
Strong fields


Chair: Ulf Saalmann

09:00  09:25

Gergana D. Borisova
(Max Planck Institute for Nuclear Physics Heidelberg)
The role of initialstate electron correlation for strongfield ionization
Electron correlation plays a fundamental role in lightmatter interaction. To gain new insights into the role of the initial state for the ionization process, we conducted a twocolor extreme ultraviolet (XUV)infrared (IR) experiment using a reaction microscope (ReMi) to study IR strongfield ionization out of selectively prepared doubly excited states in helium in the XUV energy region between 59 eV and 80 eV, with XUV light provided by the freeelectron laser in Hamburg FLASH. Both single and doubleionization have been observed and the impact of different ionization mechanisms will be discussed, also in comparison with model calculations.

09:35  10:00

Soumi Dutta
(Max Planck Institute for the Physics of Complex Systems (MPIPKS))
Attoclock Induced Electron Dynamics
Theoretical and experimental studies on intense laser atom interaction have drawn many interests over the past few decades. We consider strongfield tunnel ionization and explore two problems dealing with the ionized electron’s dynamics in the presence of infrared, highintensity, ellipticallypolarized laser pulses applied to an attoclock setup.
In the first problem, we obtain the electron’s dynamics from a static potential, describing the complicated fielddriven dynamics by a simple timeindependent Hamiltonian. For the second problem, we set up an analytical expression for the ‘attoclock offset angle’.
We use the timedependent KramersHenneberger (KH) potential, which provides an exact description of the electron’s trajectory, and show how some approximations within the KH potential lead to the static potential and the analytical offset angle. We also elucidate good agreement of our theory with the numerical results obtained from classical equations of motions. Finally, the comparison with available experimental data has led to interestingly new tunnelexit conditions different from the conventional models.

10:10  10:35

Patrick Rupprecht
(Max Planck Institute for Nuclear Physics Heidelberg)
Effective exchangeinteraction control in molecules with ultrashort laser pulses
While electronelectron interactions play a fundamental role in any atom beyond hydrogen, they also govern molecular structure and reactivity. We introduce and experimentally demonstrate a general concept to control multielectron interaction by intense, ultrashort laser fields. In particular, strong coupling to excited states allows to modify the effective exchange energy by infrared (IR)induced valence orbital mixing. For a proofofprinciple, we focus on the sulfur hexafluoride molecule SF$_6$, considering the coupling of a sulfur 2p core hole with a valence excited electron, using a combination of soft xray and IR laser pulses. The IR laser intensity represents a control knob to tune the effective exchange interaction energy, resulting in a characteristic change in the spinorbitsplit oscillator strength ratio that is directly quantified in the experiment. Such direct control of effective electronic interactions and correlation is a significant step towards laserdirected chemistry on the
fundamental electronic level with singleatomic site selectivity.

10:45  11:15

Coffee break


Chair: Christian Ott

11:15  11:40

Jonathan Dubois
(Max Planck Institute for the Physics of Complex Systems (MPIPKS))
Making nonadiabatic photoionization adiabatic
We consider the process of tunnel ionization of atoms driven by circularly polarized (CP) pulses. An intuitive semiclassical picture of tunnel ionization by a static laser field is an electron ionizing through the potential barrier induced by the laser field with constant energy, referred to as the adiabatic ionization. When the laser field alternates in time, such as it is the case for CP pulses, the energy of the electron changes in time, and at the tunnel exit, it is distributed in a range of energy on the order of the ponderomotive energy of the laser. The adiabatic picture no longer holds, and the ionization process is referred to as nonadiabatic. Extensive theoretical and experimental studies are performed in the attosecond community to probe and understand these nonadiabatic effects in photoionization.
Our goal is to understand nonadiabatic processes in CP pulses using electron trajectories in the combined laser and Coulomb fields. We map the electron dynamics in a frame which rotates with the laser field, referred to as the rotating frame (RF). Our results show that in the RF, counterintuitively, the energy of the electron is constant during tunnel ionization, and as a consequence follows the picture of adiabatic ionization. This allows us to understand and predict, for instance, the role played by ring currents in atoms and the shape of the laser envelope, and to shed light on classicalquantum correspondence.
J. Dubois, U. Saalmann, J.M. Rost.

11:50  12:15

Ulf Saalmann
(Max Planck Institute for the Physics of Complex Systems (MPIPKS))
What is the Coulomblasercoupling time? (Is there any?)

12:30  13:30

Lunch break

13:30  14:00

Informal discussions


Timedependent dynamics


Chair: Panagiotis Giannakeas

14:00  14:25

Maximilien Barbier
(Max Planck Institute for the Physics of Complex Systems (MPIPKS))
Dynamics of wave packets through a timedependent absorbing barrier
The coupling between a moving atom and a laser beam can strongly modify the dynamical properties of the atom. A complete description of the state of the atom must take both its centerofmass as well as its internal dynamics into account, and may thus be rather challenging to achieve. In this talk we discuss how the dynamical properties of the atom can be conveniently studied within the framework of matterwave absorption. We present a model that describes the interaction between a nonrelativistic structureless quantum particle and a thin timedependent absorbing barrier. The barrier represents a localized laser beam with a timedependent intensity, and is characterized by a timedependent aperture function that embeds the absorbing properties of the barrier. The main advantage of this model is that it is exactly solvable. We derive in particular a simple analytical expression of the Husimi distribution of the particle in the case where the aperture function consists of a succession of two narrow Lorentzians, hence representing a double slit in time. This allows us to analytically investigate the phasespace structure of the resulting diffraction pattern.

14:35  15:00

Mohammad Reza Eidi
(Max Planck Institute for the Physics of Complex Systems (MPIPKS))
A fast and highly accurate optimizer for stype Gaussian basis sets
I will talk about a new algorithm that finds quickly the best optimal configuration of stype Gaussians that represent the wavefunction of singleelectron systems with high accuracy.

15:10  15:35

Lipeng Chen
(Max Planck Institute for the Physics of Complex Systems (MPIPKS))
Wavepacket dynamics at conical intersection and its spectroscopic manifestation
The effect of a dissipative environment on the ultrafast nonadiabatic dynamics at conical intersections is nowadays well understood. Yet, the molecular wavepacket dynamics at conical intersection has not been directly characterized and monitored via femtosecond nonlinear spectroscopy. We have combined the hierarchy equation of motion, the multiple Davydov ansatz, and the Multiconfigurational Ehrenfest dynamics with the nonlinear response function formalism to simulate fourwavemixing (4WM) signals for a set of conical intersection systems which are coupled to the thermal bath. Our results show that the signals indeed visualize evolutions of the wave packet at conical intersections, and these evolutions are quite sensitive to the systembath coupling strengths.

15:45  16:15

Coffee break


Open Systems


Chair: Matthew Eiles

16:15  16:40

Sebastian Gemsheim
(Max Planck Institute for the Physics of Complex Systems (MPIPKS))
Prerequisites for time emergence in quantum and classical mechanics
Is time fundamental or emergent? This is an old question but to date the answer remains elusive. Advocating for the latter, we put forth a set of basic prerequisites necessary for its emergence and examine their implications in quantum and classical mechanics.
In such timeless approaches, the unitary dynamics are derived from the description of a constrained global state of a bipartite system, comprised of a quantum `clock' and a generic quantum `system’. Guided by the fact that every determination of change in time is made in relation to the reading of a clock, the central notion is that of a system state conditioned on the state of a clock. As a result, the concept of time or, more concretely, dynamics emerges from the inherent correlation between both subsystems.
In particular, we demonstrate the emergence of the Schrödinger, von Neumann and Liouville equation for pure quantum states, quantum density operators and classical phase space densities, respectively. A numerical illustration is given for a paradigmatic model from atomic physics.

16:50  17:15

Vladislav Sukharnikov
(DESYCFEL)
Generalized second quantization of open quantum systems
Second quantization is traditionally applied to model closed quantum systems of identical particles. There are several benefits associated with this approach, among them dimensionality reduction due to permutation symmetry and representation of physical operators in terms of bosonic (or fermionic) creation and annihilation operators. While the advantages of the first point are evident, the bosonization is crucial when complexity of the chosen method of description directly depends on commutation relations of operators (as in Heisenberg equations of motion, phasespace methods, etc).
In order to describe realistic quantum dynamics, one has to account for the effects stemming from the interaction with the environment. The interaction of real quantum systems with the environment gives rise to numerous incoherent phenomena (decoherence, dissipation, incoherent pumping, damping, etc.), that in many situations have a significant impact on the behaviour of the system. In this case, one has to adopt the density matrix formalism, which is capable of describing incoherent processes. However, inclusion of incoherence leads to formation of states, which cannot be constructed out of symmetric pure states. This is the point where the traditional second quantization fails, since it applies only to pure quantum states. In several works, for instance in [12], it was discovered that under the assumption that dissipation preserves the permutation symmetry (i.e. particles are still identical), one may naturally introduce occupation number representation for the density matrix. However, as we have mentioned before, the traditional second quantization is not applicable. This fact motivates extension of the second quantization to open quantum systems.
In this talk, we present the method of the generalized second quantization, applicable to an open quantum system of multilevel particles. To that end, we supplement the Liouville space of the density matrix with additional algebraic objects, that enable us to formulate the procedure of quantization similar to the traditional one. As a result, we get numerous benefits, such as size reduction, bosonization and occupation number representation – all of which greatly simplify the analysis of a wide range of quantum master equations. The method proves to be a powerful and intuitive tool for the analytical and numerical analysis of various quantum master equations.
[1] M. Gegg and M. Richter, Scientific reports 7, 1 (2017).
[2] M. Bolaños and P. BarberisBlostein, Journal of Physics A: Mathematical and Theoretical 48, 445301 (2015).

17:25  17:50

Jonathan Brugger
(AlbertLudwigsUniversity Freiburg)
Dynamical signatures of longrange interactions in 1D
The dynamics of onedimensional fewparticle systems are key to understand the phenomenological differences between single and manybody systems, and ultimately the transition to the thermodynamic limes. While experimentally such systems become increasingly controllable, with particle interactions tunable from strongly attractive to strongly repulsive and short (contact) to longrange (Coulomb), exact numerical approaches are feasible but challenging.
In the first part of this talk we briefly comment on our exact treatment of two indistinguishable, Coulombinteracting bosons in a onedimensional trapping potential, by numerical diagonalization of the manybody Hamiltonian after discretization in an appropriate finite element basis. In the second part we show how the dynamics of a simple singleparticle observable allows us to distinguish longranged Coulomb from shortranged contact interactions. This bears new insights in the interactiondependence of fewbody dynamics, and can be verified via stateoftheart experiments with ultracold atoms.

19:00

Atomic Summer Camp dinner & discussions at
Fährgarten Johannstadt, KätheKollwitzUfer 23b, 01307 Dresden
