Origin of unexpected low energy structure in photoelectron spectra induced by midinfrared strong laser fields

Control of Electron Wave Packets Close to the Continuum Threshold Using Near-Single-Cycle THz Waveforms

The control of low-energy electrons by carrier-envelope-phase-stable near-single-cycle THz pulses is demonstrated. A femtosecond laser pulse is used to create a temporally localized wave packet through multiphoton absorption at a well defined phase of a synchronized THz field. By recording the photoelectron momentum distributions as a function of the time delay, we observe signatures of various regimes of dynamics, ranging from recollision-free acceleration to coherent electron-ion scattering induced by the THz field. The measurements are confirmed by three-dimensional time-dependent Schrödinger equation simulations. A classical trajectory model allows us to identify scattering phenomena analogous to strong-field photoelectron holography and high-order above-threshold ionization.

Study of the effect of higher-order dispersions on photoionisation induced by ultrafast laser pulses applying a classical theoretical method

We investigated the effect of higher order dispersion on ultrafast photoionisation with Classical Trajectory Monte Carlo (CTMC) method for hydrogen and krypton atoms. In our calculations we used linearly polarised ultrashort 7 fs laser pulses, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$6.5 \times 10^{14} \mathrm {W/cm^{2}}$$\end{document}6.5×1014W/cm2 intensity, and a central wavelength of 800 nm. Our results show that electrons with the highest kinetic energies are obtained with transform limited (TL) pulses. The shaping of the pulses with negative second- third- or fourth- order dispersion results in higher ionisation yield and electron energies compared to pulses shaped with positive dispersion values. We have also investigated how the Carrier Envelope Phase (CEP) dependence of the ionisation is infuenced by dispersion. We calculated the left-right asymmetry as a function of energy and CEP for sodium atoms employing pulses of 4.5 fs, 800 nm central wavelength, and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$4 \times 10^{12}\mathrm {W/cm^{2}}$$\end{document}4×1012W/cm2 intensity. We found that the left-right asymmetry is more pronounced for pulses shaped with positive Group Delay Dispersion (GDD). It was also found that shaping a pulse with increasing amounts of GDD in absolute value blurs the CEP dependence, which is attributed to the increasing number of optical cycles.

It is all about phases: ultrafast holographic photoelectron imaging

Photoelectron holography constitutes a powerful tool for the ultrafast imaging of matter, as it combines high electron currents with subfemtosecond resolution, and gives information about transition amplitudes and phase shifts. Similarly to light holography, it uses the phase difference between the probe and the reference waves associated with qualitatively different ionization events for the reconstruction of the target and for ascertaining any changes that may occur. These are major advantages over other attosecond imaging techniques, which require elaborate interferometric schemes in order to extract phase differences. For that reason, ultrafast photoelectron holography has experienced a huge growth in activity, which has led to a vast, but fragmented landscape. The present review is an organizational effort towards unifying this landscape. This includes a historic account in which a connection with laser-induced electron diffraction is established, a summary of the main holographic structures encountered and their underlying physical mechanisms, a broad discussion of the theoretical methods employed, and of the key challenges and future possibilities. We delve deeper in our own work, and place a strong emphasis on quantum interference, and on the residual Coulomb potential.

Coulomb focusing in retrapped ionization with near-circularly polarized laser field

The full three-dimensional photoelectron momentum distributions of argon are measured in intense near-circularly polarized laser fields. We observed that the transverse momentum distribution of ejected electrons by 410-nm near-circularly polarized field is unexpectedly narrowed with increasing laser intensity, which is contrary to the conventional rules predicted by adiabatic theory. By analyzing the momentum-resolved angular momentum distribution measured experimentally and the corresponding trajectories of ejected electrons semiclassically, the narrowing can be attributed to a temporary trapping and thereby focusing of a photoelectron by the atomic potential in a quasibound state. With the near-circularly polarized laser field, the strong Coulomb interaction with the rescattering electrons is avoided, thus the Coulomb focusing in the retrapped process is highlighted. We believe that these findings will facilitate understanding and steering electron dynamics in the Coulomb coupled system.

High-Harmonic and Terahertz Spectroscopy (HATS): Methods and Applications

Electrons driven from atom or molecule by intense dual-color laser fields can coherently radiate high harmonics from extreme ultraviolet to soft X-ray, as well as an intense terahertz (THz) wave from millimeter to sub-millimeter wavelength. The joint measurement of high-harmonic and terahertz spectroscopy (HATS) was established and further developed as a unique tool for monitoring electron dynamics of argon from picoseconds to attoseconds and for studying the molecular structures of nitrogen. More insights on the rescattering process could be gained by correlating the fast and slow electron motions via observing and manipulating the HATS from atoms and molecules. We also propose the potential investigations of HATS of polar molecules, and solid and liquid sources.

Quantum Interference of Glory Rescattering in Strong-Field Atomic Ionization

During the ionization of atoms irradiated by linearly polarized intense laser fields, we find for the first time that the transverse momentum distribution of photoelectrons can be well fitted by a squared zeroth-order Bessel function because of the quantum interference effect of glory rescattering. The characteristic of the Bessel function is determined by the common angular momentum of a number of semiclassical paths termed as glory trajectories, which are launched with different nonzero initial transverse momenta distributed on a specific circle in the momentum plane and finally deflected to the same asymptotic momentum, which is along the polarization direction, through post-tunneling rescattering. Glory rescattering theory based on the semiclassical path-integral formalism is developed to address this effect quantitatively. Our theory can resolve the long-standing discrepancies between existing theories and experiments on the fringe location, predict the sudden transition of the fringe structure in holographic patterns, and shed light on the quantum interference aspects of low-energy structures in strong-field atomic ionization.

Low-Energy Electron Emission in the Strong-Field Ionization of Rare Gas Clusters

Clusters and nanoparticles have been widely investigated to determine how plasmonic near fields influence the strong-field induced energetic electron emission from finite systems. We focus on the contrary, i.e., the slow electrons, and discuss a hitherto unidentified low-energy structure (LES) in the photoemission spectra of rare gas clusters in intense near-infrared laser pulses. For Ar and Kr clusters we find, besides field-driven fast electrons, a robust and nearly isotropic emission of electrons with <4  eV kinetic energies that dominates the total yield. Molecular dynamics simulations reveal a correlated few-body decay process involving quasifree electrons and multiply excited ions in the nonequilibrium nanoplasma that results in a dominant LES feature. Our results indicate that the LES emission occurs after significant nanoplasma expansion, and that it is a generic phenomenon in intense laser nanoparticle interactions, which is likely to influence the formation of highly charged ions.

Resonance-like enhancement in high-order above threshold ionization of atoms and molecules in intense laser fields

We investigate the high-order above-threshold ionization (HATI) of atoms (Ar and Xe) and molecules (N₂ and O₂) subjected to strong laser fields by numerically solving time-dependent Schrödinger equation. It is demonstrated that resonance-like enhancement of groups of adjacent peaks in photoelectron spectrum of HATI is observed for Ar, Xe, and N₂, while this peculiar phenomenon is absent for O₂, which is in agreement with experimental observation [ Phys. Rev. A88, 021401 (2013)]. In addition, analysis indicates that resonance-like enhancement in HATI spectra of atoms and molecules is closely related to excitation of the high-lying excited states.

Emergence of a Higher Energy Structure in Strong Field Ionization with Inhomogeneous Electric Fields

Studies of strong field ionization have historically relied on the strong field approximation, which neglects all spatial dependence in the forces experienced by the electron after ionization. More recently, the small spatial inhomogeneity introduced by the long-range Coulomb potential has been linked to a number of important features in the photoelectron spectrum, such as Coulomb asymmetry, Coulomb focusing, and the low energy structure. Here, we demonstrate using midinfrared laser wavelength that a time-varying spatial dependence in the laser electric field, such as that produced in the vicinity of a nanostructure, creates a prominent higher energy peak. This higher energy structure (HES) originates from direct electrons ionized near the peak of a single half-cycle of the laser pulse. The HES is separated from all other ionization events, with its location and width highly dependent on the strength of spatial inhomogeneity. Hence, the HES can be used as a sensitive tool for near-field characterization in the "intermediate regime," where the electron's quiver amplitude is comparable to the field decay length. Moreover, the large accumulation of electrons with tuneable energy suggests a promising method for creating a localized source of electron pulses of attosecond duration using tabletop laser technology.

Numerical Detector Theory for the Longitudinal Momentum Distribution of the Electron in Strong Field Ionization

The lack of analytical solutions for the exit momentum in the laser-driven tunneling theory is a well-recognized problem in strong field physics. Theoretical studies of electron momentum distributions in the neighborhood of the tunneling exit depend heavily on ad hoc assumptions. In this Letter, we apply a new numerical method to study the exiting electron's longitudinal momentum distribution under intense short-pulse laser excitation. We present the first realizations of the dynamic behavior of an electron near the so-called tunneling exit region without adopting a tunneling approximation.

Efficient time-sampling method in Coulomb-corrected strong-field approximation

One of the main goals of strong-field physics is to understand the complex structures formed in the momentum plane of the photoelectron. For this purpose, different semiclassical methods have been developed to seek an intuitive picture of the underlying mechanism. The most popular ones are the quantum trajectory Monte Carlo (QTMC) method and the Coulomb-corrected strong-field approximation (CCSFA), both of which take the classical action into consideration and can describe the interference effect. The CCSFA is more widely applicable in a large range of laser parameters due to its nonadiabatic nature in treating the initial tunneling dynamics. However, the CCSFA is much more time consuming than the QTMC method because of the numerical solution to the saddle-point equations. In the present work, we present a time-sampling method to overcome this disadvantage. Our method is as efficient as the fast QTMC method and as accurate as the original treatment in CCSFA. The performance of our method is verified by comparing the results of these methods with that of the exact solution to the time-dependent Schrödinger equation.

Recollision dynamics in nonsequential double ionization of atoms by long-wavelength pulses

Recollision dynamics and electron correlation behavior are investigated for several long laser wavelengths (1200-3000 nm) in nonsequential double ionization (NSDI) of helium using three-dimensional classical ensembles. Numerical results show that for these long wavelengths NSDI events are mainly from the multiple-return trajectory which is different from the case of 800 nm. Moreover, with increasing laser wavelength NSDI events move from the diagonal to the two axes in the correlated electron momentum distributions, and finally form an experimentally observed prominent V-shaped structure [Phys. Rev. X 5, 021034 (2015)] in the first and third quadrants. Back analysis indicates that the asymmetric energy sharing between the two electrons at recollision is responsible for the formation of the prominent V-shaped structure of 3000 nm.

Laser-Driven Recollisions under the Coulomb Barrier

Photoelectron spectra obtained from the ab initio solution of the time-dependent Schrödinger equation can be in striking disagreement with predictions by the strong-field approximation (SFA), not only at low energy but also around twice the ponderomotive energy where the transition from the direct to the rescattered electrons is expected. In fact, the relative enhancement of the ionization probability compared to the SFA in this regime can be several orders of magnitude. We show for which laser and target parameters such an enhancement occurs and for which the SFA prediction is qualitatively good. The enhancement is analyzed in terms of the Coulomb-corrected action along analytic quantum orbits in the complex-time plane, taking soft recollisions under the Coulomb barrier into account. These recollisions in complex time and space prevent a separation into sub-barrier motion up to the "tunnel exit" and subsequent classical dynamics. Instead, the entire quantum path up to the detector determines the ionization probability.

Intra-half-cycle interference of low-energy photoelectron in strong midinfrared laser fields

Using semiclassical models with implementing interference effects, we study the low-energy photoelectron intra-half-cycle interferences among nonscattering trajectories and multiple forward scattering trajectories of atoms ionized by a strong mid-infrared laser field. Tracing back to the initial tunneling coordinates, we find that up to three kinds of forward scattering trajectories have substantial contributions to the low-energy photoelectrons. Those multiple forward scattering trajectories depend sensitively on the initial transverse momentum at the tunnel exit and they lead to sign reversal of the transverse momentum of the electrons. We show that the interference of the nonscattering trajectory and the triple-scattered trajectory has the largest contribution to the low-energy structure in mid-infrared laser fields. It is also shown that the long-range Coulomb potential has a significant effect on the low-energy photoelectron interference patterns.

Unraveling nonadiabatic ionization and Coulomb potential effect in strong-field photoelectron holography

Strong field photoelectron holography has been proposed as a means for interrogating the spatial and temporal information of electrons and ions in a dynamic system. After ionization, part of the electron wave packet may directly go to the detector (the reference wave), while another part may be driven back and scatters off the ion(the signal wave). The interference hologram of the two waves may be used to extract target information embedded in the collision process. Unlike conventional optical holography, however, propagation of the electron wave packet is affected by the Coulomb potential as well as by the laser field. In addition, electrons are emitted over the whole laser pulse duration, thus multiple interferences may occur. In this work, we used a generalized quantum-trajectory Monte Carlo method to investigate the effect of Coulomb potential and the nonadiabatic subcycle ionization on the photoelectron hologram. We showed that photoelectron hologram can be well described only when the effect of nonadiabatic ionization is accounted for, and Coulomb potential can be neglected only in the tunnel ionization regime. Our results help paving the way for establishing photoelectron holography for probing spatial and dynamic properties of atoms and molecules.

Long-Range Coulomb Effect in Intense Laser-Driven Photoelectron Dynamics

In strong field atomic physics community, long-range Coulomb interaction has for a long time been overlooked and its significant role in intense laser-driven photoelectron dynamics eluded experimental observations. Here we report an experimental investigation of the effect of long-range Coulomb potential on the dynamics of near-zero-momentum photoelectrons produced in photo-ionization process of noble gas atoms in intense midinfrared laser pulses. By exploring the dependence of photoelectron distributions near zero momentum on laser intensity and wavelength, we unambiguously demonstrate that the long-range tail of the Coulomb potential (i.e., up to several hundreds atomic units) plays an important role in determining the photoelectron dynamics after the pulse ends.

Carrier-envelope phase dependence of molecular harmonic spectral minima induced by mid-infrared laser pulses

The spectral minima in harmonic spectra of H2+ induced by mid-infrared laser pulses are numerically investigated based on two models of Born-Oppenheimer (BO) and non-Born-Oppenheimer (NBO) approximations. The simulation results show that, with the variation of the mid-infrared laser's carrier-envelope phase (CEP), the spectral minima positions (SMPs) are fixed for the BO model, while oscillate periodically for the NBO model. This can be understood by the two-center-destructive-interference theory via the detailed investigation to several physical quantities for each CEP case, such as SMPs, effective potential, internuclear separation and the electron's de Broglie wavelength at the time for interference occurring. The fittings to these quantities' CEP-dependent curves demonstrate that they follow a variation law in the form of a sine function.

Two-Color Strong-Field Photoelectron Spectroscopy and the Phase of the Phase

The presence of a weak second-harmonic field in an intense-laser ionization experiment affects the momentum-resolved electron yield, depending on the relative phase between the ω and the 2ω component. The proposed two-color "phase-of-the-phase spectroscopy" quantifies for each final electron momentum a relative-phase contrast (RPC) and a phase of the phase (PP) describing how much and with which phase lag, respectively, the yield changes as a function of the relative phase. Experimental results for RPC and PP spectra for rare gas atoms and CO_{2} are presented. The spectra demonstrate a rather universal structure that is analyzed with the help of a simple model based on electron trajectories, wave-packet spreading, and (multiple) rescattering. Details in the PP and RPC spectra are target sensitive and, thus, may be used to extract structural (or even dynamical) information with high accuracy.

Momentum Distribution of Near-Zero-Energy Photoelectrons in the Strong-Field Tunneling Ionization in the Long Wavelength Limit

We investigate the ionization dynamics of Argon atoms irradiated by an ultrashort intense laser of a wavelength up to 3100 nm, addressing the momentum distribution of the photoelectrons with near-zero-energy. We find a surprising accumulation in the momentum distribution corresponding to meV energy and a “V”-like structure at the slightly larger transverse momenta. Semiclassical simulations indicate the crucial role of the Coulomb attraction between the escaping electron and the remaining ion at an extremely large distance. Tracing back classical trajectories, we find the tunneling electrons born in a certain window of the field phase and transverse velocity are responsible for the striking accumulation. Our theoretical results are consistent with recent meV-resolved high-precision measurements.

Two-dimensional attosecond electron wave-packet interferometry

We propose a two-dimensional interferometry based on the electron wave-packet interference by using a cycle-shaped orthogonally polarized two-color laser field. With such a method, the subcycle and intercycle interferences can be disentangled into different directions in the measured photoelectron momentum spectra. The Coulomb influence can be minimized and the overlapping of interference fringes with the complicated low-energy structures can be avoided as well. The contributions of the excitation effect and the long-range Coulomb potential can be traced in the Fourier domain of the photoelectron distribution. Because of these advantages, precise information on valence electron dynamics of atoms or molecules with attosecond temporal resolution and additional spatial information with angstrom resolution can be obtained with the two-dimensional electron wave-packet interferometry.

Low-energy photoelectrons in strong-field ionization by laser pulses with large ellipticity

The 3D photoelectron momentum distributions created by the strong-field ionization of argon atoms and naphthalene molecules with intense, large ellipticity (∼0.7) femtosecond laser pulses are studied. The experiment reveals the presence of low-energy electrons for randomly oriented naphthalene, but not for argon. Our theory shows that the induced dipole part of the cationic potential facilitates the creation of the low-energy electrons. We establish the conditions in terms of laser pulse parameters and molecular properties for which this type of low-energy electrons can be observed and point to applications thereof.

Above-threshold ionization by few-cycle phase jump pulses

We theoretically investigate the above-threshold ionization of hydrogen atoms driven by few-cycle phase jump laser pulses. By numerically solving the three-dimensional time-dependent Schrödinger equation, we demonstrate that the phase jump plays an important role in the ionization process. The cutoff of the photoelectron energy spectrum can extend to a range of very high energy, and the yield of the photoelectrons can be dramatically enhanced by choosing proper phase jump times. Both the classical simulations and Fourier transform method are used to understand the spectra features found in our investigation.

Ionization with low-frequency fields in the tunneling regime

Strong-field ionisation surprises with richness beyond current understanding despite decade long investigations. Ionisation with mid-IR light has promptly revealed unexpected kinetic energy structures that seem related to unanticipated quantum trajectories of the electrons. We measure first 3D momentum distributions in the deep tunneling regime (γ = 0.3) and observe surprising new electron dynamics of near-zero momentum electrons and extremely low momentum structures, below the eV, despite very high quiver energies of 95 eV. Such level of high-precision measurements at only 1 meV above the threshold, despite 5 orders higher ponderomotive energies, has now become possible with a specifically developed ultrafast mid-IR light source in combination with a reaction microscope, thereby permitting a new level of investigations into mid-IR recollision physics.

Trajectory-resolved Coulomb focusing in tunnel ionization of atoms with intense, elliptically polarized laser pulses

In strong-field light-matter interactions, the strong laser field dominates the dynamics. However, recent experiments indicate that the Coulomb force can play an important role as well. In this Letter, we have studied the photoelectron momentum distributions produced from noble gases in elliptically polarized, 800 nm laser light. By performing a complete mapping of the three-dimensional electron momentum, we find that Coulomb focusing significantly narrows the lateral momentum spread. We find a surprisingly sensitive dependence of Coulomb focusing on the initial transverse momentum distribution, i.e., the momentum at the moment of birth of the photoelectron. We also observe a strong signature of the low-energy structure in the above threshold ionization spectrum.

Scaling of the low-energy structure in above-threshold ionization in the tunneling regime: theory and experiment

A calculation of the second-order (rescattering) term in the S-matrix expansion of above-threshold ionization is presented for the case when the binding potential is the unscreened Coulomb potential. Technical problems related to the divergence of the Coulomb scattering amplitude are avoided in the theory by considering the depletion of the atomic ground state due to the applied laser field, which is well defined and does not require the introduction of a screening constant. We focus on the low-energy structure, which was observed in recent experiments with a midinfrared wavelength laser field. Both the spectra and, in particular, the observed scaling versus the Keldysh parameter and the ponderomotive energy are reproduced. The theory provides evidence that the origin of the structure lies in the long-range Coulomb interaction.

Enhancement of vibrational excitation and dissociation of H2(+) in infrared laser pulses

We study vibrational excitations, dissociation, and ionization of H(2)(+) in few-cycle laser pulses over a broad wavelength regime. Our results of numerical simulations supported by model calculations show a many orders-of-magnitude enhancement of vibrational excitation and dissociation (over ionization) of the molecular ion at infrared wavelengths. The enhancement occurs without any chirping of the pulse, which was previously applied to take account of the anharmonicity of the molecular vibrations. The effect is related to strong-field two- and higher-order photon transitions between different vibrational states.

Application of discrete variable representation to planar H2+ in strong xuv laser fields

We present an efficient and accurate grid method to study the strong field dynamics of planar H(2)(+) under Born-Oppenheimer approximation. After introducing the elliptical coordinates to the planar H(2)(+), we show that the Coulomb singularities at the nuclei can be successfully overcome so that both bound and continuum states can be accurately calculated by the method of separation of variables. The time-dependent Schrödinger equation (TDSE) can be accurately solved by a two-dimensional discrete variable representation (DVR) method, where the radial coordinate is discretized with the finite-element discrete variable representation for easy parallel computation and the angular coordinate with the trigonometric DVR which can describe the periodicity in this direction. The bound states energies can be accurately calculated by the imaginary time propagation of TDSE, which agree very well with those computed by the separation of variables. We apply the TDSE to study the ionization dynamics of the planar H(2)(+) by short extreme ultra-violet (xuv) pulses, in which case the differential momentum distributions from both the length and the velocity gauge agree very well with those calculated by the lowest order perturbation theory.

Low yield of near-zero-momentum electrons and partial atomic stabilization in strong-field tunneling ionization

We measure photoelectron angular distributions of single ionization of krypton and xenon atoms by laser pulses at 1320 nm, 0.2-1.0×10(14) W/cm(2), and observe that the yield of near-zero-momentum electrons in the strong-field tunneling ionization regime is significantly suppressed. Semiclassical simulations indicate that this local ionization suppression effect can be attributed to a fraction of the tunneled electrons that are released in a certain window of the initial field phase and transverse velocity are ejected into Rydberg elliptical orbits with a frequency much smaller than that of the laser; i.e., the corresponding atoms are stabilized. These electrons with high-lying atomic orbits are thus prevented from ionization, resulting in the substantially reduced near-zero-momentum electron yield. The refined transition between the Rydberg states of the stabilized atoms has implication on the THz radiation from gas targets in strong laser fields.

Probing the longitudinal momentum spread of the electron wave packet at the tunnel exit

We present an ellipticity-resolved study of momentum distributions arising from strong-field ionization of helium. The influence of the ion potential on the departing electron is considered within a semiclassical model consisting of an initial tunneling step and subsequent classical propagation. We find that the momentum distribution can be explained by including the longitudinal momentum spread of the electron at the exit from the tunnel. Our combined experimental and theoretical study provides an estimate of this momentum spread.

Direct visualization of laser-driven electron multiple scattering and tunneling distance in strong-field ionization

Using a simple model of strong-field ionization of atoms that generalizes the well-known 3-step model from 1D to 3D, we show that the experimental photoelectron angular distributions resulting from laser ionization of xenon and argon display prominent structures that correspond to electrons that pass by their parent ion more than once before strongly scattering. The shape of these structures can be associated with the specific number of times the electron is driven past its parent ion in the laser field before scattering. Furthermore, a careful analysis of the cutoff energy of the structures allows us to experimentally measure the distance between the electron and ion at the moment of tunnel ionization. This work provides new physical insight into how atoms ionize in strong laser fields and has implications for further efforts to extract atomic and molecular dynamics from strong-field physics.

Characteristic spectrum of very low-energy photoelectron from above-threshold ionization in the tunneling regime

We report an experimental and theoretical study of very low-energy photoelectrons in tunneling ionization process from noble gas atoms interacting with ultrashort intense infrared laser pulses. A universal peak structure with electron energy well below 1 eV in the photoelectron spectrum, corresponding to the double-hump structure in the longitudinal momentum distribution, is identified experimentally for all atomic species. Our quantum and semiclassical analysis reveal the role of long-range Coulomb potential in the production of this very low-energy peak structure.

Classical simulations of electron emissions from H2+ by circularly polarized laser pulses

With the classical fermion molecular dynamics model (FMD), we investigated electron emissions from H(2)(+) by circularly polarized laser pulses. The obtained electron momentum distribution clearly shows an angular shift relative to the expected direction for H(2)(+) aligned parallel to the polarization plane, which is in good agreement with the recent experimental result. By tracing the classical trajectory, we provide direct evidence for the electron delayed emission with respect to the instant when the electric field is parallel to the molecular axis, which was regarded as the origin of the angular shift in the electron momentum distribution. Furthermore, we find that the angular shift decreases with increasing the laser wavelength.

Electron-energy bunching in laser-driven soft recollisions

We introduce soft recollisions in laser-matter interaction. They are characterized by the electron missing the ion upon recollision in contrast with the well-known head-on collisions responsible for high-harmonic generation or above-threshold ionization. We demonstrate analytically that soft recollisions can cause a bunching of photoelectron energies through which a series of low-energy peaks emerges in the electron yield along the laser polarization axis. This peak sequence is universal, it does not depend on the binding potential, and is found below an excess energy of one tenth of the ponderomotive energy.

Low-energy structures in strong field ionization revealed by quantum orbits

Experiments on atoms in intense laser pulses and the corresponding exact ab initio solutions of the time-dependent Schrödinger equation (TDSE) yield photoelectron spectra with low-energy features that are not reproduced by the otherwise successful work horse of strong field laser physics: the "strong field approximation" (SFA). In the semiclassical limit, the SFA possesses an appealing interpretation in terms of interfering quantum trajectories. It is shown that a conceptually simple extension towards the inclusion of Coulomb effects yields very good agreement with exact TDSE results. Moreover, the Coulomb quantum orbits allow for a physically intuitive interpretation and detailed analysis of all low-energy features in the semiclassical regime, in particular, the recently discovered "low-energy structure" [C. I. Blaga, Nature Phys. 5, 335 (2009) and W. Quan, Phys. Rev. Lett. 103, 093001 (2009).