Reports

The following networking activities (STSM, Virtual Mobilities) have been approved to receive support from the European Union, through COST Action CA18222 funds (for a wider description, follow the links below or take a look to the News section ‘STSM/VM results’).

Theoretical assignment of the electronic spectra of aqueous uracil and state specific analysis of solvation effects

Mr Branislav Milovanović, Faculty of Physical Chemistry, University of Belgrade, Belgrade, RS.
Host Institution: Institute Rudjer Bošković (HR)

Due to their biological relevance, DNA nucleobases excited state dynamics is extensively examined both experimentally and theoretically. Understanding competing photophysical mechanisms that can lead to and prevent light-inducted damage of the DNA nucleobases is of interest because of their widespread potential biological and/or technological applications. Determination of the character of large number of the excited electronic states is a challenging problem for nucleobases and other polyatomic molecules in general. Thus, resolving this problem is one step forward in understanding physical consequences underlying electronic excitation for these systems. Using previously developed procedure (Phys. Chem. Chem. Phys. 2019, 21, 22782) we are able to automatically determine character of the excited electronic states of nucleobase uracil both in gas phase and aqueous solution.

This procedure informs us about: theoretical assignment of the electronic spectra; state specific analysis of solvation effects; effects of vibrational averaging, temperature and solvent-induced structural changes; state specific solvent shifts for the solvent sensitive nπ* electronic states. Electronic absorption spectra profiles are calculated and general remarks can be drawn: (1) vibrational averaging and temperature increase produce comparable effects on all examined excited states energetically stabilizing them; (2) bright electronic states (ππ* states) contributing to spectral profile are stabilized by the introduction of electrostatic effects while dark (nπ* states) are strongly destabilized; (3) apparent stabilization of the bright electronic states is present when introducing solvent effects while this effects are rather complex in the case of dark and delocalized states (Rydberg transitions).

Acquired data should provide more convenient insights into absorption spectra of the uracil and therefore foundation for studying photodynamics of uracil in aqueous solution and the possibility of the excited state driven N-H bond fission. Hopefully, detailed characterization of the electronic states involved in the relaxation of gas phase and aqueous uracil structures will shed new light on the active role of the chemical environment in the exited state deactivation events of nucleobases.

Total effect of solvation on the computed UV absorption spectra for uracil. Gas phase spectra at 0 K (black) and 298 K (red), spectra with electrostatic COSMO environmental effect (blue) and COSMO solution/optimized COSMO ground state geometries (green). Vertical bars represent reference geometry vertical excitation energies. Insets represents influence of the solvation on the energy of the selected excited states and chemical structure of uracil.

Charge migration encoded in quantumpath interferometry of molecular system

Ms Leena Abusin, King’s College London (London), UK.
Host Institution: Sorbonne University Pierre and
Marie Curie Campus (FR)

The objective of this STSM was to device a code that would retrieve the ultra fast dynamical information after ionisation of molecules subject to ultrafast laser fields. The process of inonisation can result in attosecond resolved spectroscopy such as High Harmonic Generation. Here the electron of the ion recombines with itself after ionization thus releasing a XUV photon. Recreating harmonic spectra will be the main application of the code.

Development of Artificial Intelligence-based Approaches for Ultrafast X-ray Photoelectron Spectroscopy in Large Molecules

Dr. Solène Oberli, from Ecole Polytechnique Fédérale de Lausanne (Lausanne) (CH)
Host institution: Universidad Autonoma de Madrid (ES)
Dates: From 2021-03-26 – to 2021-05-26

Molecules are composed of nuclei and electrons which form chemical bonds. Their motions are as fast as 10-15 and 10-18 seconds, respectively, and govern the outcome of chemical reactions. In order to observe these ultrafast processes, we thus need very special and powerful tools. In particular, X-ray Free Electron Lasers (XFELs) are able to generate highly intense ultrashort X-ray pulses – or bursts of light – by accelerating electrons up to relativistic speed and sending them through a sequence of magnets. To probe in real time the electron and nuclear rearrangements, pulse duration should be in the same order or shorter than these motions. In addition to this extremely high temporal resolution, X-ray pulses offer atomic spatial resolution, which allows us to trigger and probe the dynamics at a particular site in the molecule. X-rays interact primarily with core electrons which are deeply localized close to the nuclei. The binding energy of these electrons is specific of the element, such that we can control which atom is ionized in the molecule by varying the energy of the X-ray photon. Moreover, the binding energy of core electrons is sensitive to the local chemical environment. The X-ray Pholoelectron Spectroscopy (XPS) technique exploits this property: by measuring the energy of the electron ejected from the molecule by the X-ray, we can observe the local electron redistribution and changes in nuclear geometry while the chemical reaction takes place.

In this project, we studied the dynamics taking place in cationic states of a molecule using a time-resolved XPS. In this way, we are able to grab a ‘movie’ of the coupled electron and nuclear dynamics. In order to simulate the processes in play, we use a surface-hopping semiclassical propagation scheme1, in which the electron motion is treated quantum mechanically, while the nuclear dynamics is performed classically using a swarm of independent trajectories. These simulations are very expensive and may be hard to converge. To overcome this limitation, we started to supplement the propagation code with a Deep-Learning algorithm to accelerate the calculations and made it suitable for the treatment of larger molecules. Deep-Learning models are based on Neural Networks which are biology-inspired statistical algorithms. We believe that the use of Artificial Intelligence-based methods coupled to traditional quantum/semi-classical algorithms is a promising avenue for studying ultrafast phenomena in complex systems, and more generally to investigate the fundamental properties of matter.

Probing ultra-fast coherent dynamics in core-excited and Rydberg states with relativistic attosecond transient absorption

Dr. Felipe Zapata, from Lund University (SE)
Host Institution: Universidad Autonoma de Madrid (ES)
Dates: From 2021-05-03 – to 2021-06-18

Femtochemistry opened the possibility of studying molecular motion using ultra-short (10⁻1515s) laser pulses. Such studies enable the comprehension of the relative atomic rearrangement during chemical reactions. Attophysics, as a natural continuation of femtochemistry, aims to unravel the electron motion within atoms and molecules by using attosecond (10-18s) laser pulses of coherent extreme ultraviolet (XUV) radiation. Attosecond transient absorption spectroscopy (ATAS) can be used to study electron coherence and motion in atoms. In a typical ATAS scenario, an intense laser field is use to prepare an electron wave packet in a specific state of the target system, while an
XUV pulse is used to probe the dynamics. By varying the time delay between pulses, timedependent light-matter interaction phenomena can be explored.
Du to the complexity of electron dynamics, the development of theoretical methods are crucial to understand the results obtained experimentally. All theoretical studies so far are based on nonrelativistic ATAS theory. Nevertheless, these theories are not sufficient to properly describe phenomena such as the spin-orbit coupling. The importance of the spin–orbit coupling was demonstrated by the first ATAS experiment, which targeted krypton.
The recent theoretical developments carried out in our group at Lund University based on the relativistic time-dependent Dirac equation allows us to describe spin-orbit coupling beyond the simplistic but well established “few states models”.
Prof. Fernando Martin’s group expertise on ATAS, together with the novel theoretical methods developed in Lund, allow us to simulate spin-orbit resolved attosecond transient absorption spectra. Figure 1 contains the preliminary ATAS investigation of the 2s22p6-2s2p6np autoionization series in neon calculated with the relativistic time-dependent configuration interaction singles method.

Figure 1. Left panel: autoionizing 2s22p6-2s2p6np series in neon. Right panel: ATAS spectra with
different time delays calculated with the RTDCIS method.

Enabling photoelectron spectroscopy experiments of gas-phase biomolecular ions in a new crossed-beam apparatus at the PLEAIDES beamline of the SOLEIL synchrotron

Dr. Lucas Schwob, Hamburg (DE)
Host Institution: SOLEIL Synchrotron (FR)
Dates: From 2021-06-23 – to 2021-07-05

There has been a long-standing effort to investigate building blocks of biological macromolecules by spectroscopic techniques based on synchrotron radiation, such as photoelectron spectroscopy (PES), near-edge X-ray absorption fine structure (NEXAFS) and photoelectron-photoion coincidence (PEPICO). Unfortunately, the studies on small building blocks often cannot be directly extrapolated to the macromolecules that they form. For example, the electronic structure of polypeptides (proteins) is strongly influenced by peptide bonds that link the constituent amino acids. Nevertheless, producing large and fragile biomolecules, such as proteins and oligonucleotides (DNA strands), isolated under high vacuum conditions in order to perform spectroscopic techniques is experimentally challenging.

The scientists of the host institute at the PLEIADES beamline are currently developing a new custom-made apparatus to perform photoelectron-photoion coincidence experiments of electrosprayed biomolecular ions. Briefly, the new PLEAIDES instrument consists of an atmospheric pressure ESI source, a double radiofrequency ion-funnel and a rectilinear quadrupole ion guide. The ions delivered by the source are further transferred and refocused into the interaction region inside the EPICEA vacuum chamber by a custom-made system of lenses and deflectors hosted inside a differentially pumped chamber, designed and built on PLEAIDES. In the interaction region, the ion beam crossed orthogonally the photon beam from the synchrotron beamline.

During this STSM, we have thoroughly assessed all aspects of this new instrument, from optimizing the production and transmission of electrosprayed biomolecular ions, to testing several configurations of synchronous timing for pulsing the ion beam and the spectrometer electrodes’ voltage. The work achieved during the STSM paves the way for the future improvement of the setup which should give its first results this year.

 

Figure 1: Result of measurement performed in PEPICO mode with protonated Tannic Acid and 90 eV photons. a) ion time-of-flight spectra. b) Ion detector image. The vertical line is the extracted ion beam and the horizontal line is photoionized residual gas. c) electron detector image, essentially from residual gas.

Time-resolved Auger electron spectroscopy (TR-AES)

Dr. Bruno Tenorio, Danmarks Tekniske Universitet (DK)
Host Institution: Università degli Studi di Trieste (IT)
Dates: From 2021-07-11 – to 2021-08-09

The STSM had as main purpose the development of a collaboration project targeting the accurate simulation of autoionization processes as well as double core hole spectroscopic effects. The project is a collaboration led by Prof. Sonia Coriani from the Technical University of Denmark and Prof. Piero Decleva from the University of Trieste.

The results described in the report have been published in scientific journals where the activities funded by COST have been acknowledged. More details can also be found here: https://attochem.qui.uam.es/?p=3023.

Development of theoretical methodologies for the description and interpretation of time-resolved spectroscopic and scattering experiments in the femto- and attosecond time scale

Ms Anna Kristina Schnack-Petersen, Technical University of Denmark
Host Institution: Norwegian University of Science and Technology (NO)
Dates: From 2021-08-30 – to 2021-10-15

Simulations of molecular dynamics is of great importance when attempting to understand reaction mechanism. In order to perform these simulations molecular gradients for both ground and excited states must be available at a sufficiently high level of accuracy in order to offer useful insights into the molecular dynamics. A very popular method for predicting molecular properties is the coupled cluster singles and doubles (CCSD) method, which not only provides results in good agreement with experiments but is also feasible even for larger systems. For this STSM a new implementation of molecular gradients at the CCSD level of theory in the open source program eT has been optimized in order to ensure an efficient implementation feasible even for larger systems. Thus the first step towards molecular dynamics at the CCSD level of theory has been achieved.

Attosecond transient absorption spectroscopy via Wannier orbitals

Mr Giovanni Consalvo Cistaro, Universidad Autónoma de Madrid (ES)
Host Institution: École polytechnique fédérale de Lausanne (CH)
Dates: From 2021-09-15 – to 2021-10-15

When dealing with Bloch functions, it is common to have an interaction potential that diverges at some points of the reciprocal space. This is because the manifold in which the electrons wavepacket move presents discontinuities. As a consequence, it is very difficult to reach convergence for the electron dynamics simulations.
Wannier90 is a post-processing DFT tool that allows the interpolation of band structures and wave functions of an extended system. It has been developed by the host institution and provides a good solution to avoid discontinuities since the wavefunctions are interpolated with wannier functions, that are well localized in space; thus, their representation in reciprocal space does not present any discontinuity.
The program developed at the home institution during the last years propagates the electronic density matrix of an extended system under the presence of an external field. It needs as a starting point the bands and the position operator in the basis of Bloch functions, all observables that can be extracted after a wannierization. Building an interface of this program with wannier90 can lead to the possibility to perform realistic calculations to compare with experiments.

The results regarding this collaboration are mainly related to the observable of calculations performed with the code developed in Madrid, the ATA spectra, as well as some outputs derived from the calculation, for example, the density of electrons in direct and reciprocal space. The ATA spectra can be compared results already published. We can notice that there are differences in the spectra, related mostly to the change in the energy dispersion of the system, and some similarities. The behavior of the Van Hove singularity is similar, with some difference related to a different slope of the energy dispersion.
The movement of the electron in reciprocal space results is mostly the same. In the resolution given by the images, it is not possible to notice any significant difference.

Capturing satellite states in the Photoelectron Spectrum (CAPS)

Ms. Torsha Moitra, from Technical University of Denmark (DK)
Virtual Mobility
Dates: From 2021-09-07 – to 2021-10-12

The project has been successfully completed by fulfilling all the objectives in the proposal as described in the attached report. During this period a manuscript detailing the theory and results has been prepared and submitted, titled “Multi-electron excitation contributions towards primary and satellite states in the photoelectron spectrum.” (https://chemrxiv.org/engage/chemrxiv/article-details/620431b3a6fb4d0a2950892f). The work is now published in PCCP (https://pubs.rsc.org/en/content/articlelanding/2022/CP/D1CP04695K).

Potential of ultrafast electron diffraction to study the correlated electronic and nuclear motion in Uracil:Numerical Simulation

Dr Purbaj Pant, ELI Beamlines (CZ)
Virtual Mobility
Dates: From 2021-09-15 – to 2021-10-15

The virtual mobility grant has been a huge support in the pandemic era to eastablish the collaborating efforts. Through this grant I have managed to initiate a collaboration in a
completely new domain of research, which is far from my previous experiences and academic trainings. The results obtained within this Virtual Mobility are described in the VM report.

Attosecond transient absorption spectroscopy in topological materials via Wannier orbitals

Mr Giovanni Consalvo Cistaro, Universidad Autónoma de Madrid (ES)
Host Institution: École polytechnique fédérale de Lausanne (CH)
Dates: From 2021-12-01 – to 2021-12-15

The main goal of this short stay was to understand the main properties of topological materials, the classification that is used in literature to distinguish them and, as a final step, choose one or more that seems promising for its geometrical and topological properties, in order to find track of the non trivial topology directly from the ATA spectrum.

Weyl semimetals have some symmetry broken because of the spin-orbit coupling, and this produce two nodes in the Brillouin zone with non trivial topology. The fact that there are two nodes is caused by the properties of the spin-orbit coupling, that splits one node into two, and breaks the symmetry of the crystal. The Berry curvature is not zero, and this makes these materials interesting for a future study from their ATA spectra. We focused on type I Weyl semimetals, which have a band dispersion similar to the one of graphene, but the cone exists in 3D and this makes the material more appealing for its properties (in particular, TaAs and TaP).

Design of EUV pulse shaper

Ms. Krishna Khakurel, from ELI-Beamlines (Dolní Břežany, CZ)
Virtual Mobility
Dates: From 2022-04-01 to 2022-05-30

During the period of the virtual mobility intense discussion with the industrial partner AXO Dresden has been conducted.  The work done in the grant period shall be further extended and will be published in a peer reviewed journal. With the industrial partner, a proper grant application shall be made in order to seek the funding for the development of the proposed system.

Theoretical Study of the Ultrafast Hydrogen Migration in Photoionized Glycine

Mr Kossi Kety, from Gustave Eiffel University (Champs-sur-Marne, FR)
Host institution: Universidad Autonoma de Madrid (ES)
Dates: From 2022-05-02 – to 2022-06-30

The STSM will consist of two main tasks: the description of the ionization by the attosecond XUV pulse and the simulation of the following electron-proton dynamic

The main output of this scientific mission is the simulation of the hydrogen migration in glycine subjected to a specific attosecond pulse train of 1.5 fs. The effect of the near infra-red (NIR) probe will be included latter in order to achieve the full theoretical description of the XUV-pump/NIR-probe experiment. This time dependent theoretical modelling of charge migration in a biomolecule, which is the first step of our project contributes significantly to the COST Action 18222 objectives as specified in the Memorandum of Understanding (MoU).

This particular STSM will be the starting point of a collaboration between the research group of Prof. Fernando Martín at “Universidad Autonoma de Madrid”, the Theoretical Chemistry group of “Laboratoire de Modélisation et Simulation Multi Echelle” at “ Université Gustave Eiffel”, the “Institut des Sciences Moléculaires d’Orsay” and “Laboratoire Interactiions, Dynamiques et Lasers” at “Université Paris-Saclay”.

Ultrafast dynamics of solvated iron tris-bipyridine

Mr. Hui-Yuan Chen, from École polytechnique fédérale de Lausanne (EPFL) (Lausanne, CH)
Host Institution: Deutsches Elektronen-Synchrotron DESY (DE)
Dates: From 2022-05-09 – to 2022-05-27

We successfully established state-of-the-art time-resolved transient absorption spectroscopy for liquid nanosheets in this short-term scientific mission. The OH stretching mode of liquid water in real-time was revealved for the first time thanks to the sub-5 fs resolution. The mission was achieved by joint expertise from groups in Hamburg (CFEL), Lausanne (EPFL), and France (CEA). Previous literature only reports this mode via vibrational spectroscopy; however, it’s never been temporally resolved due to the lack of short pulse in the deep ultraviolet (DUV) region to probe the OH bond. Moreover, the characteristic lifetime of the OH-stretching mode, around 50 fs, is interestingly on the same scale to the formation lifetime of the OH radical in ionized water.

Ultrafast dynamics of solvated iron tris-bipyridine

Dr. Hugo Marroux, Commissariat à l’Energie Atomique et renouvelable (Gig sur Yvette , France)
Host Institution: DESY – Deutsches Elektronen-Synchrotron DESY – CFEL (DE)
Dates: From 2022-05-08  to 2022-05-28

This STSM allowed us to explore new behaviour of water using a deep UV probe with unique time resolution. We believe that the dynamics observed is due to stimulated Raman transitions in the OH stretching mode of the solvated water. This high frequency vibration is excessively complicated to observe due to its high frequency and fast dephasing in solution. Its investigation will bring new insight to the dynamics behaviour of water that is, we believe, otherwise inaccessible. We are currently undertaking steps to further analyse data and confirm this first assignment. These results will probably be the subject of a publication.

Entanglement of electrons and nuclei during ultrafast excitation

Dr. Natalia Gelfand, Hebrew University (Jerusalem , IL)
Host Institution: University of Liege (BE)
Dates: From 2022-05-17 – to 2022-05-30

During this short-term scientific mission, a most compact representation of the molecular wave function obtained with SVD was applied to N2 and LiH. These molecules were chosen because they remarkably differ in their adiabatic electronic dynamics in the energy range accessible by a UV pulse. A paper describing the results on the early time electronic dynamics is under preparation.

Investigation of photodissociation of molecular ions relevant for chemistry: new data sets

Dr Vladimir Sreckovic, Institute of Physics (Belgrade, Serbia)
Host Institution: Department of Applied Physics of the Technical University – Sofia (BG)
Dates: From 2021-08-30 – to 2021-10-15

Atomic and molecular (A&M) data and databases have become essential for diagnostics and development of models and simulations of complex physical processes and for the interpretation of data provided by. It is important to highlight the need of examining the optical characteristics in many fields, especially during the modelling of those systems. If we have the relevant data, we could simulate spectral characteristics.
The processes of photodissociation in the cases of non-symmetric and symmetric systems involving hydrogen and alkali atoms, ions, molecules and molecular-ions will be investigated. Our aim (STSM Sofia) is to obtain spectroscopic data for those systems. We will obtain the partial and average cross-sections and corresponding spectral absorption rate coefficients. The results i.e. the obtained data could be used for further applications, e.g., for some modelling or experiments like PLEIADES (SOLEIL synchrotron).

Ultrafast dynamics of push-pull nitroanilines tracked with few-femtosecond XUV-NIR spectroscopy

Mr David Sorribes, Universidad Complutense de Madrid (ES)
Host Institution: Politecnico di Milano (IT)
Dates: From 2022-07-01  to 2022-07-31

Capturing real-time excitonic dynamics: Attosecond Transient Reflection Spectroscopy in hBN

Dr Mikhail Malakhov, Universidad Autonoma de Madrid (ES)
Virtual Mobility
Dates: From 2022-07-04 – to 2022-07-31

In this action, we propose to simulate pump-probe experiments for two-dimensional hBN in collaboration with the group of Prof. Dr. M. Lucchini, a leading scientist in the study of excitons with attosecond spectroscopy techniques. The main idea is to develop a new tool for a code developed at UAM that enables us to calculate the reflectivity (instead of the absorption), as this is the suitable observable in order to investigate the transient dynamics in two-dimensional materials. We will design a potential attosecond transient reflection spectroscopy (ATRS) experiment with Prof. Dr. M. Lucchini and investigate via numerical simulations the possibility to observe the formation of excitons.

Set-up of a photoelectron spectroscopy beamline for experiments with trains of attosecond pulses

Mr Yingxuan Wu, Politecnico di Milano (Milan, Italy)
Host Institution: Universidad Complutense de Madrid (ES)
Dates: From 2022-07-03 to 2022-07-08

The research group of the host institution “Centro de Láseres Ultrarrápidos” (CLUR) of the Universidad Complutense de Madrid is currently developing a beamline for photoelectron(ion) spectroscopy with trains of attosecond XUV pulses. This temporal resolution allows for the direct observation and control of electron motion in molecules opening up the possibility for the ultimate real-time control of chemical reactions, one of the central problems in modern chemistry [1].

The High-order Harmonic Generation (HHG) in gases is a highly nonlinear process in which extreme-ultraviolet (XUV) radiation is emitted during the interaction of an intense laser pulse with a gaseous medium. This process is at the basis for the generation of attosecond pulses [2].

Thanks to this STSM, we reached the goal to generate attosecond train pulses (ATP) through the HHG process. The results of this STMS represent the first step on the development of attosecond beamline moving towards time-resolved imaging spectroscopy with time scales ranging from few hundreds of attoseconds up to tens of femtoseconds.

Figure 1 Calibrated XUV photon measured with the XUV spectrometer of HHG spectra generated in Argon. The spectrum ranges from 10 eV up to 60 eV, generating a comb of odd harmonics from the HH9 to HH37. The blue line represents the harmonics generated using a 5mm gas cell and the orange line using a 3 mm gas cell. For both case the gas pressure inside the gas cell was set to be 70 mbar. The two spectrums have been measured using the same voltage on the MCP and phosphor screen.

 

 

 

 

References:

[1] R. Borrego-Varillas, M. Lucchini, M. Nisoli, Rep. Prog. Phys., 85, 066401 (2022)

[2] McPherson A, Gibson G, Jara H, Johann U, Luk T S, McIntyre I A, Boyer K and Rhodes C K, J. Opt. Soc. Am. B, 4, 595–601 (1987)

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