Rescattering double ionization of D2 and H2 by intense laser pulses

Controlling Fragmentation of the Acetylene Cation in the Vacuum Ultraviolet via Transient Molecular Alignment

An open-loop control scheme of molecular fragmentation based on transient molecular alignment combined with single-photon ionization induced by a short-wavelength free electron laser (FEL) is demonstrated for the acetylene cation. Photoelectron spectra are recorded, complementing the ion yield measurements, to demonstrate that such control is the consequence of changes in the electronic response with molecular orientation relative to the ionizing field. We show that stable C2H2+ cations are mainly produced when the molecules are parallel or nearly parallel to the FEL polarization, while the hydrogen fragmentation channel (C2H2+ → C2H+ + H) predominates when the molecule is perpendicular to that direction, thus allowing one to distinguish between the two photochemical processes. The experimental findings are supported by state-of-the art theoretical calculations.

Two-Body Metastable Dissociation of n-Pentane and n-Hexane Triplet Dications in Intense Femtosecond-Laser Fields

Mass spectra of n-pentane and n-hexane ionized through femtosecond-laser pulses were measured using a time-of-flight mass spectrometer. Fragment ions ejected with large kinetic energies were identified as side peaks in which a two-body dissociation pathway, C₅H₁₂⁺⁺ → C₂H₅⁺ + C₃H₇⁺, was identified for n-pentane, and two for n-hexane, C₆H₁₄⁺⁺ → C₂H₅⁺ + C₄H₉⁺ and C₃H₇⁺ + C₃H₇⁺, based on momentum matching of the fragments. The two-body dissociation pathways were observed when the polarization direction of the linearly polarized laser light was perpendicular to the molecular axis. However, when the polarization direction was parallel to the molecular axis or the laser light was circularly polarized, these signals were weak or difficult to identify. These results suggest that the two-body dissociation pathways are caused by nonsequential double ionization (NSDI), which begins with ionization from the π-type second highest occupied molecular orbital (HOMO-1) via the laser electric field perpendicular to the molecular axis rather than bonding the σ-type HOMO. Quantum chemical calculations show that the dication has a triplet metastable state with the same formula as the neutral state (i.e., ³[CH₃-(CH₂)_n-CH₃]⁺⁺). Therefore, the relevant two-body dissociation channels open through transition states with the (HOMO)¹(HOMO-1)¹ electron configuration and the estimated kinetic energy release values correlate with those observed.

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.

Steering electron correlation time by elliptically polarized femtosecond laser pulses

Electron correlation is ubiquitous across diverse physical systems from atoms and molecules to condensed matter. Observing and controlling dynamical electron correlation in photoinduced processes paves the way to the coherent control of chemical reactionsand photobiological processes. Here, we experimentally investigate dynamics of electron correlation in double ionization of neon irradiated by intense elliptically polarized laser pulses. We find a characteristic, ellipticity-dependent, correlated electron emission along the minor axis of the elliptically polarized light. This observation is well reproduced by a semi-classical ensemble model simulation. By tracing back the corresponding electron trajectories, we find that the dynamical energy sharing during the electron emission process is modified by the ellipticity of the laser light. Thus, our work provides evidence for a possible ultrafast control of the energy sharing between the correlated electrons by varying the light ellipticity.

Timing Recollision in Nonsequential Double Ionization by Intense Elliptically Polarized Laser Pulses

We examine correlated electron and doubly charged ion momentum spectra from strong field double ionization of neon employing intense elliptically polarized laser pulses. An ellipticity-dependent asymmetry of correlated electron and ion momentum distributions has been observed. Using a 3D semiclassical model, we demonstrate that our observations reflect the subcycle dynamics of the recollision process. Our Letter reveals a general physical picture for recollision impact double ionization with elliptical polarization and demonstrates the possibility of ultrafast control of the recollision dynamics.

High-order above-threshold dissociation of molecules

Above-threshold ionization of atoms in strong laser fields is extensively studied for its overwhelming importance and universality. However, its counterpart, above-threshold dissociation of molecules in strong laser fields, is hard to be observed, although it has been predicted for decades. In this paper, by measuring the momenta of photoelectron and dissociative fragments coincidently, we successfully obtained distinct nuclear energy peaks of the high-order above-threshold dissociation, which must appear simultaneously with the above-threshold ionization. The coexistence of high-order above-threshold dissociation and high-order above-threshold ionization in molecular dissociative ionization offers a perspective to disentangle the complex electron–nuclear correlation in molecules and to image the molecular orbitals, and so on.

Electrons bound to atoms or molecules can simultaneously absorb multiple photons via the above-threshold ionization featured with discrete peaks in the photoelectron spectrum on account of the quantized nature of the light energy. Analogously, the above-threshold dissociation of molecules has been proposed to address the multiple-photon energy deposition in the nuclei of molecules. In this case, nuclear energy spectra consisting of photon-energy spaced peaks exceeding the binding energy of the molecular bond are predicted. Although the observation of such phenomena is difficult, this scenario is nevertheless logical and is based on the fundamental laws. Here, we report conclusive experimental observation of high-order above-threshold dissociation of H₂ in strong laser fields where the tunneling-ionized electron transfers the absorbed multiphoton energy, which is above the ionization threshold to the nuclei via the field-driven inelastic rescattering. Our results provide an unambiguous evidence that the electron and nuclei of a molecule as a whole absorb multiple photons, and thus above-threshold ionization and above-threshold dissociation must appear simultaneously, which is the cornerstone of the nowadays strong-field molecular physics.

Isotope Effect in Tunneling Ionization of Neutral Hydrogen Molecules

It has been recently predicted theoretically that due to nuclear motion light and heavy hydrogen molecules exposed to strong electric field should exhibit substantially different tunneling ionization rates [O. I. Tolstikhin, H. J. Worner, and T. Morishita, Phys. Rev. A 87, 041401(R) (2013)]. We studied that isotope effect experimentally by measuring relative ionization yields for each species in a mixed H_{2}/D_{2} gas jet interacting with intense femtosecond laser pulses. In a reaction microscope apparatus, we detected ionic fragments from all contributing channels (single ionization, dissociation, and sequential double ionization) and determined the ratio of total single ionization yields for H_{2} and D_{2}. The measured ratio agrees quantitatively with the prediction of the generalized weak-field asymptotic theory in an apparent failure of the frozen-nuclei approximation.

The effect of molecular alignment on correlated electron dynamics in nonsequential double ionization

The electron-electron correlation in nonsequential double ionization (NSDI) from aligned molecules by linearly polarized 800 nm laser pulses has been investigated with the three-dimensional classical ensemble model. The result shows that for the perpendicular alignment the two electrons involved in NSDI more likely exit the molecule into the opposite hemispheres as compared to the parallel alignment, which agrees well with the experimental result [Phys. Rev. Lett. 95, 203003 (2005)]. This alignment effect is qualitatively explained based on the suppressed potential barriers which are different for parallel molecules and perpendicular molecules. Additionally, the intensity dependence of the alignment effect is also explored.

Internuclear-distance dependence of electron correlation in nonsequential double ionization of H(2)

Using three-dimensional classical ensembles, we have investigated the internuclear distance dependence of nonsequential double ionization (NSDI) of H(2) molecules by an 800 nm, 1x10(14) W/cm(2) laser pulse. For the internuclear distance R ranging from 2 to 12 a.u., the NSDI of H(2) provides rich correlation patterns in the two-electron momentum distributions. These correlation patterns essentially reveal different microscopic dynamics in NSDI process. Moreover, our calculations show that R approximately 4 a.u. is the critical distance for double ionization yield of H(2). These results are qualitatively explained based on the classical barrier expression model and back analysis.

Manipulating nonsequential double ionization via alignment of asymmetric molecules

Using classical three-dimensional ensembles, we demonstrate that nonsequential double ionization of HeH+ molecules in an intense laser field can be manipulated by controlling the alignment of the molecular axis relative to the laser field. Both the symmetry of the correlated electron momentum spectrum in the direction parallel to the laser field and the total double ionization yield strongly depend on the angle between the molecular axis and the laser field. When the molecular axis is aligned parallel to the laser field, double ionization is most probable and the correlated electron momentum spectrum parallel to the laser field from nonsequential double ionization exhibits the most asymmetry with respect to the minor diagonal.

Warm target recoil ion momentum spectroscopy for fragmentation of molecular hydrogen by ultrashort laser pulses

We demonstrate warm target recoil ion momentum spectroscopy for the fragmentation dynamics of the warm hydrogen molecules at room temperature. The thermal movement effect of the warm molecule is removed by using a correction algorithm in the momentum space. Based on the reconstructed three-dimensional momentum vectors as well as the kinetic energy release spectra, different vibrational states of the H(2)(+) ground state are clearly visible and the internuclear separation for charge resonance enhanced ionization of the second electron is identified. The results show adequate accordance with the former experiments using other techniques.

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.

Numerical observation of the rescattering wave packet in laser-atom interactions

We present a full-quantum nonperturbative method to study the electron rescattering process in the intense laser-atom interactions. We separate the ionized wave function from the background by solving the time-integral equation. Imposing the incoming boundary condition on the wave function, we reproduce the motion of the rescattering wave packet predicted by the rescattering theory. Our calculated rescattering energies differ significantly from the semiclassical ones. The difference would be substantial for the evaluation of the rescattering induced dynamics such as the molecular dissociation.

Dynamics of light-field control of molecular dissociation at the few-cycle limit

We studied the laser-molecule interaction dynamics that leads to the asymmetric D+ ion ejection in the dissociative ionization of D2 molecules observed recently in Kling et al. [Science 312, 246 (2006)10.1126/science.1126259]. By changing the carrier-envelope phase, we showed that the asymmetry is a consequence of manipulating the initial ionization and the rescattering of the electrons within one optical cycle of the laser. The result illustrates the feasibility of coherent control of reaction dynamics at the attosecond time scale.

Attosecond strobing of two-surface population dynamics in dissociating H2+

Using H2+ and D2+, we observe two-surface population dynamics by measuring the kinetic energy of the correlated ions that are created when H2+ (D2+) ionize in short (40-140 fs) and intense (10(14) W/cm2) infrared laser pulses. Experimentally, we find a modulation of the kinetic energy spectrum of the correlated fragments. The spectral progression arises from a hitherto unexpected spatial modulation on the excited state population, revealed by Coulomb explosion. By solving the two-level time-dependent Schrödinger equation, we show that an interference between the net-two-photon and the one-photon transition creates localized electrons which subsequently ionize.

Control of Electron Localization in Molecular Dissociation

We demonstrated how the subcycle evolution of the electric field of light can be used to control the motion of bound electrons. Results are presented for the dissociative ionization of deuterium molecules (D2 --> D+ + D), where asymmetric ejection of the ionic fragment reveals that light-driven intramolecular electronic motion before dissociation localizes the electron on one of the two D+ ions in a controlled way. The results extend subfemtosecond electron control to molecules and provide evidence of its usefulness in controlling reaction dynamics.

Attosecond probing of vibrational dynamics with high-harmonic generation

The numerical solution of the time-dependent Schrodinger equation for vibrating hydrogen molecules in few-cycle laser pulses shows that high-harmonic generation is sensitive to the laser-induced vibrational motion. More intense harmonics are generated in heavier isotopes, the difference increasing with the harmonic frequency. Analytical theory reveals a dependence of the harmonics on the vibrational autocorrelation function. With the help of a genetic algorithm, the nuclear motion can be reconstructed from the harmonic spectra with sub-fs time resolution.

Effects of molecular structure on ion disintegration patterns in ionization of O2 and N2 by short laser pulses

We demonstrate that the structure of the outermost orbitals of oxygen and nitrogen can be observed in the angular distribution of coincident ion pairs generated by the double ionization of these molecules by 8 fs laser pulses. We do this by establishing that these ions emerge from well defined excited electronic states of O2+2 and N2+2 respectively and that they are produced dominantly through a process which involves electron rescattering. The angular distributions of the ions from the two targets are very different, reflecting the different structures of the outermost orbitals of the two molecules.

Influence of molecular structure on double ionization of N2 and O2 by high intensity ultrashort laser pulses

The electron momentum correlation after nonsequential double ionization of N2 and O2 in ultrashort light pulses at light intensities near 1.5 x 10(14) W/cm(2) has been investigated. The experimental results reveal distinctive differences between the molecular species and between molecules and atoms of similar ionization threshold. We provide evidence that recollision double ionization is the essential mechanism and trace the origin of the differences back to the symmetry of the orbitals occupied by the valence electrons.