Sequential Double Ionization Research Articles

Improved model for the dissociative sequential double ionization of nitrogen molecules by an intense few-cycle infrared laser pulse

Improved model for the dissociative sequential double ionization of nitrogen molecules by an intense few-cycle infrared laser pulse

Theory of strong-field sequential double ionization of polyatomic molecules

Theory of strong-field sequential double ionization of polyatomic molecules

Probing vibronic coherence in charge migration in molecules using strong-field sequential double ionization

Probing vibronic coherence in charge migration in molecules using strong-field sequential double ionization

Pronounced momentum shift against the radiation pressure in strong-field sequential double ionization

Pronounced momentum shift against the radiation pressure in strong-field sequential double ionization

Internal collision double ionization of molecules driven by co-rotating two-color circularly polarized laser pulses

The double ionization process of molecules driven by co-rotating two-color circularly polarized fields is investigated with a three-dimensional classical ensemble model. Numerical results indicate that a considerable part of the sequential double ionization (DI) events of molecules occur through internal collision double ionization (ICD), and the ICD recollision mechanism is significantly different from that in non-sequential double ionization (NSDI). By analyzing the results of internuclear distances R = 5 a.u. and 2 a.u., these two recollision mechanisms are studied in depth. It is found that the dynamic behaviors of the recollision mechanisms of NSDI and ICD are similar. For NSDI, the motion range of electrons after the ionization is relatively large, and the electrons will return to the core after a period of time. In the ICD process, electrons will rotate around the parent ion before ionization, and the distance of the electron motion is relatively small. After a period of time, the electrons will come back to the core and collide with another electron. Furthermore, the molecular internuclear distance has a significant effect on the electron dynamic behavior of the two ionization mechanisms. This study will help to understand the multi-electron ionization process of complex systems.

Double-Slit Interference in the Ion Dynamics of Dissociative Photoionization.

The ion momentum distribution in the x-ray-induced dissociative photoionization of molecules is investigated, treating the ionization analytically under the Born-Oppenheimer approximation and simulating numerically the ion motion via the Schrödinger equation. The ion-photoelectron entanglement transfers information of the electronic interference to the ion dynamics. As a consequence, the ion momentum distributions of dissociative molecular photoionization present Young's double-slit interference when the photoelectron emission angle is fixed. We demonstrate that double-slit interference signatures persist in the ion longitudinal momentum shift even when the information of the correlated photoelectron is lost, which is the case for heteronuclear molecules when an additional photoelectron recoil momentum arises due to the different ion masses. For the case of sequential double ionization, we show that double-slit interference in the ion dynamics can be utilized for coherent control of the molecular dynamics.

Modeling the sequential dissociative double ionization of O2 by ultrashort intense infrared laser pulses

A density matrix approach for sequential double ionization (DM-SDI) of molecules has been developed recently and was applied to the ${\mathrm{N}}_{2}$ molecule [Yuen and Lin, Phys. Rev. A 106, 023120 (2022)]. In this article, we extended the DM-SDI model to ${\mathrm{O}}_{2}$, which is a more complicated system to model than ${\mathrm{N}}_{2}$, due to its electronic structures and spin-orbit and laser couplings in the manifold of doubly charged states. We obtained a good agreement on the kinetic energy release spectrum of ${\mathrm{O}}^{+}+{\mathrm{O}}^{+}$ from previous experiments. Thanks to the low computational cost of the model, we explored the mechanism behind the ionization and dissociation dynamics as well as the effects of lasers on the spectrum. This work will pave the way to model sequential dissociative double ionization of larger molecules and to probe molecular dynamics by measuring kinetic energy release spectra from this process.

Ion momentum distributions from sequential double ionization of Ar in elliptically polarized laser fields

In this paper, we utilize a classical ensemble model with Heisenberg-core potential to study sequential double ionization (SDI) of Ar atom by an elliptically polarized laser field. The results show that for random laser phases, as the laser wavelength increases, the ion momentum distribution gradually evolves from a six-band structure at 800 nm to an eight-band structure at 1600 nm. When the laser phase is stable, the ion momentum distribution from 1600 nm laser field exhibits a ten-band structure. These multi-band structures directly reflect the subcycle ionization dynamics of electrons in an elliptically polarized laser field. There is a significant shift among the outer three bands of ion momentum distrbutions from different laser phases, which leads to the fact that only one band is observed in the outer region of the ion momentum distribution for the case of random laser phases. By analyzing the ionization times of the two electrons, it is found that for the case of random phases, the inner bands of the ion momentum distributions originate from those combinations of electron ionization bursts with the ionization time difference of 0.5 cycle, and the outer bands arise from those combinations of ionization bursts with the ionization time difference of 1, 2 and 3 cycles. For 800 nm, the middle band corresponds to those combinations of ionization bursts with the ionization time differences of 1.5 and 2.5 cycles. For 1600 nm, there are two bands in middle regime. One is from the combination with the ionization time difference of 1.5 cycles, and the other is from those combinations with the ionization time difference of 2.5 and 3.5 cycles. These results indicate that in the case of long wavelength and phase-stable laser, the subcycle dynamics in sequential double ionization of atoms is more likely to be observed.

Electron correlation dynamics in atomic Kr excited by XUV pulses and controlled by NIR laser pulses of variable intensity

We have investigated the possibility to track and control correlation dynamics of valence electrons in krypton (Kr) initiated by the absorption of one extreme ultraviolet (XUV) photon. In this investigation, pump-probe experiments have been performed where monochromatized single high-harmonics at photon energies 29.6, 32.8, and 35.9 eV have been used as pump to populate different intermediate excited states. A temporally delayed near-infrared (NIR) pulse probes the population of various decay channels via the detection of Kr2+ ion yields and its transient profiles. We observe that by varying the NIR pulse intensity within a range from to W cm−2, the shape of the Kr2+ transient profile changes significantly. We show that by varying the intensity of the NIR pulse, it is possible—(i) to control the ratio between sequential and non-sequential double ionization of Kr; (ii) to selectively probe quantum beating oscillations between Kr satellite states that are coherently excited within the bandwidth of the XUV pulse; and (iii) to specifically probe the relaxation dynamics of doubly excited (Kr) decay channels. Our studies show that the contribution of different ionization and decay channels (i)–(iii) can be altered by the NIR pulse intensity, thus demonstrating an efficient way to control the ionization dynamics in rare gas atoms.

Ultrafast Two-Electron Orbital Swap in Li Initiated by Attosecond Pulses

A universal mechanism of ultrafast 2-electron orbital swap is discovered through 2-photon sequential double ionization of Li. After a 1s electron in Li is ionized by absorbing an extreme ultraviolet photon, the other 2 bound electrons located on 2 different shells have either parallel or antiparallel spin orientations. In the latter case, these 2 electrons are in the superposition of the singlet and triplet states with different energies, forming a quantum beat and giving rise to the 2-electron orbital swap with a period of several hundred attoseconds. The orbital swap mechanism can be used to manipulate the spin polarization of photoelectron pairs by conceiving the attosecond-pump attosecond-probe strategy and thus serves as a knob to control spin-resolved multielectron ultrafast dynamics.

Electron Correlations in Sequential Two-Photon Double Ionization of an Ar Atom

Sequential two-photon ionization is a process that is experimentally accessible due to the use of new free-electron laser sources for excitation. For the prototypical rare Ar gas atoms, a photoelectron spectrum (PES) corresponding to the second step of the sequential two-photon double ionization (2PDIII) at a photon excitation energy of 65.3 eV was studied theoretically with a focus on the consequences of electron correlations in the considered process. The calculation predicts many intense lines at low photoelectron energies, which cannot be explained on the basis of a one-electron approximation. The processes that lead to the appearance of these lines include many-electron correlations, either in the first or second step of photoionization. A significant fraction of the intensity of the low-energy part of PES is associated with the Auger decay of the excited states formed at the second step of 2PDI. The shape of the low-energy part of the 2PDIII PES is expected to be dependent on both the energy of photon excitations and the flux of the exciting beam.

Two-XUV-photon double ionization of neon

Two-XUV-photon double ionization of Ne, induced by an intense few-pulse attosecond train with a \ensuremath{\sim}4 fs envelope duration is investigated experimentally and theoretically. The experiment is performed at ELI-ALPS (Extreme Light Infrastructure Attosecond Light Pulse Source) utilizing the recently constructed 10 Hz gas phase high-order harmonic generation SYLOS GHHG-COMPACT beamline. A total pulse energy up to \ensuremath{\sim}1 \ensuremath{\mu}J generated in argon in conjunction with high-reflectivity optics in the XUV region allowed the observation of the doubly charged state of Ne induced by 40 eV central XUV-photon energies. The interaction of the intense attosecond pulse train with Ne is also theoretically studied via second-order time-dependent perturbation theory equations of motion. The results of this work, combined with the feasibility of conducting XUV-pump--XUV-probe experiments, constitute a powerful tool for many potential applications. Those include attosecond pulse metrology as well as time-resolved investigations of the dynamics underlying direct and sequential double ionization and their electron correlation effects.

Density-matrix approach for sequential dissociative double ionization of molecules

Intense femtosecond infrared (IR) laser pulses have been used in recent years to study the breakup dynamics of molecules. However, observables such as kinetic energy release and branching ratios of molecular fragments are often difficult to predict by theory, making information of the molecular dynamics difficult to retrieve. In this work, we develop a simple model for sequential double ionization of molecules based on a density-matrix approach. The model describes tunneling ionization of the neutral and the ion as well as laser couplings between the ionic states simultaneously. Population of different doubly charged states can be obtained at a low computational cost. We applied our model to ${\mathrm{N}}_{2}$ and obtained a good agreement on the kinetic energy release spectrum with a previous experiment. This theoretical development could open up the opportunities to use intense short IR laser pulses with coincidence measurement to probe molecular dynamics.

Single and sequential double ionization of NO radical in intense laser fields.

We examine the dependences of the single and double ionization probabilities of NO radical on the angle between the NO axis and the laser polarization direction in an intense laser field (790nm, 100fs, 1-10 × 1014 W/cm2) and show that the double ionization is enhanced when the NO axis is parallel to the laser polarization direction. We reveal that the angular dependence of the sequential double ionization probability is determined by the shape of the 5σ orbital of NO+ from which the second photoelectron is emitted in the ionization from NO+ to NO2+. We also reveal that the fast oscillation in the probability of the tunnel ionization of NO originating from a coherent superposition of the two spin-orbit components in the electronic ground X2Π state is described well based on the molecular Ammosov-Delone-Krainov (MO-ADK) theory in which the time evolution of the electron density distribution of the 2π orbital is taken into account.

Non-dipole effects on angular distribution of photoelectrons in sequential two-photon double ionization of Ar atom and K<sup>+</sup> ion

Owing to the development of XUV and X ray of the free-electron lasers, the photoelectron angular distribution in the sequential two-photon double ionization has received increasing attention of theorists and experimentalists, because it provides the valuable information about the electronic structure of atom or molecule systems and allows the obtaining of additional information about mechanisms and pathways of the two-photon double ionization. In this paper, the expression of the sequential two-photon double ionization process of the photoelectron angular distributions, including the non-dipole effects, is obtained based on the multi-configuration Dirac-Fock method and the density matrix theory, and the corresponding calculation code is also developed. Based on the code, the sequential two-photon double ionization process of the 3p and 2p shells of Ar atom and K<sup>+</sup> ion are studied, in which, the dipole and the non-dipole parameters of photoelectron angular distribution are investigated systematically. It is found that the angular distributions of the first- and second-step electrons in sequential two-photon double ionization are similar and the two photoionization processes affect each other. Near the ionization threshold, the photoionization cross-sections and anisotropy parameters for the 3p shell and the 2p shell show a large difference. While away from the threshold, the cross-section and angular anisotropy parameters of the 3p and 2p shells show similar behaviors. At the position of Cooper minimum of the photoionization cross section, the contribution of the electric dipole is suppressed, and the non-dipole effect is obvious. The non-dipole effect leads to a forward-backward asymmetric distribution of photoelectrons relative to the direction of incident light. The results of this paper will be helpful in studying the nonlinear processes of photon and matter interaction in the XUV range.

Ab Initio Direct Dynamics.

The reactivity and dynamics of molecular systems can be explored computationally by classical trajectory calculations. The traditional approach involves fitting a functional form of a potential energy surface (PES) to the energies from a large number of electronic structure calculations and then integrating numerous trajectories on this fitted PES to model the molecular dynamics. The ever-decreasing cost of computing and continuing advances in computational chemistry software have made it possible to use electronic structure calculations directly in molecular dynamics simulations without first having to construct a fitted PES. In this "on-the-fly" approach, every time the energy and its derivatives are needed for the integration of the equations of motion, they are obtained directly from quantum chemical calculations. This approach started to become practical in the mid-1990s as a result of increased availability of inexpensive computer resources and improved computational chemistry software. The application of direct dynamics calculations has grown rapidly over the last 25 years and would require a lengthy review article. The present Account is limited to some of our contributions to methods development and various applications. To improve the efficiency of direct dynamics calculations, we developed a Hessian-based predictor-corrector algorithm for integrating classical trajectories. Hessian updating made this even more efficient. This approach was also used to improve algorithms for following the steepest descent reaction paths. For larger molecular systems, we developed an extended Lagrangian approach in which the electronic structure is propagated along with the molecular structure. Strong field chemistry is a rapidly growing area, and to improve the accuracy of molecular dynamics in intense laser fields, we included the time-varying electric field in a novel predictor-corrector trajectory integration algorithm. Since intense laser fields can excite and ionize molecules, we extended our studies to include electron dynamics. Specifically, we developed code for time-dependent configuration interaction electron dynamics to simulate strong field ionization by intense laser pulses. Our initial application of ab initio direct dynamics in 1994 was to CH2O → H2 + CO; the calculated vibrational distributions in the products were in very good agreement with experiment. In the intervening years, we have used direct dynamics to explore energy partitioning in various dissociation reactions, unimolecular dissociations yielding three fragments, reactions with branching after the transition state, nonstatistical dynamics of chemically activated molecules, dynamics of molecular fragmentation by intense infrared laser pulses, selective activation of specific dissociation channels by aligned intense infrared laser fields, angular dependence of strong field ionization, and simulation of sequential double ionization.

Sequential double ionization of molecules by strong laser fields simulated with time-dependent configuration interaction.

A new time-dependent configuration interaction method has been developed for simulating strong field sequential double ionization of molecular systems. Ionization of the neutral is simulated by time-dependent configuration interaction with single excitations (TD-CIS) and an absorbing boundary. At each time step, the ionized part of the wavefunction from the TD-CIS calculation is transferred to a second time-dependent configuration interaction simulation for ionization of the cation to the dication. The second simulation uses a CISD-IP wavefunction that consists of singly ionized configurations and singly excited, singly ionized configurations (TD-CISD-IP). The transfer between the TD-CIS and TD-CISD-IP simulations is accomplished by partitioning the first ionization rate into contributions from individual orbitals or by singular value decomposition of the absorbed wavefunction. Sequential double ionization simulations have been carried out for HBr in five cycle 800nm linearly polarized pulses and HI (with spin-orbit coupling) in four cycle 800nm circularly polarized pulses, with intensities chosen so that the population of the neutral was depleted by the mid-pulse. The singular value decomposition of the cation produced by the first ionization is dominated by a single component for the two orientations considered. The population of the cation rises and then falls as it is ionized to the dication. Depending on the pulse shape and field strength, the ionization of the cation to the dication can continue for several half cycles. For HI with circularly polarized light, the rates for both the first and second ionization peak when the electric field is aligned with the pπ orbital.

Saturating multiple ionization in intense mid-infrared laser fields

The interpretation of experimental data from novel mid-infrared few-cycle laser sources requires an understanding of ionization mechanisms and knowledge about related ion yields. Experimental studies have indicated sequential double ionization as the dominant process above 1014 W cm−2. These results contradict a recent prediction that in this spectral region, non-sequential processes dominate the double ionization of xenon up to intensities of about 1015 W cm−2. In either case, the ratio of doubly to singly charged xenon yield reported in previous studies has been limited to a few percent, indicating a regime well below the onset of saturation of the double ionization process. We present an experimental study of double ionization of xenon and krypton atoms exposed to intense near four-cycle pulses at 3.2 μm. Our experiments rely on the ion microscopy technique, which facilitates the detection of ions originating from a restricted region within the interaction volume, thereby reducing the impact of focal averaging. Our measurements suggest that at intensities of close to 1.2 × 1014 W cm−2, double ionization of xenon and krypton is already significantly saturated. In particular, we find a doubly to singly charged yield ratio of about 75 percent for xenon and 25 percent for krypton. We compare our results with the predictions of different models accounting for the effects of volume averaging and focal geometry. We find that in the deeply saturated regime of our experiment, the Perelomov–Popov–Terentyev theory significantly underestimates the observed double ionization yield.

Simultaneous double ionization of C60 molecules in single-cycle pulses

We theoretically investigate simultaneous double ionization of C60 Buckminsterfullerene clusters within the strong field approximation, taking into account two-body effects like Coulomb blocking. Our analysis suggests that for infrared single-cycle pulses, simultaneous double ionization becomes comparable in magnitude to sequential double ionization. Additionally, estimates show that Coulomb blocking weakens with increasing cluster size and field strength.

Double ionization of hydrogen molecules in a high-intensity linearly polarized laser pulse

The double ionization of a hydrogen molecule in a high-intensity linearly polarized laser pulse is systematically investigated. It is found that the process of the double ionization of the hydrogen molecules is dominated by the sequential double ionization, whereas the spectrum of the electron momentum exhibits an obvious correlation behavior of electrons and this correlation behavior exists with the variation of the nuclear distance. Based on the analysis of electron trajectories, we demonstrate that this correlation is caused by the ultrafast electronic ionization within a quarter optical period of the laser field. Especially, the correlation of the electron momenta depends on the laser pulse’s negative vector potential, which is corresponding to the ionization instants of the two electrons.