Nonsequential Double Ionization Of Atoms Research Articles

Modulation of electron correlation dynamics in atomic non-sequential double ionization by counter-rotating two-color circularly polarized laser fields.

We investigate the ultrafast electron correlation effects during non-sequential double ionization (NSDI) of argon subjected to a combined femtosecond field composed of counter-rotating two-color circularly polarized (TCCP) pulse laser using a 3D classical ensemble model (CEM). Our simulation results reveal that manipulation of the carrier-envelope phase (CEP) of the external driving field modulates the dynamical behavior of the two electrons, resulting in a notable sensitivity of their momentum distribution to the relative phase of two components of the counter-rotating TCCP field. Through inversion analysis, we uncover the capability to direct electrons toward a single direction, thereby facilitating focused ion-electron collisions on the attosecond timescale. Furthermore, we observe that adjusting the CEP exerts a substantial influence on the NSDI ionization process, modifying the relative contributions of the recollision-induced ionization (RII) mechanism and recollision excitation with subsequent ionization (RESI) mechanism.

Manipulating nonsequential double ionization of atoms by parallel polarized three-color laser fields

Nonsequential double ionization (NSDI) of He atoms in a parallel polarized three-color field is investigated by using a three-dimensional classical ensemble model. The driving field is composed of 1600-nm and 800-nm laser pulses with equal intensity. A weak 400-nm laser pulse is used as a controlling field. The results indicate that in the correlated electron momentum distribution and ion momentum distribution, the electron pairs and ions of the first returning recollision (FRR) trajectory, the odd-returning recollision (ORR) trajectory (excluding FRR), and the even-returning recollision (ERR) trajectory are located in different regions separated well from each other. The electron pairs from FRR trajectories mainly distribute around the origin, and those electron pairs from ORR and ERR trajectories respectively cluster in the first quadrant and the third quadrant. With the increase of the phase of the controlling field, the proportion of FRR trajectories in NSDI first increases and then decreases, and the proportions of those trajectories with the returning number more than one first decrease and then increase, which leads to the fact that with the increase of the phase of the controlling field, the anticorrelated emissions first increase and then decrease and correspondingly the ion momentum distribution evolves from a double-hump to a triple-hump and then to a double-hump structure. Moreover, NSDI from multiple-returning recollision trajectories mainly occur through recollision-induced direct ionization (RDI) mechanism, while NSDI from the FRR trajectories mainly occurs through recollision-induced excitation with subsequent ionization (RESI) mechanism. Thus the dominant NSDI ionization mechanism can also be controlled by changing the phase of the controlling field.

Nonsequential double ionization of atoms driven by few-cycle counter-rotating two-color circularly polarized laser fields

Using a classical ensemble model, we investigated the dependence of nonsequential double ionization (NSDI) on the relative phase driven by few-cycle counter-rotating two-color circularly polarized (TCCP) laser fields, which consist of 1600-nm laser pulse and 800-nm laser pulse. The numerical results reveal that NSDI probability is strongly dependent on the relative phase of counter-rotating TCCP fields, which shows a sin-like behavior of period 2π. The inversion analysis shows that the returning probability of the first electron is very sensitive to the relative phase of counter-rotating TCCP fields, resulting in dependence of NSDI probability on the relative phase. Furthermore, the dynamic behavior of two electrons can be regulated by adjusting the relative phase, leading to the sensitive dependence of momentum distribution of Ar2+ ions on the relative phase of counter-rotating TCCP fields. In addition, our results show that the traveling electron can be controlled to return to the parent ion core from a relatively single direction, which leave an imprint on the momentum distribution of Ar2+ ions.

Nonsequential double ionization of atoms driven by spatially inhomogeneous laser fields

<sec>Using a three-dimensional classical ensemble method, we investigate the nonsequential double ionization (NSDI) of xenon atoms from the near infrared wavelength to the mid-infrared wavelength in spatially inhomogeneous laser fields, and compare the results with those from spatially homogeneous laser fields. The results show that the NSDI probability curves from spatially inhomogeneous laser field and spatially homogeneous laser field at short wavelength are similar to each other. With the laser wavelength increasing, NSDI at the high intensities is more and more suppressed for spatially inhomogeneous laser field.</sec><sec>Compared with the result from the spatially homogeneous laser field, the final emission angle of two electrons from the NSDI exhibits a very strongly correlated characteristic in the spatially inhomogeneous field, especially at a longer laser wavelength, the final emission angles of two electrons are almost both concentrated around <inline-formula><tex-math id="M2">\begin{document}${0^\circ }$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20230548_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20230548_M2.png"/></alternatives></inline-formula>, meaning that the two electrons are always emitted into the same direction parallelly. Moreover, effective recollision of the NSDI is always dominated by first return of the first electron from the near infrared to the mid-infrared inhomogeneous laser fields, however, the transition from the first return dominance to the second return dominance occurs in the spatially homogeneous laser fields. Further, we reveal the more details of the ultrafast dynamics of the correlated electrons in the spatially inhomogeneous laser field by back-tracing the classical trajectories of NSDI.</sec>

Nonsequential double ionization of alkaline-earth metal atoms by intense mid-infrared femtosecond pulses.

A systematic study of nonsequential double ionization (NSDI) of alkaline-earth metal atoms with mid-infrared femtosecond pulses is reported. We find that the measured NSDI yield shows a strong target dependence and it is more suppressed for alkaline-earth metal with higher ionization potential. The observation is attributed to the differences in the recollision induced excitation and ionization cross sections of alkaline-earth metals. This work indicates that NSDI of alkaline-earth metals can be generally understood within recollision picture and sheds light on ultrafast control of electron correlation and dynamics of ionic excited states during NSDI of atoms with complex structures.

Laser-Induced Inelastic Diffraction from Strong-Field Double Ionization.

In this Letter, we propose a novel laser-induced inelastic diffraction (LIID) scheme based on the intense-field-driven atomic nonsequential double ionization (NSDI) process and demonstrate that, with this LIID approach, the doubly differential cross sections (DDCSs) of the target ions, e.g., Ar^{+} and Xe^{+}, can be accurately extracted from the two-dimensional photoelectron momentum distributions in the NSDI process of the corresponding atoms. The extracted DDCSs exhibit a strong dependence on both the target and the laser intensity, in good agreement with calculated DDCSs from the scattering of free electrons. The LIID scheme may be extended to molecular systems and provides a promising approach for imaging of the gas-phase molecular dynamics induced by a strong laser field with unprecedented spatial and temporal resolution.

Recollision dynamics in nonsequential double ionization of atoms by long-wavelength pulses

Recollision dynamics and electron correlation behavior are investigated for several long laser wavelengths (1200-3000 nm) in nonsequential double ionization (NSDI) of helium using three-dimensional classical ensembles. Numerical results show that for these long wavelengths NSDI events are mainly from the multiple-return trajectory which is different from the case of 800 nm. Moreover, with increasing laser wavelength NSDI events move from the diagonal to the two axes in the correlated electron momentum distributions, and finally form an experimentally observed prominent V-shaped structure [Phys. Rev. X 5, 021034 (2015)] in the first and third quadrants. Back analysis indicates that the asymmetric energy sharing between the two electrons at recollision is responsible for the formation of the prominent V-shaped structure of 3000 nm.

Nonsequential double ionization with time-dependent renormalized-natural-orbital theory

Recently introduced time-dependent renormalized natural orbital theory (TDRNOT) is tested on non-sequential double ionization (NSDI) of a numerically exactly solvable one-dimensional model He atom subject to few-cycle, 800-nm laser pulses. NSDI of atoms in strong laser fields is a prime example of non-perturbative, highly correlated electron dynamics. As such, NSDI is an important "worst-case" benchmark for any time-dependent few and many-body technique beyond linear response. It is found that TDRNOT reproduces the celebrated NSDI "knee," i.e., a many-order-of-magnitude enhancement of the double ionization yield (as compared to purely sequential ionization) with only the ten most significant natural orbitals (NOs) per spin. Correlated photoelectron spectra - as "more differential" observables - require more NOs.

Time-dependent multiconfiguration methods for the numerical simulation of photoionization processes of many-electron atoms

Numerical simulations present an indispensable way to the understanding of complex physical processes. In quantum mechanics where the theoretical description is given in terms of the time-dependent Schrodinger equation the road is, however, difficult for any but the simplest systems. This is particularly true if one considers photoionization processes of atoms and molecules which, at the same time, require an accurate description of bound and continuum states and, therefore, an extensive region of space to be sampled during the calculation. As a consequence, direct simulations of photoionization processes are currently only feasible for systems containing up to three electrons. Despite this fundamental restriction, many physical effects can be essentially described by single- and two-electron models, among them are high-order harmonic generation and non-sequential double-ionization of atoms and molecules. A plethora of numerical investigations have been performed on atomic and molecular hydrogen and helium in the last two decades, and these have had a strong impact on the current understanding of photoionization. On the other hand, there are processes which are characterized by the interplay of a larger number of electrons, such as tunnel ionization, the Auger effect and, to give a more recent example, the temporal delay between the photo-emission of electrons from different shells of neon and krypton. The many-electron character of these effects complicates the accurate, time-resolved simulation and, so far, no universally applicable method exists. This review presents two theoretical methods which are promising candidates for closing this gap–the multiconfigurational time-dependent Hartree-Fock (MCTDHF) method and the time-dependent restricted active space configuration interaction (TD-RASCI) method. Both represent the wavefunction in a linear subspace of the many-body Hilbert space and follow particular strategies to avoid the exponential problem. This makes it possible to treat a much larger number of electrons than with the direct techniques mentioned above.

Calibration of distorted wave Born approximation for electron impact excitation of Ne and Ar at incident energies below 100 eV

We calibrate the distorted wave Born approximation (DWBA) for electron impact excitation processes empirically. Differential cross sections (DCSs) for the excitation of the 2p53s, 2p53p, 2p54s and 2p54p configurations of Ne and the 3p54s and 3p54p configurations of Ar by electron impact are calculated using DWBA for incident energies between 20 and 100 eV. The calculated results are compared with the absolute experimental measurements and other theoretical results. We found that the structure of the DCS can be well reproduced by the DWBA model while the magnitude is overestimated for most cases considered here. The differences in magnitude between DWBA and experiment are used to test the calibration of DWBA such that the DWBA can be used to describe laser-induced electron impact excitation processes. These processes are involved in the non-sequential double ionization of atoms in strong laser fields.

The role of multi-return induced collision-ionization in atomic nonsequential double ionization

The nonsequential double ionization of helium by high-intensity (1.5×1015 W/cm2) few-cycle laser pulses is investigated using a three-dimensional semi-classical rescattering model, with emphasis on the longitudinal correlated momentum spectra. We find that the percentage of opposite-hemisphere emission in this case is remarkably higher than those produced by laser pulses with medium-intensity (e.g., 7×1014 W/cm2) and low-intensity (e.g., 2.5×1014 W/cm2) those procuced by the laser pulses pulses with, and also the same intensies but longer laser pulses. Meanwhile, the V-shape structure is more pronounced. We identify the underlying mechanisms with tracing back the individual trajectories, and find that the single-and the multi-return induced collision-ionization are mainly responsible for the opposite- and the same-hemisphere emission, respectively. The dependences of these two different mechanisms on peak intensity and pulse length are predicted.

Anticorrelated electrons from high-intensity nonsequential double ionization of atoms

The nonsequential double ionization (NSDI) yield and final electron-electon correlation are examined for a laser wavelength of 800 nm over a wide range of laser intensities, using three-dimensional fully classical models. It is shown that in the transition region between NSDI and sequential ionization, turn-on stage recollisions can lead to having a majority of anticorrelated (back-to-back) electron pairs.

Using two methods to study non-sequential double ionization

The semiclassical rescattering model and the quantum-mechanical S matrix model are used to study the non-sequential double ionization of atoms and molecules in strong laser fields.Based on these two models, we can calculate ionization rate with different laser intensities, recoil ions momentum distribution and two-electron energy distribution. The numerical simulation results show that the semiclassical rescattering model coincides with the quantum-mechanical S matrix model when non-sequential double ionization of atoms and molecules is researched in strong laser fields.

Quantitative rescattering theory for nonsequential double ionization of atoms by intense laser pulses

Laser-induced electron recollisions are fundamental to many strong field phenomena in atoms and molecules. Using the recently developed quantitative rescattering theory, we demonstrate that the nonsequential double ionization NSDI of atoms by lasers can be obtained quantitatively in terms of inelastic collisions of the target ions with the free returning electrons where the latter are explicitly given by a spectrum-characterized wave packet. Using argon atoms as target, we calculated the NSDI yield including contributions from direct e,2e electron-impact ionization and electron-impact excitation accompanied by subsequent field ionization. We further investigate the dependence of total NSDI on the carrier-envelope phase of few-cycle laser pulses, and showed that the effect can be experimentally observed by measuring the yield of doubly charged ions only.

Ellipticity effects and the contributions of long orbits in nonsequential double ionization of atoms

Using a semiclassical model based on tunneling, electron kinematics in the laser field, and phase space volume, we evaluate the ion and electron momentum distributions in nonsequential double ionization of a model atom by an elliptically polarized laser field. For ellipticities exceeding approximately 0.3, we find that the shortest quantum orbit (having the shortest travel time) no longer dominates the double-ionization rate, in contrast to the case of linear polarization. Simultaneously we observe significant violations of symmetry patterns in the ion-momentum distributions and the electron-electron momentum correlations, compared with the case of linear polarization. These violations are very sensitive to the laser pulse duration because of the absence of the late returns for very short pulses. Some of these qualitative effects in the photoelectron distributions should be experimentally detectable. Observation would verify the significance of the late returns and the underlying quantum-orbit concept.

Nonsequential Double Ionization of Atoms in Strong Laser Pulses

It is now possible to produce laser pulses with reproducible pulse shape and controlled carrier envelope phase. It is discussed how that can be explored in double ionisation studies. To this end we solve numerically the Schrodinger equation for a limited dimensionality model which nevertheless treats electron repulsion qualitatively correctly and allows to study correlation effects due to the Coulomb repulsion.

Theory of laser-driven double-ionization of atoms at Ti:sapphire laser wavelengths

Progress in the theoretical understanding of non-sequential double-ionization of atoms is reviewed from its beginnings with Kuchiev's work in the late 1980s and Corkum's work in the early 1990s to the present day. The crucial role of laboratory experiment as a persistent stimulus to theoretical endeavour is underlined but the predictive roles of simple, yet fundamental, theory and also of a full quantum mechanical description are not forgotten. A theoretical forward look is provided.