# Atomic Physics 2016

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.

### Stabilization of atoms in a presence of quantized electromagnetic field

Bogatskaya, Anna

It is known from [1, 2] that the influence of high intensity laser fields causes the significant reconstruction of atomic system resulting in stabilization phenomenon, i.e. population trapping in high-lying Rydberg states. One of the possible ways to study the structure of such reconstructed (dressed) by the laser field quantum systems is to analyze their spontaneous emission. We have developed the new approach how to introduce spontaneous emission into the Schrödinger equation. The proposed approach is applied to examine the dynamics of field-driven atomic system in the regime of interference stabilization (IS). Main predictions of IS theory are testified and confirmed. Specific features of atomic system dynamics in two-color laser field are studied when high-frequency laser field causes the coupling of the ground atomic state with a Rydberg state by means of Rabi oscillations, while low-frequency field provides the stabilization phenomenon and population trapping in Rydberg states. 1) Fedorov, M. V. and Movsesian, A. M. 1988. J. Phys. B, 21, L155 2) Fedorov, M. V., Tehranchi, M.-M., and Fedorov, S. M. 1996. J. Phys. B, 29, 2907

### Applications in Molecular, Bio-medical and Material Sciences at ELI Beamlines

Brooks, Christopher

Research Program 4 (RP4) at ELI Beamlines develops applications in Molecular, Bio-Medical and Materials Sciences. We are in the process of realizing a unique set of capabilities for time resolved experiments based on the high power lasers, secondary light sources and a comprehensive set of pump beams. Early experimental capabilities in the VUV and soft X-ray range include time-resolved Magneto-Optical Ellipsometry in the VUV range, Coherent Diffractive Imaging and Atomic Molecular and Optical (AMO) science. In the X-ray range we are developing instruments for time resolved Scattering, Diffraction, Absorption Spectroscopy and Phase Contrast Imaging as well as Pulse Radiolysis. The secondary X-ray sources for these experiments will be driven by the L1 (100 mJ, 1 kHz , <20 fs) and L3 (PW class, 10 Hz) lasers. The L1 laser will also be used directly to develop applications in optical spectroscopy such as Stimulated Raman Scattering. These applications are all interesting by themselves but the potentially greatest advantage of ELI Beamlines will be the possibility to combine these methods with perfect synchronization for complete investigations of complex phenomena. Commissioning and enabling experiments will begin already in early 2016 using our conventional fs support lasers, user based commissioning with the L1 laser will proceed during 2016 and 2017 and user operations will start in 2018.

### Fast photoelectrons from clusters in intense IR lasers

Camacho Garibay, Abraham

### Strong Field processes driven by spatially inhomogeneous and homogeneous fields

Chacón Salazar, Alexis Agustín

We present theoretical calculations of single/double-electron ionization (S/DEI) and high order harmonic generation (HHG) driven by spatially homogeneous and inhomogeneous fields from different atomic, molecular and solid targets. In terms of an atomic system interacting with homogeneous fields, we focus our study on the above threshold ionization process by means of Lewenstein's model and extend it to diatomic molecular systems. The latter gives us a way to understand how structural information about the bond-distance is related and retrieved from the photoelectron spectrum. In the context of two-electron atoms driven by spatially homogeneous field, we present novel calculations of the double-electron ionization in the mid-infrared regime (3.1 microns of wavelength) from argon atom at relative low-peak intensity (8x10^13 W/cm^2). Our results shown that the re-collision ionization impact scenario is the responsible physical process of the double ionization of Ar. In addition we present a novel method to compute the emitted HHG radiation from a periodic crystal-solid lattice structure. Our model is able to capture all the fundamental features of the standard HHG experimental data so far reported in literature. Furthermore, the main advantage of this model is the new physical description of the nonlinear harmonic process in terms of localized atom excitation from the valence band to the conduction one by means of Wannier-Bloch picture.

### Application of Gaussian wave packet dynamics to high-harmonic generation and above-threshold ionization

Choi, Nark Nyul

Gaussian Wave Packet Dynamics (GWPD) has great advantages as a tool for time-dependent approach to semiclassical dynamics: 1. the evolution of GWP is determined by classical equations of motion, 2. no boundary conditions are needed to be imposed at asymptotic regions, and 3. there is a possibility of avoiding the exponential increase of computational resource as the degrees of freedom of a system grow [1]. However, GWPD suffers from severe limitations: it is valid only for a limited short time, and it cannot describe the classically forbidden processes such as tunneling and diffraction. These are owing to the broadening of GWP as time goes on, which breaks the validity of GWPD based on quadratic approximation to the potential. As a solution to this problem, it has been suggested to re-initialize the GWPs, i.e., to transform the broadened GWPs to linear combinations of the initial GWPs with a narrow width after evolving in every short time-interval [2,3]. Following [3], we develop a GWPD with re-initialization in a Gabor frame. First of all, our method is shown to work well for a system with tunneling through a static potential barrier. And we demonstrate the validity and advantages of the method as a time-dependent Schrődinger equation solver by applying to the higher-harmonic generation spectrum and the above-threshold ionization spectrum of the one-dimensional atomic model exposed to ultrafast strong laser pulses [4]. [1] E. J. Heller, J. Chem. Phys. 62, 1544 (1975). [2] F. Grossmann, Phys. Rev. Lett. 85, 903 (2000); X. Kong, A. Markmann, V. S. Batista, J. Phys. Chem. 120, 3260 (2016). [3] L. M. Andersson, J. Chem. Phys. 115, 1158 (2001). [4] Q. Su and J. H. Eberly, Phys. Rev. A 44, 5997 (1991).

### Ultrafast build-up of a Fano resonance in helium in the time domain

Donsa, Stefan

Asymmetric absorption lines in atomic spectra were first observed by Beutler [1] and are nowadays frequently encountered in many areas. In atomic and molecular systems they emerge as an interference effect between direct ionization and a delayed ionization from an auto-ionizing state as first theoretically described by Fano [2]. They are a direct consequence of electronic correlations which lie at the heart of auto ionization processes. Recently there was an increased theoretical interest in the time-dependent build-up of the characteristic line-profile [3-6]. We report on a joint experimental and theoretical study of the build-up of a Fano-resonance profile in the time domain for the prototype system of helium [7]. This was achieved by depleting the auto-ionizing state with a delayed strong near-infrared few-cycle laser pulse after the excitation by an attosecond XUV pulse. Thereby the formation of the absorption profile was stopped at the time of depletion. We validated the experimental protocol with ab-initio calculations and found that the optical gating is very effective and that the depletion takes place in a very short time-interval. References [1] H. Beutler, Z. Physik \textbf{93}, 177 (1935). [2] U. Fano, Nuovo Cimento \textbf{12}, 154 (1935). [3] M. Wickenhauser et al., Phys. Rev. Let. \textbf{94}, 023002 (2005). [4]Th. Mercouris, Y. Komninos, C. A. Nicolaides, Phys. Rev. A \textbf{75}, 013407 (2007). [5] W.-C. Chu, C. D. Lin, Phys Rev. A \textbf{82}, 053415 (2010). [6] L. Argenti et al., Phys. Rev. A \textbf{87}, 053405 (2013). [7] A. Kaldun et al., Science \textit{accepted}, (2016).

### Graphene quantum dots: A promising candidate for detecting single electron processes

Ghosh, Siddharth

Luminescent carbon nanodots (CND) are a recent addition to the family of carbon nanostructures. Interestingly, a large group of CNDs are fluorescent in the visible spectrum and possess single dipole emitters [1] with potential applications in super-resolution microscopy, quantum information science, and optoelectronics. There is a large diversity of CND’s size as well as a strong variability of edge topology and functional groups in real samples. This hampers a direct comparison of experimental and theoretical findings that is necessary to understand the unusual photophysics of these systems. We derived atomistic models of finite sized (<2.5 nm) CNDs from high resolution transmission electron microscopy (HRTEM) which are studied using approximate time-dependent density functional theory. The atomistic models are found to be primarily two-dimensional (2D) and can hence be categorised as graphene quantum dots (GQD) [2]. The electronic structure of GQDs can be suitable for resonance light scattering and achieving single-electron detection resolution in a one-dimensional fast tracking system [3]. References: [1] Ghosh, S. et al. Photoluminescence of carbon nanodots: dipole emission centers and electron-phonon coupling. Nano letters 14, 5656-5661, 2016. [2] Ghosh, S. et al. Graphene quantum dots with visible light absorption of the carbon core: insights from single-particle spectroscopy and first principles based theory. 2D Materials 3, 041008, 2016. [3] Faez, S. et al. Fast, label-free tracking of single viruses and weakly scattering nanoparticles in a nanofluidic optical fiber. ACS Nano 9, 12349, 2015.

### Light-field-driven currents in graphene

Heide, Christian

Christian Heide, Takuya Higuchi, Konrad Ullmann, Heiko B. Weber, Peter Hommelhoff Department Physik, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, www.laser.phyik.fau.de Ultrafast electron dynamics inside of solids under strong optical fields has recently found particular attention. In dielectrics and semiconductors light-field driven effects like high-harmonic generation and sub-optical-cycle interband population transfer have been reported. However, much less is known about strong field phenomena in conducting materials. Graphene is an ideal playground for studying strong-field phenomena in a conductor because of its excellent carrier mobility, much weaker screening due to a low carrier concentration compared with conventional metals and its ultrafast and broadband optical response. Here we show that one can control a residual conducting current in epitaxially grown graphene by the optical field of light [1]. In this case the electron dynamics is coherent without being affected by scattering processes, such as electron-electron or electron-phonon scattering. We found a carrier-envelope-phase-dependent current in graphene stripes, excited with few-cycle laser pulses with a peak optical field up to 3 V/nm. The amplitude of the current scales strongly nonlinearly with field strength, when the polarization is linear and parallel to the graphene stripe. In this case the CEP dependent current changes its sign at around 1.5 V/nm with increasing field strength. For circularly polarized excitation a CEP current is found, too, whereas no change in the sign of the current was observed. We drive combined interband and intraband electron dynamics with the optical field of light, leading to a light-field-waveform controlled residual conduction current after the laser pulse is gone. With linearly polarized excitation, this waveform-dependent current switches its direction when the optical field strength is increased. We identify the pivotal physical mechanism as electron quantum-path interference that takes place on the 1-femtosecond timescale. The process can be categorized as Landau-Zener-Stückelberg interferometry. For circular polarization this interference effect is absent, still light-field-driven currents are generated through modulation of individual sub-cycle transition probabilities. The main experimental features, especially the change in current direction as a function of field strength for linearly polarized excitation as well as the absence of such a change for circular polarization, are well reproduced by numerical simulations. In essence, light-field control of electrons in graphene is demonstrated, and a new mechanism for conduction current is identified. Now a door is opened for integrating two important technologies, optics and electronics, to a single platform based on controlling electrons by an optical electric field. [1] T. Higuchi, C. Heide, K. Ullmann, H. B. Weber, and P. Hommelhoff, arXiv:1607.04198 (2016).

### Theoretical modeling of strong-field non-adiabatic processes in solids

Higuchi, Takuya

Electrons in solids undergo various non-adiabatic processes when they are placed under optical electric fields that are comparable to or larger than the internal field acting on electrons inside of the solids. In this poster presentation, we discuss two theoretical aspects of such non-adiabatic processes in solids. First, we discuss a criterion discriminating between perturbative and non-perturbative processes in solids, in terms of the Keldysh adiabaticity parameter. For this, we consider strong-field-driven non-adiabatic transitions at an avoided crossing model that represents a bandgap in solids, and discuss the difference and similarity to those in atomic cases. In particular, we discuss cases where the band gap is comparable to or smaller than the driving frequency multiplied with \hbar, which is important for strong-field phenomena in narrow-gap of gapless systems, such as in graphene [1]. Experimental results underscore the results of this model (see poster by Christian Heide et al.). Furthermore we discuss high-harmonic generation in solids. We focus on the choice of basis functions, especially those of electrons experiencing intraband Bloch oscillations under strong optical fields, together with interband transitions. In such cases, the interplay between the interband and intraband dynamics becomes crucial. By choosing wave functions describing this multi-band Bloch oscillation, numerically obtained high-harmonic spectra can be well interpreted in a semi-analytical manner [2]. [1] T. Higuchi, C. Heide, K. Ullmann, H. B. Weber, and P. Hommelhoff, arXiv:1607.04198 (2016). [2] T. Higuchi, M. I. Stockman, and P. Hommelhoff, Phys. Rev. Lett. 113, 213901 (2014).

### Single-photon ionization in intense, stochastically fluctuating pulses

Kumar Giri, Sajal

### Strong field scattering from lattice defects

Medisauskas, Lukas

In atomic and molecular physics, strong field above threshold ionization (ATI) and high harmonic generation (HHG) always go hand in hand. While HHG has recently received a lot of interest in solids state systems, strong field scattering and ATI in periodic systems has gained much less attention. In this work we are looking at scattering of electrons driven by a strong laser field from lattice defects, where a single site of an infinitely periodic lattice is vacant. A Kramers-Henneberger and Floquet approaches are employed to describe the scattering in terms of number of absorbed photons. It allows to compare different physical systems that may have very different final states after scattering. The results reveal that the scattering from lattice defect is much stronger compared to atomic case. Moreover, a combination of multiphoton absorption, associated with the bands structure, and multiple scattering plateaus can be observed.

### Self-organization of charged particles in circular geometry

Nazmitdinov, Rashid

The basic principles of self-organization of one-component charged particles, confined in disk and circular parabolic potentials, are proposed. A system of equations is derived, that allows to determine equilibrium configurations for arbitrary, but finite, number of charged particles that are allocated on several rings. Our approach drastically reduces the computational effort in minimizing the energy of equilibrium configurations and demonstrates a remarkable agreement with the values provided by molecular dynamics calculations. With the increase of particle number $n>180$ we find a steady formation of a centered hexagonal lattice that smoothly transforms to valence circular rings in the ground state configurations for both potentials.

### Assessing tunneling exit characteristics with the backpropagation method

Ni, Hongcheng

We study the tunneling exit parameters of single active electron in the helium atom with the recently proposed backpropagation method upon different criteria towards defining tunneling ionization. We find, if tunneling ionization is defined by the emergence of electrons at certain assumed 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 flawed. Thereby, we provide a mapping technique that links attoclock experimental observable to the intrinsic tunneling exit time.

Ning, Qicheng

### Emergence of a higher energy structure in strong field ionization with inhomogeneous electric fields

Ortmann, Lisa

The strong field approximation (SFA) in its standard form neglects all spatial inhomogeneities. Recently, the small spatial inhomogeneity introduced by the long-range Coulomb potential has been linked to a number of important features in the photoelectron spectrum, such as Coulomb asymmetry, Coulomb focusing, and the low energy structure (LES). We theoretically investigate the effects of another type of spatial inhomogeneity, namely a small time-varying spatial dependence in the laser electric field, and find that ionization in such a field creates a prominent higher energy peak at energies above the 'classical cut-off' for direct electrons. Since this new high energy structure is highly CEP sensitive, this effect could lead to CEP determination with direct electrons. Furthermore, the accumulation of trajectories in this prominent new peak stems from electrons ionized within a single half-cycle close to the field maximum, thus opening the opportunity to use the corresponding high-energetic electrons to create localized sources of monoenergetic electron beams of sub-femtosecond duration.

### Attosecond Correlation Dynamics

Ossiander, Marcus

We measured the energy dependent photoemission delay of electrons escaping during the shake-down and shake-up processes in helium using attosecond streaking spectroscopy. Sub-attosecond precision and agreement with ab-initio calculations allows to isolate up to 6 attoseconds retardation in the shake-up photoemission process due to electronic correlations, marking the breakdown of the single-active-electron approximation for photoemission timing. The retardation is caused by a DC-Stark-shift of the asymmetric ionic final states in the instant of photoemission, possible due to the strong temporal confinement of the photoemission to within a fraction of one electric field cycle. This effect can universally be expected for all systems exhibiting polarized initial or final states and can be used for complete wavepacket reconstruction.

### Coherent control of multiphoton photoemission from single nanometric metal tips in a two-color scheme

Paschen, Timo

By focusing 1560 nm drive pulses and their second harmonic at 780 nm onto a nanotip and changing the optical phase between the two colors, we observe an emission current modulation of up to 97.5 %. Hence, we show that multi-photon photoemission including above-threshold orders can be coherently controlled and demonstrate that the photoemitted current is dependent on the optical phase of the two light fields [1]. Interestingly, despite the solid-state nature of the nanotip the amount of control of the total electron emission is extremely high and the electron current can be strongly increased or reduced due to interference between two different emission channels in the material. We present conclusive evidence that the confining nature of local field enhancement at the nanotip allows us to achieve nearly perfect control of electron emission yield. The excellent control is based on the fact that the tip singles out a local field intensity, so that the inhomogeneous intensity distribution in the laser focus can be neglected since the nanotip is much smaller than the optical beam spot size. The experimentally investigated quantum interference between different emission pathways is compared to time-dependent density functional theory (TDDFT) simulations and DFT calculations. Furthermore, promising properties of femtosecond laser-triggered electron emission are discussed, such as the superb transverse coherence [2]. As a future application prospect the concept of dielectric laser-acceleration (DLA) is introduced [3]. Here, the nanotip can serve as a well-controllable, bright and coherent source for electrons and thus represent a key element for this new technology. [1] M. Förster, T. Paschen, M. Krüger, C. Lemell, G. Wachter, F. Libisch, T. Madlener, J. Burgdörfer, and P. Hommelhoff, arXiv:1603.01516 (2016) [2] D. Ehberger, J. Hammer, M. Eisele, M. Krüger, J. Noe, A. Högele, and Peter Hommelhoff, Phys. Rev. Lett. 114, 227601 (2015). [3] J. McNeur, M. Kozak, D. Ehberger, N. Schönenberger, A. Tafel, A. Li, and P. Hommelhoff, J. Phys. B 49, 034006 (2016).

### Recovering high harmonic emission from Lorentz-force effects using noncollinear beams

Pisanty, Emilio

High harmonic radiation is a useful source of short pulses of high-frequency XUV radiation, and the cutoff frequency should increase when the driving field is stronger and has a longer wavelength. Unfortunately, in those regimes the magnetic field of the driver acts to deflect the electron excursion and prevent its recollision with the core, thus inhibiting harmonic emission. We show that the combination of two non-collinear counter-rotating circularly polarized beams produce a small forwards ellipticity, that can be used to probe, measure, control, and cancel the effect of this magnetic field. This is then a flexible scheme to re-enable harmonic emission deep in the long-wavelength and strong field regimes. [1] E. Pisanty, D.D. Hickstein, B.R. Galloway et al. High harmonic interferometry of the Lorentz force in strong mid-infrared laser fields. arXiv:1606.01931.

### Spontaneous transitions in atomic systems in the presence of high intensity laser field

Popov, Alexander

New approach to the study of the spontaneous emission of the quantum system driven by a high intensity laser field is developed. This approach is based on the accurate consideration of quantum system interacting with vacuum quantized field modes in the first order of perturbation theory, while the intense laser field is considered classically. The proposed approach is applied to study of a number of quantum systems in intense laser field (for example, strong-field atomic ionization). Accounting for spontaneous processes is relevant for the analysis of various nonlinear processes in laser plasma, starting from the spontaneous emission. On the other hand, interest in such research arises from the fact that the spectrum of atoms in a strong field is reconstructed significantly, and the spontaneous emission can serve as a diagnostic tool for these "dressed" by field atomic systems. At the same time not only atomic systems are planned to be considered but also nanophysics objects such as quantum wells, quantum wires, clusters, etc. It is supposed to extent the developed approach to the study of relaxation processes in quantum systems.

Rubisch, Andreas

### The X-Ray Movie Camera: A new experimental setup for filming ultrafast dynamcis of individual clusters

Sauppe, Mario

Imaging ultrafast dynamics in nanoscale systems resides at the forefront of X-ray science, aiming at the ultimate goal of the “molecular movie“. Here we present a novel coherent diffractive imaging experiment using high intense and ultrashort pulses of the FLASH free-electron laser. The detection setup allows to avoid the superposition of the images of initial and final state by using a two-detector geometry. Investigating single xenon nanoclusters as a modelsystem, we recorded on one detector the inital state, delivering information about size, shape and exposed intensity. On the second detector the ultrasfast dynamics is captured. Time delays from fs regime to 650 ps delay were enabled by a new multilayer mirror based, permanently at FLASH installed, split-and-delay unit called DESC. The poster gives an overview of the experimental setup and discusses first results. Publication in preparation: Mario Sauppe, Katharina Kolatzki, Bruno Langbehn, Maria Müller, Björn Senfftleben, Anatoli Ulmer, Jannis Zimbalski, Julian Zimmermann, Tobias Zimmermann, Leonie Flückiger, Tais Gorkhover, Christoph Bostedt, Cédric Bomme, Stefan Düsterer, Benjamin Erk, Marion Kuhlmann, Daniel Rolles, Dimitrios Rompotis, Rolf Treusch, Torsten Feigl, Thomas Möller, Daniela Rupp

### Electromagnetic radiation of electrons in Laguerre-Gaussian beams after strong-field tunnel ionization

Schulze, Dominik

We study the propagation and electromagnetic radiation of electrons from an ionized gas in two partially superimposed Laguerre-Gaussian beams. The dynamics of the electrons in the high intensity region of the laser fields results in a change of the angular momentum of those. Therefore the electromagnetic radiation reflects both the optical angular momentum of the beams and the position of the electrons in the beams. Depending on the initial conditions after the tunnel ionization and strength of the Laguerre-Gaussian beams, the electron can recombine into Rydberg states.

### Controlling and Observing Petahertz Electronics with Light Fields

Siegrist, Florian

The frontiers of electronics lie in the interaction of light waves with solid state materials. By applying ultrashort laser pulses, we are exploring how solid state electron phenomena can be manipulated and observed on the sub-femtosecond time-scale, corresponding to the petahertz frequency regime. This is enabled by the new regimes of optical field strength and the rapid switch-off of the excitation. Attosecond streaking provides access to the exact electric field evolution of light waves. By observing how these waves are modified by their nonlinear interaction with a material, it is possible to trace back to the exact driving term behind the changes in the light-wave: the nonlinear polarization. This nonlinear polarization gives insight into the interaction dynamics and also the exchange of energy between the light pulse and matter, which is indicated by the timing of the nonlinear response. In a follow-up experiment we want to use a few-cycle laser pulse with a defined non Gaussian time profile to look closer into these processes. We employ the approach of waveform synthesis where the broadband spectrum is divided into two individually uncompressed band parts. The phase of the light field in each channel is individually manipulated with customized chirped mirrors. Afterwards the different pulses are recombined. By varying the delay between them, various waveforms can be tailored. Another aspect that we will investigate is the transition between linear and nonlinear optical response, also in technologically relevant small-gap semiconductors. Key Publications: A. Sommer et al., Nature 534, 86 (2016). A. Schiffrin et al., Nature 493, 70 (2013).

### Influence of field-induced orbital distortion in strong-field phenomena

Spiewanowski, Maciej

### Above-threshold ionization processes in diatomic molecules driven by strong laser fields

Suarez, Noslen

In this work we develop a general theory of above threshold ionization (ATI) processes for symmetric diatomic molecules, based in the Strong Field Approximation (SFA), for direct and re-scattering transition amplitudes. Our analytical derivation allows us to disentangle the different processes contributing to the total spectra, amongst other capabilities, it allow us adjust both the internuclear separation and molecular potential in a direct and simple way. In addition, our approach directly underpins different underlying physical processes that correspond to (i) direct tunneling ionization; (ii) electron rescattering on the center of origin; and, finally, (iii) electron rescattering on a different center. Comparison of the results with the time-dependent Schrodinger equation (TDSE) are in good agreement. Our model captures also the dependence of the ATI spectra on the internuclear distance making possible to retrieve the bond length of the molecule.

### Complex-time soft recollisions in strong field ionization of atoms

Tulsky, Vasily

\underline{V.A. Tulsky}$^1$, S.V. Popruzhenko$^1$, Th. Keil$^2$, D. Bauer$^2$\\ $~^1\textit{Moscow Engineering Physics Institute, Moscow, Russia}$\\ $~^2\textit{Institute of Physics, University of Rostock, Germany}$\\ We consider recollisions of a photoelectron driven by an intense laser field with linear polarization taking into account the electron-ion interaction nonperturbatively, via the Coulomb-corrected strong field approximation [1]. In this case recollisions with a small momentum transfer can only be consistently treated as apearing in complex time and space [2,3]. The respective calculation of the Coulomb-corrected photoelectron action requires knowing the topology of branch points and branch cuts of the Coulomb potential energy considered as a function of complex time [2,3]. In this work, we discuss the choice of the integration contour in complex time taking this topology into account and show how to avoid divergences resulting from very close approaches of the photoelectron to the parent nucleus. We show also how these soft recollisions influence the direct ionization spectrum making it, under certain parameters, qualitatively different from the prediction of the plane strong field approximation [4] or of the tunneling theory. \\ $\left[1 \right]$ S.V. Popruzhenko, D. Bauer, $\textit{J . Mod. Opt. } \textbf{55}$, 2573 (2008).\\ $\left[2 \right]$ S.V. Popruzhenko, $\textit{JETP } \textbf{118}$, 580 (2014).\\ $\left[3 \right]$ E. Pisanty, M. Ivanov, $\textit{Phys. Rev. A } \textbf{93}$, 043408 (2016).\\ $\left[4 \right]$ L.V. Keldysh, $\textit{Sov. Phys. JETP } ~\textbf{20}$, 1307 (1965); F.H.M. Faisal, $\textit{J. Phys. B } \bf{6}$, L89. (1973); H.R. Reiss, $\textit{Phys. Rev. A } \textbf{22}$, 1786(1980).

### Photodynamics of few-atom doped He nanodroplets investigated by photoelectron spectroscopy and imaging

Vimal, Mekha

We present some preliminary results of our recent investigations of He nanodroplets doped with few atoms, as well as prospects of some forthcoming investigations. He nanodroplets which are superfluid clusters of He atoms have played the role of ideal spectroscopic matrices owing to their unique properties such as a high degree of solvation across a wide range of neutral and ionized atomic systems, sub-Kelvin temperatures and transparency in the UV, visible and infrared regions. However, recent investigations have shown that these host droplets exhibit very intriguing photoelectron dynamics upon direct or indirect excitation due to a plethora of quantum processes which become possible upon excitation by EUV and soft-xray photons. Even with single photon excitation, the migration and transfer of charge between the host He atoms and doped atoms or molecules leads to inter-atomic electronic dynamics which we seek to understand in detail. Here, we report some specific examples of insights gained in studying few-atom doped He nanodroplets by photoelectron imaging and spectroscopy. (This work was performed in a joint collaboration between Uni. of Freiburg, Max Planck Institute für Kernphysik Heidelberg and Indian Institute of Technology ­ Madras, India) References­ i) D Buchta et al., The Journal of Chemical Physics 139 (8), 084301. ii) D Buchta et al., The Journal of Physical Chemistry A 117 (21), 4394­4403.

### The attosecond regime of impulsive stimulated electronic Raman excitation

Ware, Matthew

We have used the Multiconfiguration Time-Dependent Hartree-Fock (MCTDHF) method to calculate transition probabilities for impulsive stimulated electronic Raman scattering (SERS) through a structured continuum. By developing a rate-equation based model for impulsive SERS, we are able to show that attosecond SERS is dominated by continuum transitions and not autoionizing resonances which has interesting corollaries to non-resonant stimulated vibrational or rotational Raman scattering. Such rate-equation models are easily extended to larger molecular systems where implementation of MCTDHF can be computationally expensive. Our results indicate that attosecond SERS transition probabilities may be understood in terms of two-photon generalized cross sections even in the high-intensity limit for extreme ultraviolet wavelengths. At x-ray wavelengths, there exists a local maxima for electronic Raman immediately below the K-edge that can be contributed to significant AC-Stark shifting, enabling SERS for long duration x-ray pulses.

### Semiclassical dynamics and the local momentum approximation for atomic ionization

Zagoya Montiel, Carlos

The semiclassical propagator in the Herman and Kluk form relies on an integral over the whole phase space of initial coordinates. In problems involving time-dependent fields, such a calculation requires a large number of initial conditions (of the order of $10^7$) in order to achieve convergence. Here, we show that one can significantly reduce the effort in the calculation by evaluating the integral not over the whole phase space of initial conditions but only over a smaller manifold $\Gamma\equiv\{({\bf q_0}, {\bf p_0})\in{\bf R}^{2N}\; |\;{\bf p}_0=\nabla \phi({\bf q}_0)\}$, with $\phi({\bf q})$ defined according to $\Psi({\bf q},t=0)=A({\bf q})\exp[i\phi({\bf q})]$. We present examples in which such an approach is appropriate.

### Machine Learning assisted classification of diffraction images

Zimmermann, Julian

Since the recent advent of short wavelength Free-Electron-Laser (FEL) it is possible to obtain high-intensity x-ray pulses with femtosecond duration. This allows for Coherent Diffraction Imaging (CDI) experiments on individual nanosized objects with a single x-ray laser shot. The LINAC Coherent Light Source (LCLS), for instance, has a repetition rate of 120 Hz and a typical hit-ratio of 20 % [1,2] . The soon-to-be opened European XFEL facility will even top that with a maximum repetition rate of 27 000 Hz [3] . This may add up to several million diffraction pattern in a single 12 hour shift. While storing a huge amount of data is easier and faster than ever, this monumental data volume represents a huge problem for the analysis of the data. We here propose a workflow routine to drastically reduce the amount of work needed for categorizing huge data-sets of diffraction patterns. First a classification and viewer tool (’The ClassificatoR’, CR) provides an efficient GUI for classifying a handpicked selection of high quality diffraction pattern. These patterns are then used as training data for a Residual Convolutional Neural Network (RCNN). The RCNN is designed to be able to classify the remaining data for efficient indexing and subsequent analysis. Training of the RCNN is based on a new technique called residual learning. It may provide a very high quality of classifications even with a relatively small training data-set [4,5] . First performance evaluations are done using data from a single-shot wide-angle scattering CDI experiment on silver clusters conducted in 2015 at the FLASH facility in Hamburg. [1] Emma, et al. First lasing and operation of an ångstrom-wavelength free-electron laser. Nat. Photonics, 4(9):641–647, 2010. ISSN 1749- 4885. doi: 10.1038/nphoton.2010.176. [2] Bostedt, et al. Linac Coherent Light Source: The first five years. Rev. Mod. Phys., 88(1):015007, 2016. ISSN 0034-6861. doi: 10.1103/ RevModPhys.88.015007. [3] Schneidmiller. Photon beam properties at the European XFEL. Technical report, XFEL, Hamburg, 2011. [4] He, et al. Deep Residual Learning for Image Recognition. 2015. [5] Szegedy, et al. Inception-v4, Inception-ResNet and the Impact of Residual Connections on Learning. 2016.