Strong-field Double Ionization Research Articles

Direct observation of entangled electronic-nuclear wave packets

We present momentum resolved covariance measurements of entangled electronic-nuclear wave packets created and probed with octave spanning phaselocked ultrafast pulses. We launch vibrational wave packets on multiple electronic states via multiphoton absorption, and probe these wave packets via strong field double ionization using a second phaselocked pulse. Momentum resolved covariance mapping of the fragment ions highlights the nuclear motion, while measurements of the yield as a function of the relative phase between pump and probe pulses highlight the electronic coherence. The combined measurements allow us to directly visualize the entanglement between the electronic and nuclear degrees of freedom and follow the evolution of the complete wavefunction. Published by the American Physical Society 2024

Probing H_{2} Double Ionization with Bicircular Laser Fields.

We present a kinematically complete study on strong-field double ionization of H_{2} molecules in two-color bicircular laser fields. The releasing times of electrons and protons are recorded with the double-hand attoclock. We observe the relative emission angles of two electrons oscillate with the kinetic energy release of protons, indicating the internal concerted four-body fragmentation. Using a three-dimensional molecular semiclassical ensemble model, we have disentangled the attosecond correlated electron emission in H_{2} double ionization. This work reveals the strong electron-nuclear coupling in the molecular bond breaking and may open up a new approach to experimentally accessing the intramolecular electron and bond dynamics with bicircular fields.

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

Initial-site characterization of hydrogen migration following strong-field double-ionization of ethanol.

An essential problem in photochemistry is understanding the coupling of electronic and nuclear dynamics in molecules, which manifests in processes such as hydrogen migration. Measurements of hydrogen migration in molecules that have more than two equivalent hydrogen sites, however, produce data that is difficult to compare with calculations because the initial hydrogen site is unknown. We demonstrate that coincidence ion-imaging measurements of a few deuterium-tagged isotopologues of ethanol can determine the contribution of each initial-site composition to hydrogen-rich fragments following strong-field double ionization. These site-specific probabilities produce benchmarks for calculations and answer outstanding questions about photofragmentation of ethanol dications; e.g., establishing that the central two hydrogen atoms are 15 times more likely to abstract the hydroxyl proton than a methyl-group proton to form H[Formula: see text] and that hydrogen scrambling, involving the exchange of hydrogen between different sites, is important in H2O+ formation. The technique extends to dynamic variables and could, in principle, be applied to larger non-cyclic hydrocarbons.

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

Strong Field Double Ionization of Formaldehyde Investigated Using Momentum Resolved Covariance Imaging and Trajectory Surface Hopping.

We use covariance velocity map imaging of fragment ions from the strong field double ionization of formaldehyde in conjunction with trajectory surface hopping calculations to determine the ionization yields to different singlet and triplet states of the dication. The calculated kinetic energy release for trajectories initiated on different electronic states is compared with the experimental values based on momentum resolved covariance measurements. We determine the state resolved double ionization yields as a function of laser intensity and pulse duration down to 6 fs (two optical cycles).

Intensity dependence of ionic-state-resolved electron-electron correlation in strong-field double ionization

In this paper, we theoretically investigate the nonsequential double ionization (NSDI) process of Ne induced by linearly polarized laser field using strong field approximation theory. It is found that, with increasing laser intensity, the correlated electron momentum distributions (CEMDs) corresponding to multiple-return-collision process, which is believed to predominate in NSDI process at high intensity, exhibit a dependence on the wave function of the intermediate ionic state. This observation indicates that the information of the intermediate ionic state in NSDI process can be extracted from CEMD at specific laser intensity.

TRecX — An environment for solving time-dependent Schrödinger-like problems

tRecX is a C++ code for solving generalized inhomogeneous time-dependent Schrödinger-type equations idΨ/dt=H[t,Ψ]+Φ in arbitrary dimensions and in a variety of coordinate systems. The operator H[t,Ψ] may have simple non-linearities, as in Gross-Pitaevskii and Hartree(-Fock) problems. Primary application of tRecX has been non-perturbative strong-field single and double photo-electron emission in atomic and molecular physics. The code is designed for large-scale ab initio calculations, for exploring models, and for advanced teaching in computational physics. Distinctive numerical methods are the time-dependent surface flux method for the computation of single and double emission spectra and exterior complex scaling for absorption. Wave functions and operators are handled by tree-structures with the systematic use of recursion on the coarse-grain level. Numerical, analytic, and grid-based discretizations can be combined and are treated on the same abstract level. Operators are specified in the input using a script language including symbolic algebra. User-friendly in- and output, error safety, and documentation are integrated by design. Program summaryProgram title: tRecX — time-dependent Recursive indeXing (tRecX=tSurff+irECS)CPC Library link to program files:https://doi.org/10.17632/zdjbnjxzrn.1Developer's repository link:https://gitlab.physik.uni-muenchen.de/AG-Scrinzi/tRecXCode Ocean capsule:https://codeocean.com/capsule/5391166Licensing provisions: GNU General Public License 2Programming language: C++Nature of problem: tRecX is a general solver for time-dependent Schrödinger-like problems, with applications mostly in strong field and attosecond physics. There are no technical restrictions on the spatial dimension of the problem with up to 6 spatial dimensions realized in the strong-field double ionization of Helium. A selection of coordinate systems is available and any Hamiltonian involving up to second derivatives and arbitrary up to three dimensional potentials can be defined on input by simple scripts.Solution method: The method of lines is used with spatial discretization by a flexible combination of one dimensional basis sets, DVR representations, discrete vectors, expansions into higher-dimensional eigenfunctions of user-defined operators and multi-center basis sets. Photo-emission spectra are calculated using the time-dependent surface flux method (tSurff) in combination with infinite range exterior complex scaling (irECS) for absorption. The code is object oriented and makes extensive use of tree-structures and recursive algorithms. Parallelization is by MPI. Code design and performance allow use in production as well as for graduate level training.

Ionic-state-resolved electron–electron correlation in strong-field double ionization

In this paper, we theoretically investigate the nonsequential double ionization (NSDI) process of Ne induced by 400 nm linearly polarized laser field with a intensity above the recollision threshold using the S-matrix theory. It is unexpected to find that, significantly different from the results for wavelength longer than 800 nm, both the electron momentum correlated distribution and the longitudinal momentum distribution of Ne2+ exhibit a strong dependence on the wave function of the intermediate ionic state from which the second electron is ionized. This observation provides a potential way to extract the information about the ionic-state wave function based on NSDI process.

Influence of Coulomb force between two electrons on double ionization of He-like atoms

In strong-field double ionization, two electrons are ionized by intense laser field. These two electrons move in the laser field and the state is described by a Coulomb–Volkov state, where the repulsive Coulomb state describes the relative motion of the two electrons and the Volkov state describes the center-of-mass motion of the two electrons in the laser field. In the frame of scattering theory, we derive a simple analytical formula of the double ionization of He-like atoms. The effect of the Coulomb force between two electrons on the double ionization process is discussed. Numerical studies disclose that the Coulomb force enhances the ionization rate of high-energy electrons but suppresses the ionization rate of the lowest-energy electrons.

Strong-field ionization of water: Nuclear dynamics revealed by varying the pulse duration

Polyatomic molecules in strong laser fields can undergo substantial nuclear motion within tens of femtoseconds. Ion imaging methods based on dissociation or Coulomb explosion therefore have difficulty faithfully recording the geometry dependence of the field ionization that initiates the dissociation process. Here we compare the strong-field double ionization and subsequent dissociation of water (both H$_2$O and D$_2$O) in 10-fs and 40-fs 800-nm laser pulses. We find that 10-fs pulses turn off before substantial internuclear motion occurs, whereas rapid internuclear motion can take place during the double ionization process for 40-fs pulses. The short-pulse measurements are consistent with a simple tunnel ionization picture, whose predictions help interpret the motion observed in the long-pulse measurements.

Momentum-resolved above-threshold ionization of deuterated water

We present momentum-resolved coincidence measurements of strong-field single and double ionization of deuterated water $({\mathrm{D}}_{2}\mathrm{O})$ using intense few-cycle laser pulses. We measure the vector momentum of electrons and ions resulting from the laser-molecule interaction. This enables us to measure the photoelectron spectrum for electrons in coincidence with fragment ions having different momenta, which can be related to different final states of the cation and dication. The photoelectron spectra for different fragment ion momenta show striking differences, which can be interpreted in terms of the removal of electrons from different molecular orbitals, including the highest occupied molecular orbital (HOMO), as well as more deeply bound ones (e.g., HOMO-2). We discuss our measurements in light of calculations which model the strong-field light matter interaction.

Dynamics of strong-field double ionization of H2 in co-rotating two-color circularly polarized laser fields

Double ionization of H2 in a co-rotating two-color circularly polarized (TCCP) laser field is theoretically investigated. By changing the ratio of electric field peak amplitudes of the TCCP laser pulses, the double ionization probability as a function of the laser intensity shows a clear knee structure, which is suppressed significantly in the case of the atom. Due to the large spatial range of the electronic initial distribution, with the analysis of classical trajectories of ionized electrons, it is found that the ionization of the electron in the farther distance increases the probability of recollision. Furthermore, the yield of nonsequential double ionization created by the recollision can be enhanced by controlling the amplitude ratio of the TCCP laser field.

Ionization induced dynamic alignment of water

Two-body dissociation resulting from strong-field double ionization of water is investigated. Two distinct features are seen in the alignment of the fragment momenta with respect to the laser polarization. One feature shows alignment of the H-OH axis with the laser polarization, while the other indicates polarization alignment normal to the H-OH axis. By analyzing kinematic differences between the OH+/D+ and OD+/H+ channels of HOD, these two alignment features are shown to result from dissociation from different states in the dication. Only dissociation from one of these states has an alignment dependence consistent with predictions of sequential strong-field tunneling ionization models. The alignment dependence of dissociation from the other state can only be explained by dynamic alignment launched by the unbending of the molecule during ionization.

Experimental Separation of Subcycle Ionization Bursts in Strong-Field Double Ionization of H_{2}.

We report on the unambiguous observation of the subcycle ionization bursts in sequential strong-field double ionization of H_{2} and their disentanglement in molecular frame photoelectron angular distributions. This observation was made possible by the use of few-cycle laser pulses with a known carrier-envelope phase, in combination with multiparticle coincidence momentum imaging. The approach demonstrated here will allow sampling of the intramolecular electron dynamics and the investigation of charge-state-specific Coulomb distortions on emitted electrons in polyatomic molecules.

Non-sequential double ionization of triatomic molecules OCS in intense laser fields

We investigated the non-sequential double ionization of the OCS molecule at both 800 nm and 1200 nm laser fields. For the double ionization with different laser wavelengths, an alternating high and low yields were observed in the yields dependence on the laser intensity. To get insight of this phenomenon, the mechanisms of the strong field double ionization were analyzed in details. An abnormal change of the mechanism was identified at 1200 nm laser fields as the laser intensity increases. With further discussions, the influence of laser wavelength on the NSDI was studied.

Internuclear distance dependence of internal-collision induced molecular nonsequential double ionization

ABSTRACT The internuclear distance (R) dependence of the correlated electron dynamics in strong-field nonsequential double ionization (NSDI) of parallel aligned diatomic molecules has been investigated. Results show the correlated electron momentum distribution (CEMD) is almost uniformly spread in the four quadrants for R = 4 a.u., while for larger R it is mainly located in the first and third quadrants. Classifying NSDI events into recollision and internal-collision processes demonstrates the CEMD of internal-collision induced NSDI is strongly dependent on R, which changes from a uniform distribution to a back-to-back emission pattern, and finally to a side-by-side emission pattern as R increases. Back analyzing of the classical trajectories shows it originates from the R dependent time delay between double ionization and collision times. When R increases, more and more internal-collision induced NSDI events occur immediately after collision, resulting in the enhanced dominance of the electron emission in the same hemisphere.

Electron correlation in channel-resolved strong-field molecular double ionization

Strong-field ionization plays a fundamental role in attosecond science as the source of electron wave packets which ultimately provide attosecond bursts of radiation as they are driven back to the atom or molecule of origin. Double ionization is an important probe of electron correlation (correlated electron dynamics), which lies at the core of attosecond science. While double ionization of atoms has been studied extensively, the double ionization of molecules is relatively unexplored---particularly for the case of polyatomic systems. We present coincidence (three and four particle) velocity map imaging measurements of molecular double ionization in 1,3-cyclohexadiene and 1,4-cyclohexadiene using few cycle ($\ensuremath{\sim}10$ fs) laser pulses. Our measurements allow us to distinguish between dissociative and nondissociative double ionization, and we find a difference in the correlation of electrons that come from the different channels.

Threshold photodissociation dynamics of NO2 studied by time-resolved cold target recoil ion momentum spectroscopy.

We study the near-threshold photodissociation dynamics of NO2 by a kinematically complete femtosecond pump-probe scheme using a cold target recoil ion momentum spectrometer. We excite NO2 to the optically bright Ã2B2 state with a 400 nm pulse and probe the ensuing dynamics via strong field single and double ionization with a 25 fs, 800 nm pulse. The pump spectrum spans the NO(X2Π) + O(3P) dissociation channel threshold, and therefore, following internal conversion, excited NO2 is energetically prepared both "above threshold" (dissociating) and "below threshold" (nondissociating). Experimentally, we can clearly discriminate a weak two-photon pump channel from the dominant single-photon data. In the single ionization channel, we observe NO+ fragments with nonzero momentum at 200 fs delay and an increasing yield of NO+ fragments with near-zero momentum at 3.0 ps delay. For double ionization events, we observe a time-varying Coulombic kinetic energy release between the NO+ and O+ fragments impulsively created from the evolving "hot" neutral ground state. Supported by classical trajectory calculations, we assign the decreasing Coulombic kinetic energy release at longer time delays to the increasing average NO-O distances in the ground electronic state during its large amplitude phase space evolution toward free products. The time-resolved kinetic energy release in the double ionization channel probes the large amplitude ground state evolution from a strongly coupled "inner region" to a loosely coupled "outer region" where one O atom is on average much further away from the NO. Both the time evolution of the kinetic energy release and the NO+ angular distributions support our assignments.