Attosecond tracing of correlated electron-emission in non-sequential double ionization

Direct observation of an attosecond electron wave packet in a nitrogen molecule

Capturing electron motion in a molecule is the basis of understanding or steering chemical reactions. Nonlinear Fourier transform spectroscopy using an attosecond-pump/attosecond-probe technique is used to observe an attosecond electron wave packet in a nitrogen molecule in real time. The 500-as electronic motion between two bound electronic states in a nitrogen molecule is captured by measuring the fragment ions with the same kinetic energy generated in sequential two-photon dissociative ionization processes. The temporal evolution of electronic coherence originating from various electronic states is visualized via the fragment ions appearing after irradiation of the probe pulse. This observation of an attosecond molecular electron wave packet is a critical step in understanding coupled nuclear and electron motion in polyatomic and biological molecules to explore attochemistry.

Resolving subcycle electron emission in strong-field sequential double ionization

Using a fully classical model, we have studied sequential double ionization (SDI) of argon driven by elliptically polarized laser pulses at intensities well in the over-barrier ionization region. The results show that ion momentum distributions evolve from the two-band structure to the four-band, six-band structure and finally to the previously obtained four-band structure as the pulse duration increases. Our analysis shows that the evolution of these band structures originates from the pulse-duration-dependent multiple ionization bursts of the second electron. These band structures unambiguously indicate the subcycle electron emission in SDI.

Channel-resolved above-threshold double ionization of acetylene

We experimentally investigate the channel-resolved above-threshold double ionization (ATDI) of acetylene in the multiphoton regime using an ultraviolet femtosecond laser pulse centered at 395 nm by measuring all the ejected electrons and ions in coincidence. As compared to the sequential process, diagonal lines in the electron-electron joint energy spectrum are observed for the nonsequential ATDI owing to the correlative sharing of the absorbed multiphoton energies. We demonstrate that the distinct channel-resolved sequential and nonsequential ATDI spectra can clearly reveal the photon-induced acetylene-vinylidene isomerization via proton migration on the cation or dication states.

Transition of correlated-electron emission in nonsequential double ionization of Ar atoms

Emission of the two electrons released from nonsequential double ionization of argon atoms is anticorrelated at lower laser intensities but is correlated at higher laser intensities. Such a transition is caused by the momentum change of recollision-induced-ionization (RII) electrons. At lower laser intensities, the Coulomb repulsion between the two RII electrons dominates the motion of electrons and pushes them leaving the laser field back-to-back. At higher laser intensities, the drift momentum obtained from the laser field dominates the motion of electrons and drives them leaving the laser field side-by-side.

Subcycle control of electron-electron correlation in double ionization

Double ionization of neon with orthogonally polarized two-color (OTC) laser fields is investigated using coincidence momentum imaging. We show that the two-electron emission dynamics in nonsequential double ionization can be controlled by tuning the subcycle shape of the electric field of the OTC pulses. We demonstrate experimentally switching from correlated to anticorrelated two-electron emission, and control over the directionality of the two-electron emission. Simulations based on a semiclassical trajectory model qualitatively explain the experimental results by a subcycle dependence of the electron recollision time on the OTC field shape.

Ionization of excited atoms by intense single-cycle THz pulses

We have employed intense, single-cycle THz pulses to explore strong-field ionization of low-lying Na Rydberg states in the low-frequency limit. At the largest fields used, F≃430  kV/cm, electrons with energies up to 60 eV are created. The field ionization threshold is greater than expected for adiabatic "over-the-barrier" ionization and is found to scale as n-3. In addition, for a given field amplitude, higher energy electrons are produced during the ionization of the most tightly bound states. These observations can be attributed to the suppression of scattering from the nonhydrogenic ion core, the long times required for Rydberg electrons to escape over the barrier in the field-dressed Coulomb potential, and the failure, in the single-cycle limit, of the standard prediction for electron energy transfer in an oscillating field. The latter, in particular, holds important implications for future strong-field experiments involving the interaction of ground-state atoms and molecules with true single-cycle laser fields.

Cavity-enhanced high-harmonic generation with spatially tailored driving fields

We theoretically and experimentally investigate high-harmonic generation in a 78-MHz enhancement cavity with a transverse mode having on-axis intensity maxima at the focus and minima at an opening in the following mirror. We find that the conversion efficiency is comparable to that achievable with a Gaussian mode, whereas the output coupling efficiency can be significantly improved over any other demonstrated technique. This approach offers additional power scaling advantages and additional degrees of freedom in shaping the harmonic emission, paving the way to high-power extreme-ultraviolet frequency combs and the generation of multi-MHz repetition-rate-isolated attosecond pulses.

Nuclear reaction induced by carrier-envelope-phase controlled proton recollision in a laser-driven molecule

Nuclear reactions induced by proton recollision with a nearby nucleus are studied in a setup where a neutral molecule is exposed to an extremely intense, few-cycle laser pulse. At the rising edge of the laser pulse, all electrons in the molecule are first ejected by field ionization, resulting in a molecule consisting of the bare nuclei only. A proton in the bare molecule is subsequently accelerated by the laser field in such a way that it recollides with a nearby, heavier nucleus, with a kinetic energy high enough to induce a nuclear reaction. As a specific example, the probability of triggering the (15)N(p,α)(12)C reaction by exposing either a (15)NH molecule or a (15)NH3 molecule to an intense laser pulse is calculated using the classical trajectory Monte Carlo method. We show that the proton recollision process can be controlled both by varying the carrier-envelope phase of the laser field and by the degree of molecular orientation. We also find that the magnetic field of the laser pulse plays a crucial role in the recollision dynamics.

Quantum effects in double ionization of argon below the threshold intensity

So far, nonsequential double ionization (NSDI) of atoms can be well understood within a semiclassical or even classical picture. No quantum effect appears to be required to explain the data observed. We theoretically study electron correlation resulting from NSDI of argon in a low-intensity laser field using a quantum-mechanical S-matrix theory. We show that quantum interference between the contributions of different intermediate excited states of the singly charged argon ion produces a transition from back-to-back to side-by-side emission with increasing laser intensity, which is in close agreement with the experimental data. For higher intensities, this transition is enhanced by the consequences of depletion of the excited states.

Strong-field double ionization through sequential release from double excitation with subsequent Coulomb scattering

We perform a triple coincidence study on differential momentum distributions of strong-field double ionization of Ar atoms in linearly polarized fields (795 nm, 45 fs, 7×10(13)  W/cm2). Using a three-dimensional two-electron atomic-ensemble semiclassical model including the tunneling effect for both electrons, we retrieve differential momentum distributions and achieve a good agreement with the measurement. Ionization dynamics of the correlated electrons for the side-by-side and back-to-back emission is analyzed separately. According to the semiclassical model, we find that the doubly excited states are largely populated after the laser-assisted recollision and large amounts of double ionization dominantly takes place through sequential ionization of doubly excited states at such a low laser intensity. Compared with the Coulomb-free and Coulomb-corrected sequential tunneling models, we verify that electrons can obtain an energy as large as ∼6.5U p through Coulomb scattering in the combined laser and doubly charged ionic fields.

Unified approach to probing Coulomb effects in tunnel ionization for any ellipticity of laser light

We present experimental data that show significant deviations from theoretical predictions for the location of the center of the electron momenta distribution at low values of ellipticity ε of laser light. We show that these deviations are caused by significant Coulomb focusing along the minor axis of polarization, something that is normally neglected in the analysis of electron dynamics, even in cases where the Coulomb correction is otherwise taken into account. By investigating ellipticity-resolved electron momenta distributions in the plane of polarization, we show that Coulomb focusing predominates at lower values of ellipticity of laser light, while Coulomb asymmetry becomes important at higher values, showing that these two complementary phenomena can be used to probe long-range Coulomb interaction at all polarizations of laser light. Our results suggest that both the breakdown of Coulomb focusing and the onset of Coulomb asymmetry are linked to the disappearance of Rydberg states with increasing ellipticity.

Boosting photoabsorption by attosecond control of electron correlation

Electron correlation plays an essential role in a wide range of fundamentally important many-body phenomena in modern physics and chemistry. An example is the importance of electron-electron correlation in multiple ionization of multielectron atoms and molecules exposed to intense laser pulses. Manipulating the dynamic electron correlation in such photoinduced processes is a crucial step toward the coherent control of chemical reactions and photobiological processes. The generation of an attosecond extreme ultraviolet (EUV) pulse may enable such controls. Here, we show for the first time, from full-dimensional ab initio calculations of double ionization of helium in intense laser pulses (λ = 780 nm), that the electron-electron interactions can be instantaneously tuned using a time-delayed attosecond EUV pulse. Consequently, the probability of producing energetic electrons from excessive photoabsorption can be enhanced by an order of magnitude, by the attosecond control of electron-electron correlation.

Coherent control at its most fundamental: carrier-envelope-phase-dependent electron localization in photodissociation of a H2(+) molecular ion beam target

Measurements and calculations of the absolute carrier-envelope-phase (CEP) effects in the photodissociation of the simplest molecule, H2(+), with a 4.5-fs Ti:sapphire laser pulse at intensities up to (4±2)×10(14)  W/cm2 are presented. Localization of the electron with respect to the two nuclei (during the dissociation process) is controlled via the CEP of the ultrashort laser pulses. In contrast to previous CEP-dependent experiments with neutral molecules, the dissociation of the molecular ions is not preceded by a photoionization process, which strongly influences the CEP dependence. Kinematically complete data are obtained by time- and position-resolved coincidence detection. The phase dependence is determined by a single-shot phase measurement correlated to the detection of the dissociation fragments. The experimental results show quantitative agreement with ab initio 3D time-dependent Schrödinger equation calculations that include nuclear vibration and rotation.

Contribution of recollision ionization to the cross-shaped structure in nonsequential double ionization

With the three-dimensional classical ensemble model, we investigate the correlated electron emission in nonsequential double ionization (NSDI) of argon atoms by few-cycle laser pulses. Our calculations well reproduce the experimentally observed cross-shaped structure in the correlated two-electron momentum spectrum [ Nature Commun. 3, 813 (2012)]. By tracing these NSDI trajectories, we find that besides the process of recollision-induced excitation with subsequent ionization just before the next field maximum, the recollision ionization also significantly contributes to the cross-shaped structure.

Fully differential single-photon double ionization of neon and argon

Triply differential cross sections are calculated for one-photon double ionization of neon and argon at various photon energies and electron energy sharings by using a frozen-core treatment to represent the remaining electrons of the residual ion. Angular distributions agree well with all existing experimental data, showing that in spite of its simplicity the method can treat the double ionization of complex targets reliably. A comparison of the cross sections for helium, neon, and argon into the same final state symmetry at the same relative excess energies reveals a distinctive signature of the role of electron correlation in each target.

Attosecond-recollision-controlled selective fragmentation of polyatomic molecules

Control over various fragmentation reactions of a series of polyatomic molecules (acetylene, ethylene, 1,3-butadiene) by the optical waveform of intense few-cycle laser pulses is demonstrated experimentally. We show both experimentally and theoretically that the responsible mechanism is inelastic ionization from inner-valence molecular orbitals by recolliding electron wave packets, whose recollision energy in few-cycle ionizing laser pulses strongly depends on the optical waveform. Our work demonstrates an efficient and selective way of predetermining fragmentation and isomerization reactions in polyatomic molecules on subfemtosecond time scales.