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The European physical journal / D 75(7), 205 (2021). doi:10.1140/epjd/s10053-021-00218-0, Ionization of laser-dressed atomic helium is investigated with focus on photoelectron angular distributions stemming from two-color multi-photon excited states. The experiment combines extreme ultraviolet (XUV) with infrared (IR) radiation, while the relative polarization and the temporal delay between the pulses can be varied. By means of an XUV photon energy scan over several electronvolts, we get access to excited states in the dressed atom exhibiting various binding energies, angular momenta, and magnetic quantum numbers. Furthermore, varying the relative polarization is employed as a handle to switch on and off the population of certain states that are only accessible by two-photon excitation. In this way, photoemission can be suppressed for specific XUV photon energies. Additionally, we investigate the dependence of the photoelectron angular distributions on the IR laser intensity. At our higher IR intensities, we start leaving the simple multi-photon ionization regime. The interpretation of the experimental results is supported by numerically solving the time-dependent Schr��dinger equation in a single-active-electron approximation., Published by Springer, Heidelberg

We demonstrate a time-domain method to quantitatively retrieve the angular dependence of laser-induced electron rescattering from the measured rotational half-revival signals of molecules with a reaction microscope. The method is based on the measured channel-resolved ion yields of single ionization and electron-rescattering-induced double ionization and the in situ measured alignment distribution of molecules. From the measured data for ${\mathrm{CO}}_{2}$, we retrieve the angular dependence of electron rescattering in nonsequential double ionization for both nondissociative and dissociative cases. The results imply that the angular dependence of electron rescattering is affected by the state of the parent ion populated during the first ionization step. The method demonstrated here opens up the possibility to quantitatively retrieve from measured data which molecular orbitals are involved in the interaction of strong laser fields with molecules.

Laser induced x-ray fluorescence were observed against laser polarization ellipticity. While emission from krypton peaks at linear polarization, a signature of recollision, emission from neon shows opposite trend. We attribute it to two competing processes.

We describe the results of experiments and simulations performed with the aim of extending photoelectron spectroscopy with intense laser pulses to the case of molecular compounds. Dimer frame photoelectron angular distributions generated by double ionization of ${\mathrm{N}}_{2}\ensuremath{-}{\mathrm{N}}_{2}$ and ${\mathrm{N}}_{2}\ensuremath{-}{\mathrm{O}}_{2}$ van der Waals dimers with ultrashort, intense laser pulses are measured using four-body coincidence imaging with a reaction microscope. To study the influence of the first-generated molecular ion on the ionization behavior of the remaining neutral molecule we employ a two-pulse sequence comprising of a linearly polarized and a delayed elliptically polarized laser pulse that allows distinguishing the two ionization steps. By analysis of the obtained electron momentum distributions we show that scattering of the photoelectron on the neighboring molecular potential leads to a deformation and rotation of the photoelectron angular distribution as compared to that measured for an isolated molecule. Based on this result we demonstrate that the electron momentum space in the dimer case can be separated, allowing us to extract information about the ionization pathway from the photoelectron angular distributions. Our work, when implemented with a variable pulse delay, opens up the possibility of investigating light-induced electronic dynamics in molecular dimers using angularly resolved photoelectron spectroscopy with intense laser pulses.

We establish a generalized picture of the phase sensitivity of laser-induced directional bond breaking using the H_{2} molecule as the example. We show that the well-known proton ejection anisotropy measured with few-cycle pulses as a function of their carrier-envelope phases arises as an amplitude modulation of an intrinsic anisotropy that is sensitive to the laser phase at the ionization time and determined by the molecule's electronic structure. Our work furthermore reveals a strong electron-proton correlation that may open up a new approach to experimentally accessing the laser-sub-cycle intramolecular electron dynamics also in larger molecules.

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.

We report on the results of a joint experimental and numerical study on the sub-cycle laser field-driven electron dynamics that underlie the population of highly excited electronic states in multiply ionized argon dimers by electron recapture processes. Our experiments using few-cycle laser pulses with a known carrier-envelope phase (CEP) in combination with reaction microscopy reveal a distinct CEP-dependence of the electron emission and recapture process and, furthermore, a small but significant CEP-offset to the scenario in which no excited argon dimers are produced. With the help of classical ensemble trajectory simulations we trace down these different CEP-dependencies to subtle differences in the laser-driven sub-cycle electron trajectory dynamics that involve in both cases the transfer of an electron from one argon ion across the system boundary to the neighboring ion and its transient capture on this ion.

We experimentally investigate dissociation of ${\mathrm{H}}_{2}^{+}$ in the (near-)zero proton energy region. Based on our experimental results we conclude that the underlying mechanism for the production of protons with such low energy in strong two-color and broadband laser fields is a stimulated Raman-scattering-based zero-photon-dissociation process, ${\mathrm{ZPD}}_{\text{stR}}$, taking place on the electronic ground state. It is furthermore shown that in the (near-)zero energy region the asymmetry in proton ejection induced by asymmetric laser fields is due to the interplay of several processes, rather than only pathway interferences, with vibrational trapping (or bond hardening) taking a key role.

We demonstrate two-dimensional control over the chargelocalization in H2 dissociation using elliptically polarized laser pulses. The influences of the CEP and the laser phase at the instant of ionization are investigated.

We introduce and employ the molecular attoclock method. This allows us to simultaneously trace the nuclear and electron dynamics during H2 fragmentation, and to CEP-control the two-electron emission dynamics on sub-cycle time scales., High Intensity Lasers and High Field Phenomena, 26–28 March, 2018, Strasbourg, France

We experimentally achieve two-dimensional CEP-control of bond breaking in H2 dissociation with elliptically polarized pulses. The lab and molecular frame of reference proton ejection asymmetries are compared.

We demonstrate kinematically complete measurements on frustrated double ionization of argon atoms in strong laser fields with a reaction microscope. We found that the electron trapping probability after strong field double ionization is much higher than that after strong field single ionization, especially in case of high laser intensity. The retrieved electron momentum distributions of frustrated double ionization show a clear transition from the nonsequential to the sequential regime, similar to those of strong field double ionization. The dependence of electron momentum width on the laser intensity further indicates that the second released electron has a dominant contribution to frustrated double ionization in the sequential regime.

We use orthogonally polarized two-color (OTC) laser pulses to separate quantum paths in the multiphoton ionization of Ar atoms. Our OTC pulses consist of 400 and 800 nm light at a relative intensity ratio of 10∶1. We find a hitherto unobserved interference in the photoelectron momentum distribution, which exhibits a strong dependence on the relative phase of the OTC pulse. Analysis of model calculations reveals that the interference is caused by quantum pathways from nonadjacent quarter cycles.

We report experiments on the direct observation of molecular oxygen formation from ${\mathrm{CO}}_{2}$ in strong laser fields with a reaction microscope. Our accompanying simulations and pump-probe measurements suggest that ${\mathrm{CO}}_{2}$ molecules undergo bending motion during strong-field ionization which supports the molecular oxygen formation process. The observation of molecular oxygen formation from ${\mathrm{CO}}_{2}$ may trigger further experimental and theoretical studies on such processes with laser pulses, and provides hints in studies of the ${\mathrm{O}}_{2}$ and ${\mathrm{O}}_{2}{}^{+}$ abundance in ${\mathrm{CO}}_{2}$-dominated planetary atmospheres.

We investigate sequential double ionization of helium by intense near-circularly polarized few-cycle laser pulses using a semiclassical ionization model with two independent electrons. Simulated ${\mathrm{He}}^{2+}$ ion momentum distributions are compared to those obtained in recent benchmark experiments [M. S. Sch\"offler, X. Xie, P. Wustelt, M. M\"oller, S. Roither, D. Kartashov, A. M. Sayler, A. Baltuska, G. G. Paulus, and M. Kitzler, Phys. Rev. A 93, 063421 (2016)]. We study the influence of a number of pulse parameters such as peak intensity, carrier-envelope phase, pulse duration, and second- and third-order spectral phase on the shape of the ion momentum distributions. Good agreement is found in the main features of these distributions and of their dependence on the laser pulse duration, peak intensity, and carrier-envelope phase. Furthermore, we find that for explaining certain fine-scale features observed in the experiments, it becomes important to consider subtle timing variations in the two-electron emissions introduced by small values of chirp. This result highlights the possibility of measuring and controlling multielectron dynamics on the attosecond time scale by fine tuning the field evolution of intense close-to-single-cycle laser pulses.

Photoelectron circular dichroism (PECD) was investigated for methyloxirane with intense few-cycle pulses. The observed PECD shows a dependence on electron energy. This may signify an influence of the chiral potential on the outgoing electron’s trajectory.

Electron correlation in sequential double ionization of helium is studied experimentally with the ``attoclock'' technique. Control of the two-electron emission dynamics is demonstrated on the time scale of a half laser cycle.

We report on the observation of subcycle interferences of electron wave packets released during strong field ionization of $\text{H}_{2}$ with cycle-shaped two-color laser fields. With a reaction microscope we measure three-dimensional momentum distributions of photoelectrons correlated with either $\text{H}_{2}^{+}$ or protons within different energy ranges generated by dissociation of $\text{H}_{2}^{+}$ . We refer to these different types of photoelectrons as channels. Our results show that the subcycle interference structures of electron wave packets are very sensitive to the cycle shape of the two-color laser field. We explain this behavior by the dependence of the ionization time within an optical cycle on the shape of the laser field cycle. The subcycle interference structures can be further used to obtain insight into the subcycle dynamics of molecules during strong field interaction.

The dynamics of sequential two-electron emission from helium is extracted from measured momentum distributions. Emission in between the field-maxima and on sub-laser-cycle times is identified. The corresponding structures are roughly reproduced by a semi-classical model.

We demonstrate control over the localization of high-lying Rydberg wave packets in argon atoms with phase-locked orthogonally polarized two-color laser fields. With a reaction microscope, we measure ionization signals of high-lying Rydberg states induced by a weak dc field and blackbody radiation as a function of the relative phase between the two-color fields. We find that the dc-field-ionization yield of high-lying Rydberg argon atoms oscillates with the relative two-color phase with a period of $2\ensuremath{\pi}$ while the photoionization signal by blackbody radiation shows a period of $\ensuremath{\pi}$. Accompanying simulations show that these observations are a clear signature of the asymmetric localization of electrons recaptured into very elongated (low angular momentum) high-lying Rydberg states after conclusion of the laser pulse. Our findings thus open an effective pathway to control the localization of high-lying Rydberg wave packets.

We demonstrate the detection of high-lying Rydberg states produced in strong laser fields with coincidence spectroscopy. Electron emission after the interaction of strong laser pulses with atoms and molecules is measured together with the parent ions in coincidence measurements. These electrons originate from high-lying Rydberg states with quantum numbers from $n\ensuremath{\sim}20$ up to $n\ensuremath{\lesssim}120$ formed by frustrated field ionization. Ionization rates are retrieved from the measured ionization signal of these Rydberg states. Simulations show that both tunneling ionization by a weak dc field and photoionization by blackbody radiation contribute to delayed electron emission on the nano- to microsecond scale. Furthermore, the dependence of the Rydberg-state production on the ellipticity of the driving laser field indicates that such high-lying Rydberg states are populated through electron recapture. The present experiment provides detailed quantitative information on Rydberg production in strong-field interaction.

We experimentally and theoretically investigated the deprotonation process on nanosecond to microsecond timescale in ethylene and acetylene molecules, following their double ionization by a strong femtosecond laser field. In our experiments we utilized coincidence detection with the reaction microscope technique, and found that both the lifetime of the long-lived ethylene dication leading to the delayed deprotonation and the relative channel strength of the delayed deprotonation compared to the prompt one have no evident dependence on the laser pulse duration and the laser peak intensity. Quantum chemical simulations suggest that such delayed fragmentation originates from the tunneling of near-dissociation-threshold vibrational states through a dissociation barrier on a dication electronic state along C--H stretching. Such vibrational states can be populated through strong field double ionization induced vibrational excitation on an electronically excited state in the case of ethylene, and through intersystem crossing from electronically excited states to the electronic ground state in the case of acetylene.

By tracking the electron wave function of a molecule in a laser field at a distance from the bound system we define rescattering electron momentum distributions within the framework of the time-dependent Schrodinger equation without making the strong field approximation. It is shown that within certain limitations the classical picture of rescattering can be upheld. The dependence of momentum distributions on molecular orientation, orbital symmetry and correlation is investigated. For orbitals with a node, a strong dependence of time and momentum distributions on molecular orientation is found.

The alignment-dependent ionization of acetylene and ethylene in short laser pulses is investigated in the framework of the time-dependent density-functional theory coupled with Ehrenfest dynamics. The molecular alignment is found to have a substantial effect on the total ionization. Bond stretching is shown to cause an increase of the ionization efficiency, i.e., enhanced ionization, in qualitative agreement with previous theoretical investigations. It is also demonstrated that the enhanced ionization mechanism greatly enhances the ionization from the inner valence orbitals, and the ionization of the inner orbitals is primarily due to their extended weakly bound density tails.

Using orthogonally polarized two-color laser fields on neon and coincidence momentum imaging we gain access to the Coulomb influence in single ionization on sub-cycle times, and demonstrate a strong electron-electron anti-correlation in double ionization.

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.

We present theoretical studies on real-time probing of the electron density evolution of an atom in a strong infrared (IR) laser field with few-cycle near-infrared (NIR) and attosecond extreme-ultraviolet (XUV) pulses. Our results indicate that the electron density near the tunneling barrier is reflected in the additional tunneling ionization yield with a time delayed NIR pulse and the electron density near the nucleus can be probed by the single photoionization yield with a time delayed XUV pulse. It turns out the NIR-probing scheme can be used to study the polarization of the system in an external IR field and the XUV probing can be additionally applied to explore excitation dynamics during and after the IR field interaction.

The removal of electrons from polyatomic molecules by ionization with intense, ultrashort laser pulses may trigger complex restructuring and fragmentation dynamics. Depending on the valence-shell from which the electrons are removed the molecular ion might be put into a certain dissociative or binding state by the ionization process. With control over the ionization process it might thus be possible to gain control over the subsequent restructuring and fragmentation process on a purely electronic level. Here we introduce two conceptually similar schemes that allow controlling the outcome of molecular restructuring and fragmentation processes in polyatomic molecules on sub-femtosecond time-scales. The first one involves recollision double ionization in few-cycle laser fields with a known carrier-envelope phase (CEP). We demonstrate experimentally CEP-control over various fragmentation reactions of a series of polyatomic molecules (acetylene, ethylene, 1,3-butadiene). As the recollision energy for a given intensity sensitively depends on the CEP, tuning of the CEP allows controlling the removal of inner-valence electrons and the controlled population of dissociative excited states. The second control scheme uses the strong preponderance of ionization from specific molecular orbitals to the alignment of the molecular axis with respect to the laser polarization direction for determining which valence level the electrons are removed from. We demonstrate experimental control over different two-body fragmentation and dissociation pathways from the cation and the dication of the acetylene molecule using the field-free alignment angle as a control knob. Finally, we turn from the demonstration of control schemes working at sub-femtosecond time-scales, for which the nuclear dynamics following the ionization are not essential, to the investigation of the coupled nuclear-electronic dynamics that in general take place for longer pulse durations. We explore experimentally the mechanism behind the surprisingly high charge states recently observed in the ionization of hydrocarbon molecules that have been explained by a multi-bond version of the well known enhanced-ionization (EI) mechanism taking place in parallel at many C–H bonds. Our experimental results are in agreement with the proposed multi-bond version of EI.

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. © 2014 American Physical Society.

The two-dimensional sub-cycle-time to electron momentum mapping provided by orthogonal two-color laser fields is applied to photoelectron spectroscopy. Using neon as the example we gain experimental access to the dynamics of emitted electron wave packets in electron momenta spectra measured by coincidence momentum imaging. We demonstrate the opportunities provided by this time-to-momentum mapping by investigating the influence of the parent ion on the emitted electrons on laser-sub-cycle times. It is found that depending on their sub-cycle birth time the trajectories of photoelectrons are affected differently by the ion's Coulomb field.

We present a theoretical analysis of the atomic photoelectron emission spectra produced by a linearly polarized sculpted laser pulse of two colors with frequencies ω and 2ω. The spectrum of the "direct" electrons with intermediate energies prominently features both intracycle and intercycle interferences. We derive a simple analytic expression for this spectral range based on a semiclassical approximation of the time-dependent distorted wave strong-field approximation generalized to strong-field ionization by a two-color pulse. We verify its applicability to approximately represent the intricate interference patterns by comparison with the exact solutions of the time-dependent Schrödinger equation and with the strong-field approximation. We show that the interference patterns can be tuned and its contrast enhanced by the additional "knob" available, the relative phase between the two frequency components. The present results confirm that two-color ionization allows resolving interference structures originating from trajectories launched within a time interval of less than 100 as [X. Xie et al., Phys. Rev. Lett. 108, 193004 (2012)]. Fil: Arbo, Diego. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomia y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomia y Física del Espacio; Argentina Fil: Nagele, Stefan. Vienna University of Technology; Austria Fil: Tong, Xiao Min. Tsukuba University; Japón Fil: Xie, Xinhua. Vienna University of Technology; Austria Fil: Kitzler, Markus. Vienna University of Technology; Austria Fil: Burgdörfer, Joachim. Vienna University of Technology; Austria

We experimentally investigate the ionization mechanism behind the formation of remarkably high charge states observed in the laser-pulse-induced fragmentation of different hydrocarbon molecules by Roither et al. [Phys. Rev. Lett. 106, 163001 (2011)], who suggested enhanced ionization occurring at multiple C-H bonds as the underlying ionization mechanism. Using multiparticle coincidence momentum imaging we measure the yield of multiply charged fragmenting ethylene and acetylene molecules at several intensities and pulse durations ranging from the few-cycle regime to 25 fs. We observe, at constant intensity, a strong increase of the proton energy with increasing laser pulse duration. It is shown that this is caused by a strong increase in the yield of highly charged parent molecular ions with pulse duration. Based on experimental evidence we explain this increase by the necessary population of precursor states in the parent ion that feature fast C-H stretch dynamics to the critical internuclear distance, where efficient ionization via enhanced ionization takes place. For increasing pulse duration these precursor ionic states are more efficiently populated, which leads in turn to a higher enhanced-ionization probability for longer pulses. Our work provides experimental evidence for the existence of a multiple-bond version of enhanced ionization in polyatomic molecules.

We experimentally and theoretically investigated the applications of attosecond electron wave packets on probing ultrafast electron dynamics of atoms and controlling electron recollision induced double ionization of atoms and fragmentation of hydrocarbon molecules.

Angstrom and attosecond control of free electron wave packets is one of the pinnacles of attosecond science. Orthogonally polarized two-color (OTC) laser fields allow to control the motion of field-ionizing electronic wave packets both in time and space[1]. In OTC pulses time and space are connected and thus an attosecond time scale is established in the polarization plane for both the emitted and the re-colliding wave packets[2,3]. © 2013 IEEE., 2013 Conference on Lasers and Electro-Optics Europe and International Quantum Electronics Conference, CLEO/Europe-IQEC 2013, 12 May 2013 through 16 May 2013, Munich

We experimentally and theoretically demonstrate a self-referenced wave-function retrieval of a valence-electron wave packet during its creation by strong-field ionization with a sculpted laser field. Key is the control over interferences arising at different time scales. Our work shows that the measurement of subcycle electron wave-packet interference patterns can serve as a tool to retrieve the structure and dynamics of the valence-electron cloud in atoms on a sub-10-as time scale. Fil: Xie, Xinhua. Vienna University of Technology; Austria Fil: Roither, Stefan. Vienna University of Technology; Austria Fil: Kartashov, Daniil. Vienna University of Technology; Austria Fil: Persson, Emil. Vienna University of Technology; Austria Fil: Arbo, Diego. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina. Vienna University of Technology; Austria Fil: Zhang, Li. Vienna University of Technology; Austria Fil: Gräfe, Stefanie. Vienna University of Technology; Austria Fil: Schöffler, Markus S.. Vienna University of Technology; Austria Fil: Burgdörfer, Joachim. Vienna University of Technology; Austria Fil: Baltuska, Andrius. Vienna University of Technology; Austria Fil: Kitzler, Markus. Vienna University of Technology; Austria

Attosecond Physics builds on the properties of electron wave packets created by strong-field ionization close to the maxima of the field oscillations which are subsequently driven by the strong laser field on a sub-cycle time scale [1]. These wave packets can lead to interference structures in energy or momentum space. While interference of wave packets created during different field cycles (inter-cycle) give rise to the well-known above-threshold ionization (ATI) peaks, interference of wave packets that are created during the same laser cycle (intra-cycle) can lead to a hologram of the binding potential [2,3].

Atomic single- and double- ionization using orthogonally polarized two-color laser fields was investigated via COLTRILMS technique. We prove that the electronic wave packets can be spatially controlled with attosecond precision., 27th International Conference on Photonic, Electronic and Atomic Collisions, ICPEAC 2011, July 27 - August 2, 2011, Belfast, UK

We demonstrate self-referenced wavefunction retrieval of a valence electron wavepacket during its creation by strong-field ionization with a sub-10-attosecond precision, based on a distinct separation of interferences arising at different time-scales.

We use cycle-sculpted two-color waveforms to drive electronic wavepackets generated by strong-field ionization from helium, neon, and argon gas atoms and analyze their momentum spectra measured by electron-ion coincidence momentum spectroscopy. Varying the relative phase of the two colors allows to sculpt the ionizing field and hence to control the emission times and motion of the wavepackets on an attosecond timescale. Using semiclassical calculations, we investigate the influence of the ionic Coulomb field onto the motion of emitted electronic wavepackets. We further show that the measured electron momentum spectra contain interference patterns created by pairs of electron wavepackets that are released within a single laser-field cycle. We experimentally distinguish these subcycle interference structures from above-threshold ionization (ATI) peaks and argue that they can be used to extract the subcycle phase evolution of the laser-driven complex bound-state wavefunction.

By controlling the electric field of strong laser pulses wave packets created by field ionization can be steered in space and time and can be used to coherently probe the parent ion on the attosecond and angstrom scale. In addition to the actual photon or electron emission signal caused by the probe, two important processes influence the observed result: (i) ionization and (ii) wave packet propagation due to the combined force of the laser electric field and the ionic binding potential. The ionic Coulomb potential is often considered only a weak perturbation to the dominant driving force of the laser field. Quantitative statements, however, can only be made when the influence of the ionic long range Coulomb potential is properly accounted for [1–3].

We manipulate the trajectories of field-ionizing electron wave packets using two-color laser pulses for controlling the Coulomb potential's influence on their three-dimensional momentum distributions.