Poster Contributions

For each poster contribution there will be one poster wall (width: 97 cm, height: 250 cm) available. Please do not feel obliged to fill the whole space. Posters can be put up for the full duration of the event.

Coherent Control of Bond Making and Binary Reaction

Amitay, Zohar

High-harmonic spectroscopy with two-color counter-rotating bi-circular fields

Ayuso, David

High-harmonic generation (HHG) is a nonlinear process that converts intense infrared (IR) radiation into high-frequency light [1,2]. It can be understood as a sequence of three steps: (1) tunnel ionization of an atom or a molecule induced by the strong IR field, (2) laser-driven acceleration of the electron in the continuum, and (3) recombination with the parent ion resulting in the emission of harmonic light. Since there is a well-defined relationship between the duration of the electron excursion and the energy released during recombination, the harmonic spectrum provides snapshots of the laser-induced dynamics in the ion [3]. Very recently, the application of elliptically polarized fields has allowed to probe molecular chirality with sub-femtosecond time resolution [4], opening new directions in high-harmonic spectroscopy [5]. In general, the chiral response of a system increases with the ellipticity (chirality) of the driving field, usually maximizing for circularly polarized light. However, the harmonic signal quickly drops with ellipticity as the liberated electron is driven in a trajectory that misses recollision with the core. In this context, the use of two-color counter-rotating bi-circular fields [6] constitutes a promising tool for exploring time-resolved chiral dynamics as they allow recombination while maximizing the chiral response. In this conference we will present our theoretical work on HHG using two circularly polarized counter-rotating driving pulses carried at the fundamental frequency and its second harmonic. Our calculations have been performed in the framework of the strong field approximation (SFA), which assumes that the electron round-trip is solely driven by the laser field. This approximation is justified as long as the intensity of the field is sufficiently high and allows us to capture the most relevant physics arising in HHG. By using the saddle-point method for HHG [7] we have performed a semi-classical analysis in terms of electron trajectories, which provides rich insight into the physical process. We will discuss the most relevant features arising in the igh-harmonic spectra of different chemical species, comparing our results with those coming from numerical simulations and with experimental data, when available. REFERENCES [1] M Ferray et al. (1988), J. Phys. B 21, L31–L35 [2] P. B. Corkum (1993), Phys. Rev. Lett. 71, 1994–199 [3] S. Baker, et al. (2006) Science 312, 424–427 [4] R. Cireasa et al. (2015) Nat. Phys. 11 654-8 [5] O. Smirnova et al. (2015) J. Phys. B, 48 234005 [6] D. B. Milošević and W. Becker (2000), Phys. Rev. A 62 011403 [7] O. Smirnova and M. Ivanov (2013), Multielectron High Harmonic Generation: simple man on a complex plane, chapter 7 in Attosecond and XUV physics, edited by T. Schultz and M. Vrakking, Wiley

Amplitude and phase shaping of femtosecond laser pulses in the ultraviolet with the help of an acousto optical modulator

Bickhardt, Sascha

Femtosecond laser pulse shaping is the key technology in quantum control. So far, we were able to demonstrate pulse shaping with subcycle temporal accuracy making use of phase and amplitude modulation of femtosecond laser pulses in the infrared spectral region [1]. The experimental demonstration of molecular strong-field control schemes was achieved [2]. Organic molecules typically show pronounced absorption bands lying in the ultraviolet (UV) spectral region. Tailoring ultrashort UV laser pulses with respect to temporal and spectral shape opens up the possibility to investigate electron dynamics within different organic molecules, e.g. chiral ones. We present the current status of our 4f and acousto optical modulator based setup for amplitude and phase shaping in the ultraviolet. [1] J.Koehler et al.: Optics Express 19 (12), 11638-11653 (2011) [2] T.Bayer et al.: Physical Review Letters 110, 123003 (2013)

Ultrafast non-adiabatic relaxation of the naphthalene molecule after iner-valence ionization by a short XUV pulse

Despré, Victor

XUV induced dynamics in molecules are nowadays accessible in experiment thanks to the development of attosecond laser pulses. The simulation of such dynamics is however, very challenging. With such photon energies range, iner-valence electrons can be removed, which, due to the strong multi-electronic effects populates a large multitude of electronic states. Furthermore, a strong non-adiabatic coupling between the electronic and the nuclear degrees of freedom is expected. To overcome these difficulties we developed a model that permits to treat the ultrafast non-adiabatic relaxation that occurs in the naphthalene molecule (C10H8) after the population of cationic eigenstates lying close to the double ionization threshold. The model consists in a vibronic-coupling Hamiltonian derived from the electronic potential energy surfaces obtain with the Algebraic Diagrammatic Construction scheme. It includes 23 cationic states and 25 normal vibrational modes. The model Hamiltonian was used to propagate nuclear wave packets on the coupled manifold of cationic states with the Multi Configuration Time Dependent Hartree methods. Our results have permitted to interpret a recent experiment performed by the group of F. Lépine (Institute of Light and Matter, Lyon). Using time-resolved electron momentum imaging, the relaxation times of several cationic states of naphthalene have been directly observed. The measure relaxations times fall in the range of few tens of femtoseconds and, counter intuitively, increase with the increase of the energy of the states. Our simulations well reproduce this behavior and show that it is a result of the increasing density of states when approaching the double ionization threshold.

THz radiation out of a xenon plasma in a doubly resonant cavity

Dietrich, Christian Markus

Strong-Field Ionisation with Tailored Two-Color Laser Fields

Eckart, Sebastian

We present our two-color laser setup and our recently published results on nonsequential double ionization of Ar by counterrotating circularly polarized two-color (780 nm & 390 nm) laser fields with intensities of up to $8.0*10^{14} W/cm^2$. Absolute and relative intensities of the two colors are tuned to tailor the combined electric field. The 3D-momenta of the fragments are recorded using Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS) as experimental technique. The double-ionization probability depends strongly on the relative intensity of the two fields. We conclude that double ionization is driven by a beam of nearly monoenergetic recolliding electrons, which can be controlled in intensity and energy by the field parameters. The electron momentum distributions show the recolliding electron as well as a second electron which escapes from an intermediate excited state of Ar+.

Single-photon ionization in intense, fluctuating pulses

Giri, Sajal Kumar

Ionization of atoms (and molecules) with high-frequency laser pulse is a linear process, known for a long time as photo-effect. This process becomes non-linear in intense pulses as available from existing and upcoming free-electron laser (FEL) machines. Here, in contrast to low-frequency laser pulses, where the non-linearity emerges as above-threshold ionization (i.e. multi-photon absorption), multiple absorption and emission of photons lead to Stark shifts and amended matrix elements. Thereby ionization rates and electron spectra become sensitive to the shape of pulse envelope and random fluctuations of the pulse. Those fluctuations are immanent to any non-seeded FEL.We study these properties for a 1-dimensional model system.

Accurate ultra-broadband amplitude and phase shaping in the visible

Hillmann, Philipp

Femtosecond laser pulse shaping is the key technology in quantum control. So far, we were able to demonstrate pulse shaping with subcycle temporal accuracy making use of phase and amplitude modulation of femtosecond laser pulses in the infrared spectral region [1]. The experimental demonstration of molecular strong-field control schemes was achieved [2]. Supercontinua exceeding one octave are a prerequisite to generate few cycle light pulses in the temporal domain. Combining supercontinuum generation spanning from the ultraviolet to near-infrared spectral region with high-throughput prism based pulse shaping [3] opens up the possibility to temporally steer wavepacket dynamics on the sub-cycle timescale expanding the technique to a broader range of electronic systems. We present the current status of our setup for ultra-broadband amplitude and phase shaping of femtosecond laser pulses characterized by transient grating frequency resolved optical gating. [1] J. Köhler et al., Optics Express 19 (12), 11638-11653 (2011) [2] T. Bayer et al., Physical Review Letters 110, 123003 (2013) [3] T. Binhammer et al., IEEE 41 (12), 1552-1557 (2006)

Towards transient absorption measurements in filaments

Jusko, Christoph

TBA

Optimisation of strong laser field-free alignment using tailored light fields

Karamatskos, Evangelos

Multi-state Photoelectron Circular Dichroism in Femtosecond Multiphoton Ionization

Kastner, Alexander

The asymmetry of photoelectron angular distributions (PADs) from randomly oriented enantiomers of chiral molecules in the ionization with circularly polarized light arises in forward/backward direction with respect to the light propagation. This effect was termed Pho- toelectron Circular Dichroism (PECD) and so far investigated using synchrotron radiation [1, 2]. We observed highly structured asymme- tries in the range of ± 10% on bicyclic Ketones [3, 4]. Due to the multi photon ionization (MPI), high order Legendre poly- nomials appear in the measured PADs. In the case of Resonance En- hanced MPI (REMPI) using different intermediate states, the con- tributions in the photoelectron spectrum depend on wavelength. By changing wavelength and intermediate states, we are able to investi- gate PECD depending on photoelectron kinetic energy. [1] I. Powis in S. A. Rice (Ed.): Adv. Chem. Phys. 138, 267-329, (2008) [2] L. Nahon et al., J. El. Spectr. 204, 322-334, (2015) [3] C. Lux et al., Chem. Phys. Chem. 16, 115-137, (2015) [4] A. Kastner et al., Chem. Phys. Chem. 17, 1119-1122, (2016)

Photoelectron momentum distributions from CEP-stable polarization-shaped bichromatic fields

Kerbstadt, Stefanie

Polarization-tailored bichromatic laser fields with commensurable center frequencies became a new twist to coherently control and study the dynamics of free electron wave packets. Here, we introduce an innovative approach for the generation of polarization-tailored bichromatic laser fields based on ultrafast pulse shaping techniques [1]. The combination of a 4f polarization pulse shaper and a composite polarizer in the Fourier plane enables the independent amplitude and phase modulation of two spectrally separated bands of a CEP-stable few-cycle whitelight supercontinuum. The setup allows us to design the spectral amplitude, phase and polarization state of the fields of both colors individually, leading to an enormous versatility of producible bichromatic waveforms. In addition, the scheme offers built-in dispersion management and shaper-based pulse diagnostics. We verify the fidelity of the generated bichromatic fields by optical characterization and present recent experimental results on two-color photoionization employing photoelectron imaging tomography. [1] S. Kerbstadt, L. Englert, T. Bayer, M. Wollenhaupt, J. Mod. Opt., accepted (2016).

Tunneling time in attosecond experiments, Time in Quantum Mechanics and the realization of the Bohr-Einstein photon box Gedanken experiment.

Kullie, Ossama

Tunneling time in attosecond and strong field experiments is one of the most controversial issues in today’s research, because of its importance to the theory of time, the time operator and the time quantum mechanics. In [1] I present a theoretical model of the tunneling time for attosecond experiment of the He atom [2]. The model offers a relation which performs an excellent estimation for the tunneling time in attosecond and strong-field experiments of the He atom [2]. The tunneling time estimation is found by utilizing the time-energy uncertainty relation and represents a quantum clock. The tunneling time is also featured as the time of passage through the barrier similar to Einsteins photon-box Gedanken experiment. This work tackles an important case study for the theory of time in quantum mechanics and is very promising for the search for a (general) time operator in quantum mechanics [3]. The work can be seen as a fundamental step in dealing with the tunneling time in strong-field and ultrafast science and is appealing for more elaborate treatments using quantum wave-packet dynamics and especially for complex atoms and molecules. [1] O Kullie 2015 Phys. Rev. A 92 052118. [2] P Eckle et al 2008 Nat.phys. 4 565. [3] M. Bauer, arxiv1608.03492v1 (2016). [4] O Kullie 2016 J. Phys. B 49, 095601.

Double-slit electron interference in strong-field ionization of neon dimer

Kunitski, Maksim

Controlled molecule imaging

Küpper, Jochen

tbd

Strong field scattering from lattice defects

Medisauskas, Lukas

Electron scattering from point defects in dielectrics driven by strong and low frequency laser fields close to the dielectric breakdown of the solid is investigated. The results reveal a strongly non-perturbative nature of the scattering process. Namely, multiphoton absorption to several bands and multiple scattering plateaus reminiscent of strong-field scattering in atoms can be observed in the energy resolved spectra. Intensity scaling of the spectra reveals band structure modification due top the strong laser field. Kramers-Henneberger frame and Floquet approaches are employed. An efficient numerical method is developed to describe the scattering process in a regime where hundreds of Floquet states become important. This approach allows to compare scattering in different physical systems that may have very different final state.

Tailoring the vacuum fluctuations of the electric field

Moskalenko, Andrey

I will discuss the time-resolved behavior of the photonic ground state and show that vacuum fluctuations of its electric field can be directly detected using the linear electro-optic effect. I will sketch the main aspects of a general paraxial theory of electro-optic sampling of quantum fields developed for this purpose. Our calculations and experimental results demonstrate that nonlinear mixing of a femtosecond near-infrared probe pulse with the multi-terahertz vacuum field in a thin electro-optic crystal leads to an increase of the signal variance with respect to the shot noise level. Moreover, with another femtosecond pump pulse interacting with the vacuum in an additional nonlinear optical crystal, we can modify the ground state of the field that leads to the generation of a pulsed squeezed vacuum state. Our findings shall pave the way for a new approach to quantum optics operating with an extreme, subcycle time resolution.

Laser Induced Magnetization Dynamics: An ab-initio perspective

Müller, Tristan

Control of laser excited magnetization dynamics, such as induced spin-currents or ultrafast demagnetization, would have a huge technological impact due to the efficiency and timescale on which such processes act. However, there is not yet a complete understanding of these phenomena, and thus first-principles calculations are required for deducing the underlying physics. We use the ab-initio method of time-dependent density functional theory (TDDFT) to study the laser-induced spin dynamics of a few atomic layers of Ni/Co on top of a substrate Al/Pt. We find two mechanisms that lead to ultrafast demagnetization of the Ni/Co: 1) spin currents carrying moment into the substrate and 2) spin-orbit mediated spin-flips. Both these mechanisms contribute roughly equally to the total demagnetization. Furthermore this work doubles as an investigation of spin-injection into different materials with revelance for spintronic applications. We find that for the Pt substrate, we also see demagnetization due to spin-orbit interaction of the injected moment.

Attosecond intra-atomic delay in solid state photoemission

Neb, Sergej

The availability of single attosecond (as) extreme ultraviolet (EUV) pulses and their application in time-resolved photoemission from solids allows timing of photoemission events. The physical origin of the observed photoemission delays is not yet fully understood and controversial theoretical models coexist. As recent experiments show electron wave packet propagation in the bulk contributes. This propagation is determined by delocalized band-like excited states in the solid and the effects are accounted for within the common models of solid state photoemission. Here we address a very novel aspect, which sheds light on the very initial stage of photoelectron emission, i.e. the very short time window the emitted electron still resides inside the atom from which it is emitted. Attosecond time-resolved photoemission from the layered van der Waals crystal WSe2 in combination with theoretical modelling shows that atom-like effects such as the centrifugal barrier and intra-atomic corrections significantly affect the delays between different emission channels. These effects are well established for the photoemission from free atoms but, most interestingly, require a revision of models for solid state photoemission, i.e. atom-like and bulk electron propagation effects must be combined to realistically model photoelectron kinematics.

Tunneling exit characteristics from classical backpropagation of an ionized wavepacket

Ni, Hongcheng

We study the tunneling exit parameters of single active electron in the helium atom with the recently proposed backpropagation method [Phys. Rev. Lett. 117, 023002 (2016)] upon different criteria towards defining tunneling ionization. We find, if tunneling ionization is characterized by the emergence of electrons at certain predefined distances from the ion, the tunneling exit parameters extracted have a number of inconsistencies; while if tunneling ionization is defined by a vanishing momentum in the instantaneous field direction, which captures both adiabatic and nonadiabatic tunneling dynamics, the tunneling exit parameters retrieved are intuitive and easy to understand. This analysis has important implications towards future numerical simulations of the attoclock experiments that commonly used trajectory-based methods starting from assumed exit time and position are imprecise. Thereby, we provide a mapping technique that links attoclock experimental observable to the intrinsic tunneling exit time.

Sub-envelope dynamics of slow electrons from non-adiabatic transitions

Ning, Qicheng

Strong field ionization with twisted photons: problems and perspectives

Paufler, Willi

Strong-field ionization of two-dimensional model systems intense, by few-cycle circularly polarized fields

Paul, Matthias

We have developed a two-dimensional numerical model system mimicking an asymmetric environment of nuclei forming a molecule. Based on this model, we investigate the role of symmetry imprinted by the nuclei on the strong-field photoelectron angular distribution. We compare our results of quantum dynamical calculations in long- and short-range potentials with those of classical Monte Carlo calculations and show how the photoelectron momentum distribution changes for different carrier-envelope phases of the strong driving field.

Free-electron quantum optics

Priebe, Katharina

Katharina E. Priebe, Christopher Rathje, Armin Feist, Sergey V. Yalunin, Sascha Schäfer, and Claus Ropers Besides being a powerful tool for time-resolved measurements of nanoscale dynamics, ultrafast transmission electron microscopy (UTEM) serves as an ideal test bench for quantum optical experiments studying the interaction with free-electron beams. Specifically, inelastic scattering between the electrons and strong optical near-fields [1] allows for a coherent manipulation of the electron quantum state [2]. The optical near-field imprints a sinusoidal phase modulation on the electron wavefunction, which is manifest in a comb of sidebands in the electron kinetic energy distribution. In this contribution, we will demonstrate how multiple near-fields can be employed to coherently control the free-electron momentum superposition states [3,4]. Furthermore, dispersive propagation translates the phase modulation into a density modulation: the electron wavefunction is self-compressed into a train of attosecond bursts. This temporal structuring of free-electron beams may find applications in electron microscopy with attosecond resolution. [1] B. Barwick \textit{et al}., Nature, \textbf{462}, 902 (2009). [2] A. Feist \textit{et al}., Nature, \textbf{521}, 200-203 (2015). [4] K. E. Echternkamp \textit{et al}. Nat. Phys \textbf{12}, 1000-1004 (2016). [5] K. E. Priebe \textit{et al}. In preparation.

Broadband Photoelectron Circular Dichroism

Ring, Tom

Photoelectron circular dichroism (PECD) describes the asymmetry in the photoelectron angular distribution (PAD) with regard to the light propagation direction after ionization with circularly polarized light. First investigated by one photon synchrotron ionization,${}^1$ we demonstrated that PECD on isotropically distributed chiral molecules is accessible via 2+1 resonance enhanced multi photon ionization (REMPI) using femtosecond laser pulses and established PECD as a highly sensitive and robust analytical tool.${}^2$ A different excitation scheme is addressed within PECD studies of electrons from above threshold ionization (ATI).${}^3$ In this contribution we present an ultra-broadband approach to PECD using white light generated by an argon filled hollow-core fiber. This way the excitation scheme changes due to the combination of different wavelengths with regard to the temporal structure of the pulses and a wide range of excess energies in the PADs can be accessed simultaneously. [1] L. Nahon et al.: \textit{J. El. Spectr. Rel. Phen.}, \textbf{204}, 322 (2015) [2] C. Lux et al.: \textit{Chem. Phys. Chem} \textbf{16}, 115 (2015) [3] C. Lux et al.: \textit{J. Phys. B} \textbf{49}, (2016)

A VMI spectrometer for nanoplasma experiments

Schomas, Dominik

Velocity Map Imaging (VMI) is a useful technique for measuring kinetic energies and angular distributions of electrons or ions simultaneously. Conventional VMI spectrometers map electrons with energies in the low eV range [1]. A number of modifications have been demonstrated to achieve higher resolution [2] or to access higher kinetic energies [3]. However, these modifications usually require a larger number of electrodes and therefore the spectrometer becomes more complicated to design and operate. Here we present a modified VMI geometry for energetic electrons up to the keV range with just two additional electrodes with respect to the traditional configuration, where one of the lens electrodes is set to ground potential. We show simulations and measurements to demonstrate the characteristics of the spectrometer. [1] Rev. Sci. Instrum. 68 (9), 1997 [2] J. Chem. Phys. 138, 2013 [3] Int. J. Mass Spect. 365–366, 2014

Ionization behavior of HeH+ in intense laser fields

Schulz, Bruno

tba

The tRecX package - an open source TDSE solver

Scrinzi, Armin

The tRecX package [1] for the TDSE of atomic and molecular multi-electron systems in external fields is an easy-to-use program for typical problems in the QUTIF community. It incorporates the recently developed tSurff (time-dependent surface flux) and haCC (hybrid anti-symmetrized Coupled Channels) techiques. A range of results using these techniques include the double-ionization of He at 800 nm wave length, the (absence of) multi-electron effects in ionization of He in elliptically polarized light (attoclock), effects of dynamic exchange on the alignment dependence of $CO_2$ ionization. On the poster, input and results for selected applications will be presented. [1] https://trecx.physik.uni-muenchen.de

Attosecond Electron Scattering in Dielectrics

Seiffert, Lennart

Scattering of electrons in dielectrics is at the heart of laser nanomachining, light-driven electronics, and radiation damage. Accurate theoretical predictions of the underlying dynamics require precise knowledge of the low-energy electron transport involving elastic and - even more important - inelastic collisions. Here, we demonstrate real-time access to electron scattering in isolated SiO$_2$ nanoparticles via attosecond streaking [1]. Utilizing semiclassical Monte-Carlo trajectory simulations [2,3] we identify that the presence of the field inside the dielectric cancels the influence of elastic scattering, enabling selective characterization of the inelastic scattering time [4]. [1] R. Kienberger et al., Nature 427, 817-821 (2004) [2] F. Süßmann et al., Nat Commun. 6, 7944 (2015) [3] L. Seiffert et al., Appl. Phys. B 122, 1-9 (2016) [4] L. Seiffert et al., submitted

Time delay in photoionization with light carrying orbital angular momentum

Wätzel, Jonas

The pioneering experiment of Schultze et al. [1] on time delay in photoemission triggered substantial experimental and theoretical activities with the aim to understand and quantitatively reproduce the results of the measurements [2-4]. Up to date various mechanisms and calculation techniques were put forward, yet disputable some differences between theory and experiment remain despite the relative simplicity of the considered targets. To add yet a new aspect to time delay in photoemission we considered using a twisted light beam, also called optical vortex. Such a beam has a phase singularity at its centre and carries orbital angular momentum (OAM) characterized by the topological charge. The spatial inhomogeneity makes it possible to transfer OAM and therefore a torque to an electron. The amount of transferable OAM is controlled by the topological charge. The use of OAM XUV laser beams to trigger photoionization implies a complete new set of optical selection rules [5] that optical transitions tuneable by the choice of the beam topological charge. We present results of calculations of the atomic time delay of the photoionization process of the argon 3p subshell initiated by a twisted light XUV pulse [6]. We show that in different directions either the co-rotating electron (relative to the field) or the counter rotating electron dominates photoionization amplitude. Furthermore the corresponding time delays are completely different in magnitude and sign, and depend sensitively on the position of the atom in the laser beam spot. Therefore, the time delay represents an interesting measure to identify the origin of the photoelectron with respect to the initial magnetic (sub-)state as well as the position in space of the ionized atom. References 1 M. Schultze et al., Delay in photoemission, Science 328, 1658 (2010). [2The pioneering experiment of Schultze et al. [1] on time delay in photoemission triggered substantial experimental and theoretical activities with the aim to understand and quantitatively reproduce the results of the measurements [2-4]. Up to date various mechanisms and calculation techniques were put forward, yet disputable some differences between theory and experiment remain despite the relative simplicity of the considered targets. To add yet a new aspect to time delay in photoemission we considered using a twisted light beam, also called optical vortex. Such a beam has a phase singularity at its centre and carries orbital angular momentum (OAM) characterized by the topological charge. The spatial inhomogeneity makes it possible to transfer OAM and therefore a torque to an electron. The amount of transferable OAM is controlled by the topological charge. The use of OAM XUV laser beams to trigger photoionization implies a complete new set of optical selection rules [5] that optical transitions tuneable by the choice of the beam topological charge. We present results of calculations of the atomic time delay of the photoionization process of the argon 3p subshell initiated by a twisted light XUV pulse [6]. We show that in different directions either the co-rotating electron (relative to the field) or the counter rotating electron dominates photoionization amplitude. Furthermore the corresponding time delays are completely different in magnitude and sign, and depend sensitively on the position of the atom in the laser beam spot. Therefore, the time delay represents an interesting measure to identify the origin of the photoelectron with respect to the initial magnetic (sub-)state as well as the position in space of the ionized atom. References 1The pioneering experiment of Schultze et al. [1] on time delay in photoemission triggered substantial experimental and theoretical activities with the aim to understand and quantitatively reproduce the results of the measurements [2-4]. Up to date various mechanisms and calculation techniques were put forward, yet disputable some differences between theory and experiment remain despite the relative simplicity of the considered targets. To add yet a new aspect to time delay in photoemission we considered using a twisted light beam, also called optical vortex. Such a beam has a phase singularity at its centre and carries orbital angular momentum (OAM) characterized by the topological charge. The spatial inhomogeneity makes it possible to transfer OAM and therefore a torque to an electron. The amount of transferable OAM is controlled by the topological charge. The use of OAM XUV laser beams to trigger photoionization implies a complete new set of optical selection rules [5] that optical transitions tuneable by the choice of the beam topological charge. We present results of calculations of the atomic time delay of the photoionization process of the argon 3p subshell initiated by a twisted light XUV pulse [6]. We show that in different directions either the co-rotating electron (relative to the field) or the counter rotating electron dominates photoionization amplitude. Furthermore the corresponding time delays are completely different in magnitude and sign, and depend sensitively on the position of the atom in the laser beam spot. Therefore, the time delay represents an interesting measure to identify the origin of the photoelectron with respect to the initial magnetic (sub-)state as well as the position in space of the ionized atom. References (1) M. Schultze et al., Delay in photoemission, Science 328, 1658 (2010). [2] J. Wätzel et al., Angular resolved time delay in photoemission, J. Phys B: At. Mol. Opt. Phys. 48, 025602 (2015) and references therein. [3] J. Dahlström et al., Theory of attosecond delays in laser-assisted photoionization, Chem. Phys. 414, 53 (2013). [4] A. S. Kheifets, Time delay in valence-shell photoioinzation of noble-gas atoms, Phys. Rev. A 87, 063404 (2013). [5] A. Picón, Photoionization with orbital angular momentum beams, Opt. Expr. 18, 3660 (2010). [6] J. Wätzel and J. Berakdar, Discerning on a sub-optical-wavelength the attosecond time delays in electron emission from magnetic sublevels by optical vortices, Phys. Rev. A 94, 033414 (2016 M. Schultze et al., Delay in photoemission, Science 328, 1658 (2010). [2] J. Wätzel et al., Angular resolved time delay in photoemission, J. Phys B: At. Mol. Opt. Phys. 48, 025602 (2015) and references therein. [3] J. Dahlström et al., Theory of attosecond delays in laser-assisted photoionization, Chem. Phys. 414, 53 (2013). [4] A. S. Kheifets, Time delay in valence-shell photoioinzation of noble-gas atoms, Phys. Rev. A 87, 063404 (2013). [5] A. Picón, Photoionization with orbital angular momentum beams, Opt. Expr. 18, 3660 (2010). [6] J. Wätzel and J. Berakdar, Discerning on a sub-optical-wavelength the attosecond time delays in electron emission from magnetic sublevels by optical vortices, Phys. Rev. A 94, 033414 (2016J. Wätzel et al., Angular resolved time delay in photoemission, J. Phys B: At. Mol. Opt. Phys. 48, 025602 (2015) and references therein. [3] J. Dahlström et al., Theory of attosecond delays in laser-assisted photoionization, Chem. Phys. 414, 53 (2013). [4] A. S. Kheifets, Time delay in valence-shell photoioinzation of noble-gas atoms, Phys. Rev. A 87, 063404 (2013). [5] A. Picón, Photoionization with orbital angular momentum beams, Opt. Expr. 18, 3660 (2010). [6] J. Wätzel and J. Berakdar, Discerning on a sub-optical-wavelength the attosecond time delays in electron emission from magnetic sublevels by optical vortices, Phys. Rev. A 94, 033414 (2016

Velocity map imaging of scattering dynamics in orthogonal two color fields

Würzler, Daniel

Tuning the relative phase of orthogonal two-color laser fields has become an important technique to get insight/control into sub-cycle electron dynamics of strong-field ionization processes. Here this technique is applied to velocity map imaging spectroscopy using an unconventional orientation with the polarization of the ionizing laser field perpendicular to and the steering field parallel to the detector surface. We measure the phase-dependent photoelectron momentum distribution of Neon and Xenon and analyse them by using semi-classical calculations in three dimensions including elastic scattering at different orders of return. The results are confirmed with the solution of three-dimensional time-dependent Schrödinger equation (3D TDSE) calculations. Thereby control over direct and rescattered electrons is demonstrated.

Fragmentation dynamics of HeH+ in strong fields

Wustelt, Philipp

We investigate the fragmentation of the HeH$^+$ molecular ion in intense laser fields. The HeH$^+$ ion is the simplest heteronuclear molecule with asymmetric electronic wave functions. Because of its lack of symmetry, HeH$^+$ is particular attractive for investigating the photochemical fragmentation induced by intense asymmetric waveforms. The HeH$^+$ ions are produced in an ion beam apparatus and exposed to intense laser pulses. The momenta and charge states of all fragments, including neutrals, are detected in coincidence. Here, we present first measurement results for the dissociation of HeH$^+$ by 800 nm multi-cycle laser pulses. We observe that He + H$^+$ is the dominating dissociation channel. For this channel both fragments are measured with respect to the initial orientation of the molecules relative to the laser polarization. The experimental results are compared to quantum-mechanical simulations in single-active-electron approximation, which include the nuclear dynamics. Here is the list of the authors: P. Wustelt1, F. Oppermann2, L. Yue3, M. Möller1, A. M. Sayler1, S. Gräfe3, G.G. Paulus1, M. Lein2 1Institute of Optic and Quantum Electronics, Friedrich-Schiller-University Jena, Germany, 2Institute for Theoretical Physics, Leibniz University Hannover, Germany, 3Institute of Physical Chemistry, Friedrich-Schiller-University Jena;

Application of the many-electron weak-field asymptotic theory on diatomic molecules: a generalized-active-space configuration interaction approach

Yue, Lun

The many-electron weak-field asymptotic theory (ME-WFAT) for static tunneling ionization [Tolstikhin et al., Phys. Rev. A 89, 013421 (2014)] is applied to diatomic molecules. In the ME-WFAT, the dependence of the ionization rate on the molecular orientation with respect to the static field direction is determined by the structure factor, which in turn depends on the asymptotic tail of of the Dyson orbital. We extract the latter by the time-dependent generalized-active-space configuration-interaction (TD-GASCI) method [Bauch et al., Phys. Rev. A 90, 062508 (2014)], which takes into account electron correlation effects systematically. Results for the orientation-dependent structure factor are presented for H$_2$ and LiH. Compared to Hartree-Fock results, the inclusion of the electron-electron correlations has a non-negligible effect on the structure factor. The results holds within the fixed-nuclei approximation and the leading-order approximation of the ME-WFAT.

Strong-field-induced breakup of small diatomics with the Floquet surface hopping method

Yue, Lun

The Floquet surface hopping method [Fiedlschuster et al., Phys. Rev. A 93, 053409 (2016)] is applied to the breakup of small molecules. In the Floquet surface hopping method, the nuclei are treated as classical particles moving on the internuclear-distance-dependent Floquet surfaces, while the electronic problem is solved fully quantum mechanically and determines the hopping probability between the Floquet surfaces.

CEP-dependent above-threshold ionization with few-cycle laser pulses at 1800 nm

Zille, Danilo

The carrier-envelope(CE)-phasemeter, which is based on a stereographic above-threshold ionization (ATI) measurement, has been proven to be a precise, real-time, single-shot CEP tagging and pulse length characterization technique at 800 nm [1]. However, for longer wavelengths, the higher ponderomotive energy increases electron return energies. This reduces the scattering probability and thus reduces the corresponding yield of the high energy back-scattered electrons detected by the CE-phasemeter, which makes the measurements challenging. Nevertheless, recent preliminary results have shown that determination of the CEP of 1800 nm pulses using stereo-ATI of Xenon is possible. Here, the stereo-ATI spectra in few-cycle pulses at 1800 nm are simulated using the semi-classic three-step model of strong-field ionization and are compared with experimental results. The simulation results provide guidelines for optimizing CE-phasemeter technology to allow measurements at longer wavelength with increased precision. [1]T.Rathje et al.J.Phys.B:At.Mol.Opt.Phys.45(2012)074003