Report on the outcomes of a Short-Term Scientific Mission

Action number: CA21101

Applicant name: Ltaief Ben Ltaief

Details of the STSM

Title: Time-resolved interatomic Coulombic decay (ICD) induced by photoelectron impact excitation in pure and doped large He nanodroplets

Start and end date: 01/09/2023 to 23/09/2023.

Description of the work carried out during the STSM

The work should shed light onto the spectroscopy of pure and doped He droplets which is a core topic of WG4. However, as we encountered a serious problem with L1 Allegra laser, we could not conduct any experiment. This beamtime will be rescheduled next year 2024.
In the meanwhile, I used my time to analyse XUV laser intensity dependent-data obtained during previous ELI beamtimes using XUV HHG laser pulses delivered by L1 Allegra Laser. The XUV intensity achieved by our experiment that time is actually comparable to the lower limit of the free electron laser (FEL) intensity, and it is enough to trigger ICD and CAI in He droplets. At least two XUV photons of hv=21.54 eV must be absorbed by the He droplet to trigger ICD; i.e. He* + He* → He + He+ + eICD, whereas, CAI requires the absorption of more than two photons by the droplet to occur. The aim of this data analysis is thus to see how ICD/CAI signals vs. XUV laser intensity scale. By comparing these analysed data to FEL data, I found that the relative ICD/CAI rates vs. XUV laser intensity scale linearly. It is expected to see a nonlinear scaling; however, this is not true as those ICD/CAI signals are mainly due to multiple-photon absorption by one single He droplet (containing many atoms) and not by one single He atom. A non-linear light-matter interaction usually manifests e.g., as a two-photon absorption by a single atom. Here, we have, however, a different absorption scenario—absorption of N>1 photons by a single He droplet that leads to N>1 excited He*’s in the droplet. The resultant signal from the decay of at least two He*’s in the droplet should therefore scale linearly as well. These results were mentioned in the revised version of a recently submitted paper to Commun. Phys. journal and COST Action COSY will be acknowledged under this COST Action grant CA21101 once the paper gets published.

Also, I analysed time resolved data that we recently obtained at ELI using pump XUV pulses and probe IR pulses driven by the Legend Laser. The obtained time-resolved signals of He+ and He2+ ions emitted from resonantly excited He droplets show a clear He nanoplasma dynamics at positive time delays. This indicates that resonant excitation of He droplets by sufficiently intense XUV pulses is sufficient for igniting He nanoplasma. These data will soon be published, and COSY will be acknowledged.

The STSM has allowed me to intensify further my collaborations with research groups at ELI Beamlines. I have learned a lot about the different activities that can be done there, especially at the E1 hall. This will allow me to submit further applications for laser beamtime and for third party funding. My stay at ELI has also allowed me to connect with the beamline and Laser scientists at the E1 and L1 Allegra halls and hold constructive scientific discussions with them about the L1 Allegra Laser.

Description of the STSM main achievements and planned follow-up activities

A plan for the further proceedings on the road to the final goal of the Torun-Athens collaboration, i.e. the ‘Supramolecular tuning of the photophysical properties of Squaraine dyes with applications to Biomedicine and Materials science’ was discussed and agreed upon. Also, the procedure to follow for the first next steps, the development of force fields for the accurate description of the target structures has been investigated and narrowed down to a specific procedure, for which first calculations have been started. We will later incorporate the basis sets specifically developed by Dr. Dariusz Kedziera for halogen-bonded structures in a previous STSM into the force field optimization procedure. It will also be necessary to develop optimized basis sets for chalcogen-bonded structures, so that both kinds of non-covalent bonding patterns will be treated on the same level of accuracy. Both activities contribute to the general objective of WG2 of developing general strategies for the investigation of molecular motion in confined systems, and to some extent to the goal of creating new open-access codes for macroscopic studies of confinement effects and contribute to the capacity building objective to provide a platform for exchange and collaboration between research groups working on confined molecular systems. In addition, they contribute to Deliverable 5 expected of WG2, namely “Quantification of binding specificity between organic compounds and confining environments”.  Finally, they are in line with the objective of the first grant period in WG2 to test computational methods addressing the dynamics of molecular systems in confined systems.

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