We implement a liquid metal ion source in a 3D coincidence momentum spectroscopy setup for studying the interaction of ionic targets with intense laser pulses. Laser intensities of up to 4 ⋅ 1016 W cm−2 allow for the observation of up to ten-fold ionization of Au+-ions and double ionization of Si2+-ions. Further, by utilizing two-color sculpted laser fields to control the ionization process on the attosecond time scale, we demonstrate the capability to resolve the recoil ion momenta of heavy metal atoms. Simulations based on a semiclassical model assuming purely sequential ionization reproduce the experimental data well. This work opens up the use of a range of metallic and metalloid ions, which have hardly been investigated in strong-field laser physics so far.

Above-threshold ionization spectra from cesium are measured as a function of the carrier-envelope phase (CEP) using laser pulses centered at 3.1  μm 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 2π. 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.

The vibrational motion of molecules represents a fundamental example of an anharmonic oscillator. Using a prototype molecular system, HeH^{+}, we demonstrate that appropriate laser pulses make it possible to drive the nuclear motion in the anharmonic potential of the electronic ground state, increasing its energy above the potential barrier and facilitating dissociation by purely vibrational excitation. We find excellent agreement between the frequency-dependent response of the helium hydride molecular cation to both classical and quantum mechanical simulations, thus removing any ambiguities through electronic excitation. Our results provide access to the rich dynamics of anharmonic quantum oscillator systems and pave the way to state-selective control schemes in ground-state chemistry by the adequate choice of the laser parameters.

The spatially dependent phase distribution of focused few-cycle pulses, i.e., the focal phase, is much more complex than the well-known Gouy phase of monochromatic beams. As the focal phase is imprinted on the carrier-envelope phase (CEP), for accurate modeling and interpretation of CEP-dependent few-cycle laser-matter interactions, both the coupled spatially dependent phase and intensity distributions must be taken into account. In this Letter, we demonstrate the significance of the focal phase effect via comparison of measurements and simulations of CEP-dependent photoelectron spectra. Moreover, we demonstrate the impact of this effect on few-cycle light-matter interactions as a function of their nonlinear intensity dependence to answer the general question: if, when, and how much should one be concerned about the focal phase?

The structure and dynamics of molecules are governed by the electric forces acting between electrons and nuclei. Intense, ultrashort laser pulses offer the possibility to manipulate these forces, on the time scales relevant for the motion of a molecule's constituents. Thus, laser fields can act, not only as a mechanism to trigger molecular dynamics, but also controlling them. The fragmentation patterns that result from the interaction testify to the laser-induced processes occurring in the molecule. In this review, we examine how a laser addresses the different degrees of freedom of a molecule, from electronic excitation to vibrations of nuclei, to rotations of the molecule. We will focus the discussion on the most fundamental systems, particularly H-2(+), H-2, and HeH+. These simple systems allow for accurate theoretical analysis of experimental results, and extrapolation of the conclusions to more complex systems. Since some of the most fundamental molecules, such as HeH+ and H-3(+) do not exist in the neutral form, we put an emphasis on experiments starting from molecular ions, but do not restrict the discussion to these. Strong-field interactions of small molecules are a test ground, not only for experimental but also for theoretical methods. The joint effort of the two scientific disciplines have delivered deep insights into fundamental concepts of molecular science. The recent developments of novel laser sources with longer wavelength, higher peak power, or repetition rates, as well as more complex targets and detection schemes, promise that the field will remain highly relevant in the decades to come.

High-harmonic generation (HHG) in crystals offers a simple, affordable and easily accessible route to carrier-envelope phase (CEP) measurements, which scales favorably towards longer wavelengths. We present measurements of HHG in ZnO using few-cycle pulses at 3.1µm. Thanks to the broad bandwidth of the driving laser pulses, spectral overlap between adjacent harmonic orders is achieved. The resulting spectral interference pattern provides access to the relative harmonic phase, and hence, the CEP.

By applying recently introduced, phase-of-the-phase spectroscopy [S. Skruszewicz et al., Phys. Rev. Lett. 115, 043001 (2015)], we analyze the phase-dependent photoelectron signal from Xe ionized in intense, parallel, twocolor (1800 nm and 900 nm) laser fields. With such a field configuration, tuning of the relative phase between the ionizing, CO, and the perturbative, 2 omega, field results in a modulation of the ionization rate, as well as modifications of the trajectories of electrons propagating in the laser-dressed continuum. Based on a semiclassical model, we confirm that phase dependencies, due to the perturbation of the ionization rate, encode the ionization times of the electrons. Here, using the fork structure, a well-known feature originating from well-defined dynamics allows us to distinguish between electrons ionized within distinct time windows. However, due to the simultaneous perturbation of the electron trajectories, the assignment of the ionization times can be distorted by up to 80 as, i.e., a 10 degrees phase shift, which is independent of the degree of the perturbation.

Using quantum-mechanical, one-dimensional, non-Born-Oppenheimer simulations we study the control over the strong-field dynamics of the helium hydride molecular ion HeH(+)due to interaction driven by short and strong two-color laser pulses. We calculate yields of two competing fragmentation channels: electron removal and dissociation. We find that by changing the relative phase of the two colors, we can select the dominating channel. Nuclear motion is decisive for explaining ionization in this target. Ionization yields are vastly underestimated when nuclear motion is excluded and they are substantially reduced in the heavier isotopologue HeD+. Coupling of the two lowest electronic states is crucial even for the ground-state dissociation process.

The helium hydride molecular ion, HeH + , is the simplest heteronuclear polar molecule and serves as a fundamental benchmark system for the investigation of multi-electron molecules and molecules with a permanent dipole [1,2]. The laser-induced fragmentation dynamics of HeH + are measured using an ion beam of helium hydride and an isotopologue at various wavelengths and intensities [3]. These results are interpreted using reduced dimensionality solutions to the time-dependent Schrödinger equation and with simulations based on Dressed surface hopping [4] (see Fig. 1 (a-b)).

The carrier-envelope phasemeter (CEPM), based on the stereographic detection of above-threshold ionization (ATI) spectra, can measure the carrier-envelope phase (CEP) for each and every single laser shot with high precision [1]. An additional and very valuable feature of the CEPM is that the duration of few-cycle pulses can be characterized simultaneously to the CEP measurements by utilizing the radius of the so-called parametric asymmetry plots (PAPs) that can be derived from the ATI spectra. Recently, this technique has further been developed and now can operate at 1800 nm [2] and at 800 nm with 100 kHz repetition rate [3].

In dieser Doktorarbeit wurde die Interaktion von drei fundamentalen Ein- und Zweielektronen-systemen in intensiven, ultrakurzen Laserfeldern experimentell untersucht und mit theoretischen Vorhersagen verglichen. Die Verwendung einer Ionenstrahlapparatur erlaubt es, einen Strahl atomarer sowie molekularer Ionen zu erzeugen und die dreidimensionalen Impulse der ionischen und neutralen Fragmente nach der Laserinteraktion in Koinzidenz zu detektieren. Um den fundamentalen Vorgang des Tunnelns von Elektronen zu verstehen und Modelle der Starkfeldionisation zu testen, wurde in dieser Arbeit die Attoclock-Methode auf das Heliumion angewendet, einem Einelektronensystem mit der doppelten Ladung des Wasserstoffatoms. Dies ermoglicht die Untersuchung des Tunnelprozesses nahe am Idealfall - dem quasistatischen Tunnelregime. Die Analyse der experimentell gemessenen Elektronenemissionswinkel als Funktion des radialen Impulses zeigt, dass die Emissionswinkel fur He+ deutlich kleiner sind als fur typischerweise genutzte Atome mit niedrigem Ionisationspotential. Die Verwendung von He+ fuhrt zu einem viel geringeren Keldysh-Parameter, was die Relevanz nichtadiabatischer Effekte, die die Interpretation der Attoclock erschweren, signifikant reduziert. Die Ergebnisse fur den Elektronenemissionswinkel sind in guter Ubereinstimmung mit theoretischen Vorhersagen aus der Losungen der TDSE sowie semiklassischer Simulationen, die keine Tunnelzeiten beinhalten. Weiterhin wurde die Doppelionisation des neutralen Heliumatoms durch nahezu zirkular polarisierte Few-Cycle-Laserpulse untersucht. Unter Nutzung semiklassischer Monte-Carlo-Simulationen wurden experimentelle und simulierte Ionenimpulsverteilungen detailliert verglichen und die Abhangigkeit der Doppelionisation von der Subzyklenform des ionisierenden Few-Cycle-Laserfeldes demonstriert. Die auf einem rein sequentiellen Ionisationsmodell basierenden Simulationen zeigen eine bemerkenswert gute Ubereinstimmung mit den experimentellen Beobachtungen und konnen die fur die Doppelionisation mit Few-Cycle-Laserpulsen charakteristische 6-Peak-Struktur der gemessenen Ionenimpulsverteilungen reproduzieren. Neben der Interaktion atomarer Systeme mit starken Laserfeldern wurde in dieser Arbeit die erste experimentelle Untersuchung des einfachsten asymmetrischen Molekuls, dem Heliumhydridion, in starken Laserfeldern durchgefuhrt. Es konnte gezeigt werden, wie der asymmetrische Aufbau und das daraus folgende permanente Dipolmoment die laserinduzierte Fragmentation des Heliumhydridions beeinflusst. Sowohl das Experiment als auch die Theorie fur Dissoziation, Einfachionisation und Doppelionisation von HeH+ und dem Isotop HeD+ demonstrieren, dass, abhangig von der Laserwellenlange, fur das asymmetrische Molekul die direkte Vibrationsanregung vor der Ionisation zur Streckung des Molekuls fuhrt. Die Dynamik von HeH+ steht im Gegensatz zu den bekannten symmetrischen Molekulen und gibt interessante Einblicke in das Verhalten asymmetrischer Systeme.

The laser-induced fragmentation dynamics of this most fundamental polar molecule HeH^{+} are measured using an ion beam of helium hydride and an isotopologue at various wavelengths and intensities. In contrast to the prevailing interpretation of strong-field fragmentation, in which stretching of the molecule results primarily from laser-induced electronic excitation, experiment and theory for nonionizing dissociation, single ionization, and double ionization both show that the direct vibrational excitation plays the decisive role here. We are able to reconstruct fragmentation pathways and determine the times at which each ionization step occurs as well as the bond length evolution before the electron removal. The dynamics of this extremely asymmetric molecule contrast the well-known symmetric systems leading to a more general picture of strong-field molecular dynamics and facilitating interpolation to systems between the two extreme cases.

A sophisticated setup for measuring laser-induced fragmentation of an ion beam of helium hydride and an isotopologue at various wavelengths and intensities enables us to study the dynamics of this most fundamental polar molecule. In contrast to the prevailing interpretation of strong-field fragmentation, in which stretching of the molecule results primarily from laser-induced electronic excitation, experiment and theory for non-ionizing dissociation, single ionization and double ionization both show that the direct vibrational excitation plays the decisive role here. We are able to reconstruct fragmentation pathways and determine the times at which each ionization step occurs as well as the bond length evolution before the electron removal. The dynamics of this extremely asymmetric molecule contrast the well-known symmetric systems yielding a much clearer picture of strong-field molecular dynamics in general and facilitating interpolation to other systems.

Utilizing a benchmark measurement of laser-induced ionization of an H$_2^+$ molecular ion beam target at infrared wavelength around 2 $μ$m, we show that the characteristic two-peak structure predicted for laser-induced enhanced ionization of H$_2^+$ and diatomic molecules in general, is a phenomenon which is confined to a small laser parameter space --- a Goldilocks Zone. Further, we control the effect experimentally and measure its imprint on the electron momentum. We replicate the behavior with simulations, which reproduce the measured kinetic-energy release as well as the correlated-electron spectra. Based on this, a model, which both maps out the Goldilocks Zone and illustrates why enhanced ionization has proven so elusive in H$_2^+$, is derived.

We investigate dissociative single and double ionization of HeH+ induced by intense femtosecond laser pulses. By employing a semi-classical model with nuclear trajectories moving on field-dressed surfaces and ionization events treated as stochastical jumps, we identify a strong-field mechanism wherein the molecules dynamically align along the laser polarization axis and stretch towards a critical internuclear distance before getting dissociative ionized. As the tunnel-ionization rate is greater for larger internuclear distance and for aligned samples, ionization is enhanced. The strong dynamical rotation is traced back to a maximum in the parallel component of the internuclear-distance-dependent polarizability tensor. Qualitative agreement with our experimental observations is found. Finally the criteria for observing the isotope effect for the ion angular distribution is discussed.

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 He2+ ion momentum distributions are compared to those obtained in recent benchmark experiments [M. S. Schoffler, X. Xie, P. Wustelt, M. Moller, 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.

A high-precision, single-shot, and real-time carrier-envelope phase (CEP) measurement at 1.8 mu m laser wavelength based on stereographic photoelectron spectroscopy is presented. A precision of the CEP measurement of 120 mrad for each and every individual laser shot for a 1 kHz pulse train with randomly varying CEP is demonstrated. Simultaneous to the CEP measurement, the pulse lengths are characterized by evaluating the spatial asymmetry of the measured above-threshold ionization (ATI) spectra of xenon and referenced to a standard pulse-duration measurement based on frequency-resolved optical gating. The validity of the CEP measurement is confirmed by implementing phase tagging for a CEP-dependent measurement of ATI in xenon with high energy resolution. (C) 2017 Optical Society of America