Search

We present measurements of above-threshold ionization (ATI) of CO2 in a pump-probe experiment where the pump pulse creates a rotational wave packet and the linearly polarized probe pulse generates the ATI spectrum as a function of the pump-probe delay time, which sweeps over the revival time of field-free alignment and its quarter fractions. The observed alignment signals, which are the electron yields of ATI by a probe pulse as a function of a delay between the pump and probe pulse, are compared with the calculated alignment parameter [Formula: see text]. The results are explained by nuclear spin statistics and the wave packet evolution of the CO2 molecule in terms of the highest occupied molecular orbital.

Relative phases of atomic above-threshold ionization wave packets were investigated in an experiment [L. J. Zipp, A. Natan, and P. H. Bucksbaum, Optica 1, 361 (2014)] exploiting interferences between different pathways in a weak probe field at half the frequency of the strong ionization pulse. In this work we explore theoretically the extraction of phase delays and time delays of attosecond wave packets formed in strong-field ionization. We perform simulations solving the time-dependent Schr\"odinger equation and compare these results with the strong-field and Coulomb-Volkov approximations. In order to disentangle short- from long-range effects of the atomic potential, we also perform simulations for atomic model potentials featuring a Yukawa-type short-range potential. We find significant deviations of the ab initio phase delays between different photoelectron pathways from the predictions by the strong-field approximation even at energies well above the ionization threshold. We identify similarities to but also profound differences from the well-known interferometric extraction of phase and time delays in one-photon ionization.

Above-threshold ionization (ATI) is one of the most fundamental processess when atoms or molecules are subjected to intense laser fields. Analysis of ATI process in intense laser fields by a Wigner-distribution-like (WDL) function is reviewed in this paper. The WDL function is used to obtain various time-related distributions, such as time-energy distribution, ionization time distribution, and time-emission angle distribution and so on, of atoms in laser field pulses with different laser parameters. For the linearly polarized laser pulses, the time-energy distribution intuitively shows from a quantum point of view the relationship between the ionization moment and the final energy and clearly reveals the origin of interference structures in the photoelectron spectrum. In particular, for linearly polarized few-cycle laser pulses, all calculated distributions show the dependence of electron behavior on the ionization time, emission direction, and carrier-envelope phase (CEP). For elliptically polarized few-cycle pulses, we calculate the angular distribution, ionization time distribution, and time-emission distribution, which are compared with the semiclassical calculations. Analysis shows that the offset angle (difference between positions of the peaks in the angular distributions obtained by two methods) in the angular distributions does not correspond to the offset time (difference between positions of the peaks in the ionization time distributions obtained by two methods) in the ionization time distributions, which implies that the attosecond angular streaking technique based on this correspondence between the offset angle and time is in principle inaccurate. Furthermore, the offset time cannot be interpreted as tunneling time.

In the regime of above-threshold ionization of gas atom in the field of laser radiation, plasma with photoelectron distribution consisting of peaks at discrete energy values is formed. It is shown that the number of longitudinal waves in such plasma coincides with the number of peaks in the distribution function. When peaks practically don't overlap, the dispersion law of each wave in the region of short waves is determined by electrons from the corresponding peak. In this case the phase and group velocities of the waves are close to the electron velocity, which corresponds to the peak maximum. It is possible to talk about such waves as an electronic sound, since the perturbations of the electron density mainly arise due to pressure perturbations. When the peaks are narrow, but having a finite width, the Cherenkov damping of waves is exponentially small. Numerical calculations the dispersion laws for of the two and four waves in photoionized xenon plasma, in which the electron distribution function consists of two or four narrow peaks are given.

Using the quantum mechanical analysis of stimulated bremsstrahlung of an electron on an electrostatic potential in the field of a strong electromagnetic wave in the generalized eikonal approximation, the simplified formula for the probability of multiphoton above-threshold ionization of a hydrogen-like atom in the laser field is obtained, taking into account the interaction of the photoelectron with the long-range Coulomb field of the residual ion.

Above-threshold ionization (ATI) of atoms in intense laser pulses produces photoelectron momentum distributions that exhibit discrete concentric circles related to the absorption of more photons than required to exceed the ionization threshold. Using numerical solutions of the one-electron time-dependent Schr\"odinger equation beyond the electric dipole approximation for circularly polarized laser pulses, we show that the geometric centers of ATI rings are counterintuitively shifted against the propagation direction of the light, i.e., in the direction opposite to the on-average transferred photon momentum. From a comparison with a simple model based on energy considerations, we identify a nondipole modification of the ac Stark effect for electron continuum states as the origin of our observation.

Clusters and nanodroplets hold the promise of enhancing high-order nonlinear optical effects due to their high local density. However, only moderate enhancement has been demonstrated to date. Here, we report the observation of energetic electrons generated by above-threshold ionization (ATI) of helium (He) nanodroplets which are resonantly excited by ultrashort extreme ultraviolet (XUV) free-electron laser pulses and subsequently ionized by near-infrared (NIR) or near-ultraviolet (UV) pulses. The electron emission due to high-order ATI is enhanced by several orders of magnitude compared with He atoms. The crucial dependence of the ATI intensities with the number of excitations in the droplets suggests a local collective enhancement effect.

Above-threshold ionization spectra from cesium are measured as a function of the carrier-envelope phase (CEP) using laser pulses centered at $3.1\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ wavelength. The directional asymmetry in the energy spectra of backscattered electrons oscillates three times, rather than once, as the CEP is changed from 0 to $2\ensuremath{\pi}$. Using the improved strong-field approximation, we show that the unusual behavior arises from the interference of few quantum orbits. We discuss the conditions for observing the high-order CEP dependence, and draw an analogy with time-domain holography with electron wave packets.

We present synthetic dynamical symmetries (SDS), composed of spatial, temporal, and parameter space operations. SDSs result in a new type of selection rules in high harmonic generation (HHG) and above threshold ionization (ATI).

Journal of physics / B 54(14), 144002 (2021). doi:10.1088/1361-6455/ac0fef, Published by IOP Publ., Bristol

We use an interferometic scheme to extract the phase distribution of the electron wave packet from above-threshold ionization in elliptically polarized laser fields. In this scheme, an electron wave packet released from a circularly polarized laser pulse acts as a reference wave and interferes with the electron wave packet ionized by a time-delayed counter-rotating elliptically polarized laser field. The generated vortex-shaped interference pattern in the photoelectron momentum distribution enables us to extract the phase distribution of the electron wave packet in the elliptically polarized laser pulse with high precision. By artificially screening the ionic potential at different ranges when solving the time-dependent Schordinger equation, we find that the angle-dependent phase distribution of the electron wave packet in the elliptically polarized laser field shows an obvious angular shift as compared to the strong-field approximation, whose value is the same as the attoclock shift. We also show that the amplitude of the angle-dependent phase distribution is sensitive to the ellipticity of the laser pulse, providing an alternative way to precisely calibrate the laser ellipticity in the attoclock measurement.

Rescattering induced by intense laser fields is one of the dominant processes in strong-field physics. It was usually described classically or semiclassically. In the present work, we show that the fanlike structures exist in photoelectron momentum distributions even though using the strong field approximation model with the short-range potential. We also investigate the dependence of fanlike structures on the laser intensity. We have performed the detailed analysis on the photoelectron momentum distributions through the quantum-orbit theory.

Femtosecond pump-probe electron and ion spectroscopy is applied to study the development of a helium nanoplasma up to the nanosecond timescale. Electrons, bound by the deep confining mean-field potential, are elevated toward the vacuum level in the nanoplasma expansion. Subsequent electron recombination gives rise to transitions between ${\mathrm{He}}^{+}$ states, resulting in autoionization. The time-resolved analysis of the energy transfer to quasifree electrons reveals a transient depletion of the Auger emission, which allows for a temporal gate to map the distribution of delocalized electrons in the developing mean field. Furthermore, we trace the recombination of delocalized electrons near the vacuum level into highly excited Rydberg states. Transient above-threshold ionization is introduced as a diagnostic tool to resolve the dynamics. Thus, the development of the electron distribution in the nanoplasma mean-field potential can be monitored via the features observed in the emission spectra.

Based on the numerical solution of the full-dimensional time-dependent Schr\"odinger equation, we study the above-threshold ionization of atomic hydrogen by subpicosecond laser pulses at wavelengths ranging from 300 nm to 800 nm, in which regime substructures of the photoelectron energy spectra resulting from quantum interference are entangled with those induced by the Freeman resonance. By analyzing the time-dependent populations and ac Stark energy shifts of the relevant atomic levels, and observing the pulse intensity dependence and the pulse length evolution of the spectra, we clearly identify the origins of the substructures exhibited in the spectra.

Photoionization of the hydrogen atom by intense and short coherent laser pulses is investigated from first principles by a numerical solution of the time-dependent Schrodinger equation in the dipole-velocity gauge. The considered photon energies are resonant to the 1 s → 2 p excitation, and the pulse intensities are high enough to induce Rabi floppings. The computed resonance-enhanced two-photon ionization spectra as well as the three-photon above threshold ionization spectra exhibit pronounced multiple-peak patterns due to dynamic interference. Fingerprints of dynamic interference can also be seen directly in the radial density of the photoelectron. The impact of the variation of the pulse intensity and photon energy on the dynamic interference is investigated, and the angular distribution of the emitted electrons is analyzed in some details. The present precise numerical results confirm our previous theoretical predictions on the two-photon ionization spectra (Demekhin and Cederbaum, 2012) made within a minimal few-level model.

We present a theoretical scenario for the atomic above-threshold ionization (ATI) in an intense laser field by investigating the Rydberg state population in real time. Rather than merely viewing the final distribution of photoelectron yield directly, we monitor the Rydberg state population by projecting the time-dependent wave function onto the bound eigen-states. The calculation shows that the population of resonant Rydberg states is closely related to the peaks in photoelectron kinetic energy spectrum (PKES). For a hydrogen atom, the highest populated Rydberg states are degenerated, exactly corresponding to the first ATI peak if one additional photon is absorbed. While for non-hydrogen atoms, e.g., Ar, the highest Rydberg states are mainly populated on specific states, e.g., 3 d ( 5 s ) and 4f in our case, also giving exact peak positions in PKES, where the state identification is obtained by the angular momentum resolved distribution of excited Rydberg states. This method provides an easy to understand picture for the resonance-enhanced effects in ATI as well as the role of atomic core potential in strong-field ionization.

A Wigner distributionlike function based on the improved strong-field approximation theory is proposed to calculate the rescattering time-energy distribution (RTED) of high-energy photoelectrons of atomic above-threshold ionization process in few-cycle laser fields with different frequencies. The RTED shows bell-like stripes and the outermost stripe is compared with semiclassical results given by the simple-man model with consideration of different positions of tunnel exit and different initial longitudinal momenta. Analysis indicates the existence of the tunnel exit. However, though it shifts farther away from the core with decreasing frequency, the position of the tunnel exit is significantly less than the prediction by adiabatic theory even for the low-frequency case which is well in the tunneling regime. Our results also imply that the effect of the tunnel exit is more important than that of the initial longitudinal momentum at the tunnel exit for the backward-scattering electrons. Moreover, the inner stripe structures in the RTED are attributed to interference between electrons with the same final energy emitted at different ionization times.

We investigate a polarization control scheme in two-color two-photon atomic ionization. Experiments employing free-electron laser or high-order harmonic generation sources have shown light polarization dependencies in above threshold ionization of atoms assisted with an IR field. These were mostly performed at relatively high photoelectron energies where the soft-photon approximation remains valid. We use here a perturbation theory approach to show the larger degree of control that can be attained when the short wavelength photon energy is below the ionization threshold. This largely unexplored energy region offers unique possibilities to achieve polarization control of the reaction.

The above-threshold ionization (ATI) of argon atoms in intense circularly polarized laser fields has been investigated by numerically solving the three-dimensional time-dependent Schr\"odinger equation and the classical Newton-Lorentz equation. There are obvious angular shifts of the photoelectron momentum distribution corresponding to the peaks in the ATI spectrum. They have little dependence on the external field and are mainly determined by the final kinetic energy. It is found that the angular shift is monotonically increasing as a function of the photoelectron final kinetic energy. Based on the classical simulations, the initial kinetic energy of electrons can be obtained by separating the contribution of the external field and Coulomb potential from the final kinetic energy. We conclude that the stronger the laser field, the bigger the initial velocity of electrons for a definite final kinetic energy. Then, based on the final kinetic energy and the laser parameters, the initial velocity can be evaluated.

Multiphoton ionization provides a clear window into the nature of electron correlations in the helium atom. In the present study, the final state energy range extends up to the region near the $N=2$ and $N=3$ ionization thresholds, where two-photon ionization proceeds via continuum intermediate states above the lowest threshold. Our calculations are performed using multichannel quantum defect theory (MQDT) and the streamlined R-matrix method. The sum and integration over all intermediate states in the two-photon ionization amplitude is evaluated using the inhomogeneous R-matrix method developed by Robicheaux and Gao. The seamless connection of that method with MQDT allows us to present high resolution spectra of the final state Rydberg resonances. Our analysis classifies the resonances above the $N=2$ threshold in terms of their group theory quantum numbers. Their dominant decay channels are found to obey the previously conjectured propensity rule far more weakly for these even parity states than was observed for the odd-parity states relevant to single photon ionization., Comment: 10 pages, 9 figures

Journal of physics / B 54(2), 025602 (2020). doi:10.1088/1361-6455/abce97, Published by IOP Publ., Bristol

Intense laser pulses lasting a few optical cycles, are able to ionize molecules via different mechanisms. One such mechanism involves a process whereby within one optical period an electron tunnels away from the molecule, and is then accelerated and driven back as the laser field reverses its direction, colliding with the parent molecule and causing correlated non-sequential double ionization (NSDI). Here we report control over NSDI via spectral-phase pulse shaping of femtosecond laser pulses. The measurements are carried out on ethane molecules using shaped pulses. We find that the shaped pulses can enhance or suppress the yield of dications resulting from electron recollision by factors of 3 to 6. This type of shaped pulses is likely to impact all phenomena stemming from electron recollision processes induced by strong laser fields such as above threshold ionization, high harmonic generation, attosecond pulse generation, and laser-induced electron diffraction. Controlling the ionisation process of molecules via ultrafast photoexcitation can provide insight into a number of nonlinear processes. This study employs shaped laser pulses to enhance or suppress dication yields resulting from electron recollision by up to a factor of six.

We report on the experimental observation of a decreased self-injection threshold by using laser pulses with circular polarization in laser wakefield acceleration experiments in a nonpreformed plasma, compared to the usually employed linear polarization. A significantly higher electron beam charge was also observed for circular polarization compared to linear polarization over a wide range of parameters. Theoretical analysis and quasi-3D particle-in-cell simulations reveal that the self-injection and hence the laser wakefield acceleration is polarization dependent and indicate a different injection mechanism for circularly polarized laser pulses, originating from larger momentum gain by electrons during above threshold ionization. This enables electrons to meet the trapping condition more easily, and the resulting higher plasma temperature was confirmed via spectroscopy of the XUV plasma emission.

Calculations on the dynamics of ions and electrons in near-infrared laser fields at intensities up to $3\ifmmode\times\else\texttimes\fi{}{10}^{23}\phantom{\rule{4pt}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$ are presented. We explore the acceleration of ions in a laser focus by conservation of canonical momentum during ionization events and by the ponderomotive force in the $f$/1 focal geometry required to reach such intensity. At intensities exceeding ${10}^{23}\phantom{\rule{4pt}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$, highly charged ions are expelled from the laser focus before they can interact with the laser pulse at peak intensity, decreasing the predicted ionization yields of deeply bound states. We consider the interaction of a tightly focused, $f$/1 laser pulse with krypton at an intensity of $3\ifmmode\times\else\texttimes\fi{}{10}^{23}\phantom{\rule{4pt}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$ and a pulse duration of 140 fs. We find that the ions and electrons are accelerated to energies in excess of 2 MeV/nucleon and 1.4 GeV, respectively. Ponderomotive expulsion of the parent ions decreases the total number of ultrarelativistic above-threshold ionization electrons produced by tunneling ionization from the $K$-shell states of krypton but does not change their energy spectrum.

We present above-threshold ionization (ATI) in the molecule nitric oxide, NO, by intense (${10}^{12}--{10}^{13}\phantom{\rule{0.28em}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$) two-color femtosecond laser pulses. Vibrationally resolved coherent interference phenomena of multiphoton and multipathway ionization processes have been investigated by measuring experimentally and modeling theoretically the molecular photoionization yields. We find that the ATI photoelectron distribution of NO, initiated from degenerate electronic and vibrational states, depends on the relative phase of the two pulses, the kinetic energy, and emission angle of the photoelectrons. This dependence codes the information of interference effects of molecular electronic states. AC Stark effects modulate interference patterns in ATI spectra. Moreover, the ATI spectra exhibit peak energy dependencies that illustrate interference effects from coherent couplings between various ionization channels. These features reflect the molecular property and the multiple channel ionization interference effects in ATI processes. The results allow to explore coherent electron-vibration processes in ultrafast molecular photoionization.

We present comparisons between quantum and classical calculations for above threshold ionization of an Argon atom interacting with a strong laser pulse. The time-dependent complete-active-space self-consistent field (TD-CASSCF) method and the classical trajectory Monte Carlo (CTMC) method are used for the cases of quantum and classical calculations, respectively. We find and discuss differences between the results obtained by the TD-CASSCF and the CTMC simulations.

Interaction of a moving electron wave packet in free space with light and nanostructures have taken much attention recently, by the emergence of photon-induced near-field electron microscopy [1–2]. Electron beams can inelastically interact by laser-induced nearfield distribution of the nanostructures, opening ways in shaping the electron wave packets and characterization of plasmon polaritons. The interaction of high energy electron waves with laser-induced excitations in the sample is generally discussed within eikonal approximations, neglecting other effects which arise from the pondermotive forces and experienced elastic recoils [3–4]. In contrast, coherent control of above threshold ionization and high harmonic generation can be understood by interferences between different quantum paths, related to ponderomotive and absorptive parts in the minimal coupling Hamiltonian [5].

The two-photon above-threshold 3p-ionization of argon for the photon energies exceeding the 3p-ionization threshold is studied using the correlation function technique. Generalized two-photon ionization cross sections were calculated taking into account many-electron correlations. The calculations demonstrate that the two-photon ionization of Ar at the 3p4 threshold is almost entirely a collective process. The decisive contribution in the above-threshold two-photon 3p-ionization at the exciting-photon energies corresponding to the 3p→ed giant resonance comes from the Auger-like many-electron correlations of e′ de″d − 3pef type.

High-order above-threshold ionization (HATI) of the ${{\mathrm{H}}_{2}}^{+}$ ion by a linearly polarized laser field is investigated theoretically using numerical solutions of the full three-dimensional time-dependent Schr\"odinger equation (TDSE). The photoelectron spectrum is calculated in two ways: by projecting the final wave function onto continuum states satisfying the incoming boundary condition and by using the window-operator method. Our numerical simulations show that the shape of the corresponding photoelectron energy spectra depends on the internuclear distance. For small internuclear distances the obtained HATI spectrum is very similar to the HATI spectrum for atomic targets. By increasing the internuclear distance, i.e., by stretching the molecule, some features emerge in the spectra. The cutoff position of the high-energy photoelectron spectrum extends from $10{U}_{p}$ (${U}_{p}$ is the electron ponderomotive energy) for small internuclear distances up to above $18{U}_{p}$ for very large internuclear distances. The second observed feature is that, with the increase of the internuclear distance, a low-energy plateau appears which is much higher than the above-mentioned extended plateau. For the internuclear distance of 18 a.u. this plateau extends up to $6{U}_{p}$. These features are further investigated by using solutions of the classical equation of motion for the electron in the strong laser field and by using quantum-orbit theory. The results of classical analysis are confirmed using two quantum-mechanical methods based on the analysis of the electron time-dependent probability density and of the electron energy-resolved probability density, obtained from the TDSE solutions.

We introduce an approximation for strong-laser-field-induced ionization of atoms, which we call atom-Volkov strong-field approximation (aV SFA). For the final state, instead of a plane wave (used in the standard well-known SFA) or a Coulomb wave (used in the Coulomb-Volkov SFA), in the aV SFA we use the exact atom-specific scattering states. We first introduce the aV SFA as an ansatz and show that it yields results that are closer to the exact results than those obtained using the SFA. After that, we systematically derive an iterative theory whose zeroth order is equal to the introduced aV SFA. We analyze its first-order correction, which corresponds to electron rescattering off the parent ion and show that, in the case of forward scattering, this correction leads to the low-energy structures in the photoelectron spectra which are observed in the experiments.