Dynamics Of Double Ionization Research Articles

Electro-Nuclear Dynamics of Single and Double Ionization of H2 in Ultrafast Intense Laser Pulses.

We present an efficient method for modeling the single and double ionization dynamics of the H2 molecule in ultrashort, intense laser fields. This method is based on a semianalytical approach to calculate the time-dependent single and double molecular ionization rates and on a numerical approach to describe the vibrational motion that takes place in the intermediate molecular ion H2+. This model allows for the prediction of the single and double ionization probabilities of the H2 molecule to be made over a wide range of frequencies and laser intensities with limited computational time while providing a realistic estimate of the energy of the products of the dissociative ionization and of the Coulomb explosion of the H2 molecule. The effect of vibrational dynamics on ionization yields and proton kinetic energy release spectra is demonstrated and, in the case of the latter, is discussed in terms of basic strong-field molecular fragmentation mechanisms.

Fragmentation dynamics of electron-impact double ionization of helium

We study the double ionization dynamics of a helium atom impacted by electrons with full-dimensional classical trajectory Monte Carlo simulation. The excess energy is chosen to cover a wide range of values from 5 eV to 1 keV for comparative study. At the lowest excess energy, i.e., close to the double-ionization threshold, it is found that the projectile momentum is totally transferred to the recoil-ion while the residual energy is randomly partitioned among the three outgoing electrons, which are then most probably emitted with an equilateral triangle configuration. Our results agree well with experiments as compared with early quantum-mechanical calculation as well as classical simulation based on a two-dimensional Bohr’s model. Furthermore, by mapping the final momentum vectors event by event into a Dalitz plot, we unambiguously demonstrate that the ergodicity has been reached and thus confirm a long-term scenario conceived by Wannier. The time scale for such few-body thermalization, from the initial nonequilibrium state to the final microcanonical distribution, is only about 100 attoseconds. Finally, we predict that, with the increase of the excess energy, the dominant emission configuration undergoes a transition from equilateral triangle to T-shape and finally to a co-linear mode. The associated signatures of such configuration transition in the electron-ion joint momentum spectrum and triple-electron angular distribution are also demonstrated.

Control of concerted back-to-back double ionization dynamics in helium.

Double ionization (DI) is a fundamental process that despite its apparent simplicity provides rich opportunities for probing and controlling the electronic motion. Even for the simplest multielectron atom, helium, new DI mechanisms are still being found. To first order in the field strength, a strong external field doubly ionizes the electrons in helium such that they are ejected into the same direction (front-to-back motion). The ejection into opposite directions (back-to-back motion) cannot be described to first order, making it a challenging target for control. Here, we address this challenge and optimize the field with the objective of back-to-back double ionization using a (1 + 1)-dimensional model. The optimization is performed using four different control procedures: (1) short-time control, (2) derivative-free optimization of basis expansions of the field, (3) the Krotov method, and (4) control of the classical equations of motion. All four procedures lead to fields with dominant back-to-back motion. All the fields obtained exploit essentially the same two-step mechanism leading to back-to-back motion: first, the electrons are displaced by the field into the same direction. Second, after the field turns off, the nuclear attraction and the electron-electron repulsion combine to generate the final motion into opposite directions for each electron. By performing quasi-classical calculations, we confirm that this mechanism is essentially classical.

Strong-field-induced single and double ionization dynamics from single and double excitations in a two-electron atom

Two-electron dynamics of an excited model atom interacting with moderately strong laser fields is analyzed in the time domain. We solve the time-dependent Schrödinger equation (TDSE) for two electrons confined to the same one-dimensional configuration space, accounting also for the electron-electron interaction. The computational method allows direct access to the time-dependent population of the relevant atomic states during and right after the interaction with a near-infrared (NIR) laser pulse. We compare the ionization dynamics of singly excited states and doubly excited states. We find that doubly-excited initial states exhibit enhanced double ionization yield, with non-trivial dynamics including contributions from direct and sequential processes, while the electrons leave the atom either back-to-back or in the same direction.

Revealing the multiorbital and excitation effects in dissociative double ionization of CO molecules with angular streaking measurement

SynopsisWe investigate the sequential double ionization dynamics for the dissociative channel C+ + O+ of CO molecules in strong circularly polarized laser fields. By using four-particle coincident measurement, the obtained ionic angular emissions and molecular frame photoelectron angular distributions of each ionization steps for various pathways reveals the significant effects of multiorbital ionization and laser-driven excitation.

Coulomb-repulsion-assisted double ionization from doubly excited states of argon

We report a combined experimental and theoretical study to elucidate nonsequential double-ionization dynamics of argon atoms at laser intensities near and below the recollision-induced ionization threshold. Three-dimensional momentum measurements of two electrons arising from strong-field nonsequential double ionization are achieved with a custom-built electron-electron-ion coincidence apparatus, showing laser intensity-dependent Coulomb repulsion effect between the two outgoing electrons. Furthermore, a previously predicted feature of double ionization from doubly excited states is confirmed in the distributions of sum of two-electron momenta. A classical ensemble simulation suggests that Coulomb-repulsion-assisted double ionization from doubly excited states is at play at low laser intensity. This mechanism can explain the dependence of Coulomb repulsion effect on the laser intensity, as well as the transition from side-by-side to back-to-back dominant emission along the laser polarization direction.

Time-dependent restricted-active-space self-consistent-field theory with space partition

Aiming at efficient numerical analysis of time-dependent (TD) many-electron dynamics of atoms involving multielectron continua, the TD restricted-active-space self-consistent-field theory with space partition (TD-RASSCF-SP) is presented. The TD-RASSCF-SP wave function is expanded in terms of TD configuration-interaction coefficients with Slater determinants composed of two kinds of TD orbitals: $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{M}$ orbitals are defined to be nonvanishing in the inner region ($\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{V}$), a small volume around the atomic nucleus, and $\stackrel{\ifmmode \check{}\else \v{}\fi{}}{M}$ orbitals are nonvanishing in the large outer region ($\stackrel{\ifmmode \check{}\else \v{}\fi{}}{V}$). For detailed discussion of the SP strategy, the equations of motion are derived by two different formalisms for comparison. To ensure continuous differentiability of the wave function across the two regions, one of the formalisms makes use of the property of the finite-element discrete-variable-representation (FEDVR) functions and introduces additional time-independent orbitals. The other formalism is more general and is based on the Bloch operator as in the $R$-matrix theory, but turns out to be less practical for numerical applications. Hence, using the FEDVR-based formalism, the numerical performance is tested by computing double-ionization dynamics of atomic beryllium in intense light fields. To achieve high accuracy, $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{M}$ should be set large to take into account the strong many-electron correlation around the nucleus. On the other hand, $\stackrel{\ifmmode \check{}\else \v{}\fi{}}{M}$ can be set much smaller than $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{M}$ for capturing the weaker correlation between the two outgoing photoelectrons. As a result, compared with more accurate multiconfigurational TD Hartree-Fock (MCTDHF) method, the TD-RASSCF-SP method may achieve comparable accuracy in the description of the double-ionization dynamics. There are, however, difficulties related to the stiffness of the equations of motion of the TD-RASSCF-SP method, which makes the required time step for this method smaller than the one needed for the MCTDHF approach.

Laser-subcycle control of sequential double-ionization dynamics of helium

Electron correlation in sequential double ionization of helium is studied experimentally with the ``attoclock'' technique. Control of the two-electron emission dynamics is demonstrated on the time scale of a half laser cycle.

Double Ionization Dynamics of Molecular Hydrogen in Ultrashort Intense Laser Fields**Supported by the National Basic Research Program of China under Grant No 2013CB922201, and the National Natural Science Foundation of China under Grant Nos 11304365, 11374329 and 11334009.

We experimentally investigate the double ionization of molecular hydrogen subjected to ultrashort intense laser pulses. The total kinetic energy release of the two coincident H+ ions, which provides a diagnosis of different processes to double ionization of H2, is measured for two different pulse durations, i.e., 25 and 5 fs, and various laser intensities. It is found that, for the long pulse duration (i.e., 25 fs), the double ionization occurs mainly via two processes, i.e., the charge resonance enhanced ionization and recollision-induced double ionization. Moreover, the contributions from these two processes can be significantly modulated by changing the laser intensity. In contrast, for a few-cycle pulse of 5 fs, only the recollsion-induced double ionization survives, and in particular, this process could be solely induced by the first-return recollision at appropriate laser intensities, providing an efficient way to probe the sub-laser-cycle molecular dynamics.

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.

Exploration of carrier-envelope phase dependence on the double ionization process of Ar and N2 in few-cycle laser fields

The double ionization dynamics of the Ar atom and the N2 molecule are investigated in few-cycle laser fields with different carrier-envelope phases (CEPs). The corresponding electron momentum distribution of Ar and N2 presents different patterns. The energy distribution of double ionized electron trajectories is also presented to further describe the collision ionization and recollision excitation sequential ionization process of double ionization. Moreover, the angular distribution of the Ar atom and the N2 molecule at the end of a pulse is also sensitive to the CEP. The numerical results indicate that the double ionization mechanisms of atoms and molecules can probably be controlled by adjusting carrier-envelope phase in few cycle fields.

Control of atomic single and double ionization dynamics using orthogonally polarized two-color laser pulses

Single- and double-ionization of neon with orthogonally polarized two-color (OTC) laser fields is investigated using the COLTRIMS method. We study the influence of the long range Coulomb potential of the parent ion on the final momentum distribution of electrons emitted during single ionization. Furthermore, we investigate nonsequential double ionization in OTC fields and demonstrate that the electron-electron correlation is highly sensitive to the sub-cycle field shape of the OTC pulses, in agreement with recent theoretical predictions.

Laser-induced breakup of helium3S1s2swith intermediate doubly excited states

Solving the time-dependent Schr\odinger equation in full dimensionality for two electrons, it is found that in the XUV regime the two-photon double ionization dynamics of He($1s2s$) is predominantly dictated by the process of resonance enhanced multiphoton ionization via doubly excited states (DESs). We have studied a pump-probe scenario where the full laser-driven breakup of the ${}^{3}S$ $1s2s$ metastable state is dominated by intermediate quasiresonant excitation to doubly excited (autoionizing) states in the ${}^{3}{P}^{o}$ series. Clear evidence of multipath interference effects is revealed in the resulting angular distributions of the ejected electrons in cases where more than one intermediate DES is populated in the process.

Confining the double ionization dynamics of argon to half of a laser cycle

The sub-cycle dynamics in non-sequential double ionization of argon are studied by combining a reaction microscope with a single-shot carrier-envelope phase measurement technique.

Double ionization of helium by intense near-infrared and VUV laser pulses

We investigate the dynamics of double ionization of He atom by an intense near-infrared and an attosecond vacuum ultraviolet (VUV) laser pulse, which are either applied in sequence or at the same time. To this end we solve the time-dependent Schr\"odinger equation for a two-electron model atom interacting with the two fields. We compare the double-ionization yields and probability density distributions, with and without the application of the attosecond pulse, for the different scenarios. The results of our numerical simulations show how ionization or excitation of the neutral atom by a preceding or simultaneously applied VUV pulse affects the double-ionization dynamics driven by the near-infrared laser pulse. The findings provide insights regarding the question if attosecond technology can be used to temporally resolve mechanisms of correlated emission of electrons in a strong laser field.

Systematic analysis of double-ionization dynamics based on four-body Dalitz plots

We report on an experimental and theoretical systematic study of double ionization of helium by ion impact in terms of four-particle Dalitz plots. Several collision systems covering a broad range of perturbation parameters $\ensuremath{\eta}$ (projectile charge to speed ratio) were investigated. With increasing $\ensuremath{\eta}$ we observe a systematic trend from features, characteristic to correlated double-ionization mechanisms, to signatures of higher-order processes not requiring electron-electron correlations [the mechanism called ``two-step--two projectile-electron interaction'' (TS-2)]. The data for the largest $\ensuremath{\eta}$ can qualitatively be amazingly well described by a simple model only including the TS-2 mechanism.

Electron impact ionization-excitation and double-ionization dynamics of He at large momentum transfer

We report a collision dynamics study on electron-impact ionization-excitation and double-ionization of He at large momentum transfer. The symmetric noncoplanar (e,2e) and (e,3-1e) cross sections have been measured for transitions to the He+ n=2 excited state and He2+doubly ionized state at impact energies of 1.2, 2.1, and 4.3 keV. An investigation of impact energy dependence of the cross sections provides definite evidence that noticeable higher-order effects are involved even at the rather high impact energies used. It has been found that second Born approximation calculations satisfactorily account for the experimental results, indicating that the second-order two-step mechanisms play crucial roles in the ionization-excitation and double-ionization processes under the kinematical conditions considered here.

Double ionization of HeH^+ molecules in intense laser fields

We present quantum mechanical calculations of double ionization of HeH(+) molecules by intense laser pulses at various intensities. The resulting two-electron momentum distributions exhibit a clear asymmetry, which depends on the laser intensity. The asymmetric charge configuration of HeH(+) is responsible for the asymmetric two-electron momentum distributions. An approach to control the dynamics of double ionization of heteronuclear molecules is proposed.

H2double ionization with few-cycle laser pulses

The temporal dynamics of double ionization of ${\text{H}}_{2}$ has been investigated both experimentally and theoretically with few-cycle laser pulses. The main observables are the proton spectra associated with the ${\text{H}}^{+}+{\text{H}}^{+}$ fragmentation channel. The model is based on the time-dependent Schr\odinger equation and treats the electronic and nuclear coordinates on the same level. Therefore it allows the ultrafast nuclear dynamics to be followed as a function of the laser pulse duration, carrier-envelope phase offset, and peak intensity. We mainly report results in the sequential double-ionization regime above $2\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0.3em}{0ex}}\text{W}\phantom{\rule{0.2em}{0ex}}{\text{cm}}^{\ensuremath{-}2}$. The proton spectra are shifted to higher energies as the pulse duration is reduced from 40 down to 10 fs. The good agreement between the model predictions and the experimental data at 10 fs permits a theoretical study with pulse durations down to a few femtoseconds. We demonstrate the very fast nuclear dynamics of the ${\text{H}}_{2}{}_{+}$ ion for a pulse duration as short as 1 fs between the two ionization events, giving $\text{H}_{2}{}^{+}$ from ${\text{H}}_{2}$ and ${\text{H}}^{+}+{\text{H}}^{+}$ from $\text{H}_{2}{}^{+}$. The carrier-envelope phase offset plays a significant role only for pulse durations shorter than 4 fs. At 10 fs, the laser intensity dependence of the proton spectra is fairly well reproduced by the model.

Time-resolved mapping of correlated electron emission from helium atom in an intense laser pulse

We apply and analyze the concept of mapping ionization time on to the final momentum distribution to the correlated electron dynamics in the nonsequential double ionization of helium in a strong laser pulse (λ=800 nm) and show how the mapping provides insight into the double ionization dynamics. To this end, we study, by means of numerical integration of the time-dependent Schrödinger equation of a fully correlated model atom, the temporal evolution of the center-of-mass momentum in a short laser pulse. Our results show that in the high intensity regime (I0=1.15×1015 W cm-2), the mapping is in good agreement with a classical model including binary and recoil rescattering mechanisms. In the medium intensity regime (I0=5×1014 W cm-2), we identify additional contributions from the recollision-induced excitation of the ion followed by subsequent field ionization (RESI).