (MCTDHF) method. The results agree with the previous theoretical report by directly solving the time-dependent Schrödinger equation. The time evolution of electronic population on the ground state, electronic energy and dipole moment are also presented and discussed.

Time-dependent formalism of double ionization of multielectron atomic targets

A time-dependent formalism for treating double ionization of atomic targets with core electrons is presented. It relies on the solution of the time-dependent Schrödinger equation with a combined orbital and grid-based representation of the time-evolving wave packet. Total and differential double ionization probabilities and cross sections are then obtained through a exterior complex scaling implementation. The method is demonstrated for single-photon double ionization of the valence electrons of beryllium atom using 500 attosecond pulsed lasers, with the inner-shell electrons held fixed in the frozen-core orbitals. The extracted cross sections are compared to existing experimental and time-independent theoretical results. The accuracy of the formalism is demonstrated in furtherance of investigation of physical processes induced by ultrashort pulses in multielectron atoms that cannot be achieved in a time-independent framework.

Ultrafast photoionization dynamics at high laser intensities in the xuv regime

We study the ionization dynamics in the soft-x-ray regime for high intensities and short pulses for excitations near the ionization threshold. Using a one-dimensional helium atom model, we compare exact numerical solutions with time-dependent Hartree-Fock results in order to identify the role of electron-electron correlations. At moderate intensities but still in the x-ray and short-pulse regime, we find that the Hartree-Fock theory reproduces well the dynamics of the ground-state occupation, while at high intensities strong correlation effects occur for excitations close to the threshold. From their characteristic momentum distributions, we can identify contributions to the double ionization from sequential three-photon and nonsequential or sequential two-photon processes. At elevated intensities these contributions deviate from their usual intensity scaling due to saturation effects, even though the total double-ionization probability stays below 10%. Furthermore, analysis of the time evolution of the momentum distribution reveals signatures of the energy-time uncertainty which indicate a coherent regime of the dynamics.

Enhanced ionization of perpendicularly aligned H2 in intense laser fields

Using three-dimensional classical ensembles, we have investigated the enhancement of double ionization of perpendicularly aligned H 2 molecules by a 800 nm laser pulse with intensity ranging from 1 × 10 14 W/cm 2 to 6 × 10 14 W/cm 2. The simulated results show that double ionization probability of H 2 strongly depends on R and reaches a maximum at an intensity independent critical distance R C ≈ 5 a.u. Furthermore, the enhancement of double ionization is more pronounced in the cases of weaker or stronger fields. These results, a well indication of the influence of molecular structures and laser–molecule interactions on double ionization of diatomic molecules, are analyzed in detail and qualitatively explained based on the field-induced barrier suppression model and back analysis.

Double ionization of He in an intense laser field via a rescattering process

We investigate the ratio of double to single ionization of He in an intense laser field based on the rescattering model. Folding the rescattering energy spectra with the electron impact inelastic cross sections, we obtain the probability of double ionization due to the nonsequential ionization process. Our results are in reasonable agreement with the experiment [Walker et al., Phys. Rev. Lett. 73, 1227 (1994)]. Furthermore, we investigate the physical insights of the nonsequential double ionization by analyzing the rescattering energy spectra at different intensities and the contributions from individual returns. This study confirms the reliability of the rescattering energy spectra obtained from ab initio calculations. The rescattering information can be used to analyze many other dynamical processes in intense laser-matter interactions, such as molecular imaging.

Elliptical Polarization and Probability of Double Ionization

The degree of elliptical polarization of intense short laser pulses is shown to be related to the timing of strong-field nonsequential double ionization. Higher ellipticity is predicted to force the initiation of double ionization into a narrower time window, and this "pins" the ionizing field strength in an unexpected way, leading to the first experimentally testable formula for double ionization probability as a function of ellipticity.

Double ionization of helium interacting with elliptically polarized laser pulse by classical ensemble simulations

This paper uses the classical ensemble method to study the double ionization of a 2-dimensional (2D) model helium atom interacting with an elliptically polarized laser pulse. The classical ensemble calculation demonstrates that the ratio of double to single ionization decreases with the increasing ellipticity of the driving field. The classical scenario shows that there are hardly any e–e recollisions with the circularly polarized laser pulse. The double ionization probability is studied for linearly and circularly polarized laser pulses. The classical numerical results are consistent with the semiclassical rescattering mechanism and in agreement with the experimental results and the quantum calculations qualitatively.

From Recollisions to the Knee: A Road Map for Double Ionization in Intense Laser Fields

We examine the nature and statistical properties of electron-electron collisions in the recollision process in a strong laser field. The separation of the double ionization yield into sequential and nonsequential components leads to a bell-shaped curve for the nonsequential probability and a monotonically rising one for the sequential process. We identify key features of the nonsequential process and connect our findings in a simplified model which reproduces the knee shape for the probability of double ionization with laser intensity and associated trends.

Quantum-state measurement of ionic Rydberg wavepackets

Nearly unipolar, sub-picosecond half-cycle pulses (HCPs) have been used to measure, via impulsive ionization, the aspects of the electronic quantum state relevant to the radial motion of Ba+ ng Rydberg wavepackets. Following laser excitation of the ionic wavepacket, the double ionization probability is measured as a function of the amplitude and delay of the HCP probe. Fourier transform of the time-dependent ionization signal facilitates the determination of the complex amplitudes of the constituent ng eigenstates in the wavepacket, enabling a full characterization of its time-dependent radial evolution. This is the first experimental demonstration of wavepacket recovery in ions, a capability that may prove to be invaluable for future measurements of quantum dynamics in multielectron systems.

Strong field double ionization: what is under the ‘knee’?

Both uncorrelated (‘sequential’) and correlated (‘nonsequential’) processes contribute to the double ionization of the helium atom in strong laser pulses. The double ionization probability has a characteristic ‘knee’ shape as a function of the intensity of the pulse. We investigate the phase-space dynamics of this system, specifically by finding the dynamical structures that regulate the ionization processes. The emerging picture complements the recollision scenario by clarifying the distinct roles played by the recolliding and core electrons. Our analysis leads to verifiable predictions of the intensities where qualitative changes in ionization occur, leading to the hallmark ‘knee’ shape.