STSM by Károly Tokési, ATOMKI (HU) with Christoph Lemell, Vienna University of Technology (AT)
On April 18, 2017 (11 days)
From HUNGARY to AUSTRIA

Photoionization using attosecond streaking technique

The goal of the STSM was the investigation of the photoionization of water using attosecond streaking technique. We have determined all necessary input parameters in order to simulate photoelectron transport through water. We calculated the total and angular differential elastic and inelastic scattering cross sections with the elastic and inelastic mean free paths.

 During the STSM we improved our classical transport code including the special boundary conditions of our liquid system. We also improved the input data of the calculation, focusing on the lower energy range of the electron transport simulation. We took into account both elastic and inelastic collisions during the simulation. For the case of the elastic scattering of electrons we used the static field approximation with non-relativistic Schrödinger partial wave analysis. For the case of inelastic scattering we used the dielectric response formalism. We developed a new computer code to study the interaction between laser and water.

 The present STSM was very successful. The proposed objectives were achieved. A classical transport code was improved, which allows the theoretical study of electron emission from liquid water excited with external laser fields by streaking technique.

We provided an important contribution to the deeper understanding of the physics of electron emission in laser-water collisions. The material science community, in particular the physics of condensed matter, and also applied research in atomic manipulation on surfaces will clearly gain from this new understanding.

The results will be published in international journals. The support of the COST Action will be greatly acknowledged in the publications.

STSM by Francisca Mota-Furtado, Royal Holloway-University of London (UK) with Bernard Piraux, Université Catholique de Louvain-la-Neuve (BE)
On March 19, 2017 (14 days)
From UNITED KINGDOM to BELGIUM

The population of Rydberg states as a function of the ellipticity of an intense field in the quasi-static limit

The creation of Rydberg states following interaction with an intense field is currently of topical interest in particular with respect to the proposed mechanisms at their origin, such as frustrated tunnelling or multi–‐photon excitation. We study the population of Rydberg states as a function of the ellipticity of the laser field and of its intensity, for long and short pulses, and we compare our results with recent theory and experiment.

For linear polarization, by solving the time dependent Schrödinger equation (TDSE), we have shown that multi–‐photon excitation plays an important role in explaining the yield of Rydberg states as a function of intensity and pulse length for intense laser fields in the quasi–‐static limit (with wavelengths as long as 1800 nm). For elliptically polarized light, we have compared our quantum results with semi–‐classical approaches and although we obtain a gaussian distribution as a function of ellipticity as predicted semi–‐classically, we see important differences with semi–‐classical predictions in the dependence on the pulse length and the intensity of the pulse.

The STSM has led to a new paper on population trapping in the quasi–‐static limit for linearly polarized pulses which has been sent for publication to Physical Review A. A second paper on the dependence on the ellipticity of the polarization of the pulse is currently in preparation.

Figure: The total excited state probability as a function of ellipticity for an 800 nm laser pulse with an intensity of 2×10¹⁴ W/cm² and for two pulse lengths (LHS) and (RHS) for a fixed pulse length but at two different intensities.

STSM by Johann Förster, Institut für Physik, Berlin (DE) with Piero Decleva, Universita’ di Trieste (IT)
On January 2, 2017 (89 days)
From GERMANY to ITALY

Molecules in short, intense elliptically polarized laser fields

Exposing molecules to intense, ultrashort laser fields is a promising tool to image and control (at this point small) molecules and possibly (in the future) chemical reactions. The orientation-dependence of ionization and elliptically polarized fields which are investigated theoretically within this STSM (for NH_3) directly reveal interesting structural information and dynamics caused by the field.

The method developed in collaboration between the groups of A. Saenz (Berlin) P. Decleva (Trieste) solving the time-dependent Schrödinger equation describing small molecules in intense laser fields (within the single-determinant approximation) is extended within this STSM. It is now possible to treat the orientation dependence of molecules belonging to symmetry groups containing degenerate irreducible representations (like the C_{2v} group of NH_3). Furthermore, time propagation using elliptically polarized laser fields is implemented.

The orientation-dependent ionization yields for linear polarization and propagation-direction-dependent ionization yields for elliptical/circular polarization (examples shown in the figure)  have been studied extensively. Ionization dominates for polarization directions parallel to the  inversion (x-)axis with a strong asymmetry visible for extremely short 2-cycle pulses. Obtained  results will be submitted for publication soon and the new code version allows for interesting future studies, e.g. ionization of chiral molecules exposed to elliptically polarized pulses.

As a result od this STSM a thesis project shall be defended at Humboldt-Univ., Berlin, under the supervision of Alejandro Saenz and with collaboration of P. Decleva (Univ. Trieste).

STSM by Patrick O’Mahony, Royal Holloway University of London (UK) with Bernard Piraux, Institute of Condensed Matter and Nanosciences (BE)
On February 5, 2017 (15 days)
From UNITED KINGDOM to BELGIUM

Sturmian bases for three-electron systems in hyperspherical coordinates

With the advent of new attosecond laser sources in the XUV it has become possible to excite several electrons simultaneously with the possibility of creating hollow atoms as was done for example with lithium using synchrotron radiation. To study such correlated wavepackets in time requires a concise and compact description of the many body problem. To this end the purpose of the visit was to initiate a new collaborative program to construct angular Sturmian functions in hyperspherical co-ordinates for 3-electron systems

We replaced the spherical coordinates  by the hyperspherical coordinates and constructed the angular Sturmian functions, , which account for the interaction of each of the electrons with the nucleus and the leading order term in the electron-electron interactions for each pair of electrons. They form a very compact complete basis when combined with radial Sturmians in the coordinate .

The STSM has led to a new code to construct the angular Sturmian functions. The next step is to construct properly anti-symmetrised basis functions so as to calculate the lowest eigenvalues for given LS states of lithium and He^– before tackling the time dependent problem of hollow atoms created by XUV pulses.

STSM by Marin Sapunar, Ruđer Bošković Institute (HR) with Piero Decleva, University of Trieste (IT) 
On May 2, 2016 (18 days)
From CROATIA to ITALY

Simulation of time-resolved photoelectron spectroscopy along nonadiabatic dynamics trajectories

The goal of the two week scientific mission was preparing a code for simulating photoelectron spectroscopy observables along nonadiabatic dynamics trajectories. To achieve this, changes had to be made in the input part of the code developed by prof. Decleva and co-workers for calculating photoionization observables to accommodate easy interfacing with output from trajectory based nonadiabatic dynamics simulations.

A new subroutine was added to the code based on the separation of the B-spline basis into two parts, a set of atom centred knots for the description of the short range interactions and a grid of linear knots for the description of the continuum. Convergence of the basis set was tested for three key geometries taken from a nonadiabatic dynamics simulation of the pyrrole molecule.

The modified code is easy to interface with a nonadiabatic dynamics program and can be used to calculate photoionization observables without losing accuracy for a wide range of geometries using a benchmark method. Once the basis set is shown to be converged on a single (or a few) geometry of a system of interest, it is safe to assume that accurate results can be obtained along an entire dynamics run.

STSM by Armin Scrinzi, Ludwig Maximilians University (DE) with Fernando Martin, Universidad Autonoma de Madrid (ES)
On April 11, 2016 (19 days)
From GERMANY to SPAIN

Absorption and electron spectra for small chemical systems

The momentum spectra of fragments emitted in laser-matter interaction are a main source of information about the internal dynamics. With tSurff a new and powerful theoretical tool was developed. The STSM was to introduce and implement the method in the group at UAM.

Implementations were designed and partially finished for emission from single- and multi-particle systems. This will, in certain applications, allow substantial speedup of calculations at UAM. Use with the XChem  code was laid out and implementation is in progress. In turn, the for LMU’s tRecX code, structural adaptions for a new interface to standard quantum chemistry structure were intiated. In a third line of development, adopting tSurff for breakup was discussed.

Outcome: the expertise for using tSurff has been successfully transferred to the UAM group. A collaboration for breakup has been agreed on. A new strategy for a quantum chemistry interface for tRecX has been adopted.

Figure: tSurff for break up: partitioning of configuration space  (from an internal report)

STSM by Alejandro Saenz, Humboldt-Universität zu Berlin (DE) with Piero Decleva, Universita’ di Trieste (IT)
On March 6, 2016 (6 days)
From GERMANY to ITALY

Photoelectron spectra for REMPI processes in (chiral) molecules

Chirality (the distinction of left and right mirror images) is one of the fascinating aspects of biology, since enzymes possess a 100% selectivity with respect to the stereochemistry of molecules. While the left-handed version of some molecule may be a very powerful pain reliever, its right-handed counterpart may be fully ignored by the human body, or, even worse, it may have fatal consequences.

Therefore, analytical tools for a very efficient and sensitive detection of the absolute configuration of a molecule is of great practical importance.

Some years ago the research group of Th. Baumert (Hassel, Germany) reported an asymmetry in the photoelectron angular distributions between two enantiomers of camphor as well as fenchone in (2+1) resonantly-enhanced multiphoton ionization. The similarity of the asymmetry may indicate some universal pattern which may pave a path to a sensitive detection tool for the absolute configuration of different enantiomers. In order to investigate this even further, it is of great interest to set up an approach that allows for the calculation of photoelectron angular distributions in resonant multiphoton ionization.

Within this STSM in Trieste a numerical approach for treating (2+1) resonantly enhanced multiphoton ionization, especially for extracting photoelectron spectra, was formulated and implemented. Two data sets for testing (hydrogen atom and the chiral molecule BrClFCH) were generated. During the STSM first tests for hydrogen atoms were performed by comparing the results with the ones of full solutions of the time-dependent Schroedinger equation. Furthermore, the STSM was used to clarify the strategy for extracting photoelectron distributions from the common Trieste/Berlin single-determinant time-dependent Schroedinger equation solver for arbitrary molecules and to prepare the resubmission of a joint publication.