STSM by Mark Stockett, Stockholm University (SE) with Steen Brøndsted Nielsen, Aarhus University (DK)
On April 23, 2017 (7 days)
From SWEDEN to DENMARK

Electronic properties of Flavin Adenine Dinucleotide anions

The purpose of this Short Term Scientific Mission (STSM) was to investigate the photo-induced dissociation of flavin adenine dinucleotide (FAD) anions in vacuo. These experiments utilized the Sep1 accelerator mass spectrometer complex at Aarhus University, Department of Physics and Astronomy. Our scientific aim was to assess the plausibility of the proposed role if FAD in the perception of magnetic fields by migratory birds. Using a combination of optical spectroscopy and mass spectrometry techniques known as action spectroscopy, we investigated the excited state photophysics and relaxation dynamics of isolated FAD anions.

At Sep1, FAD anions were transferred into vacuum by electrospray ionizaton and accelerated to 50 keV. The photo-induced dissociation (PID) mass spectrum was recorded by irradiating mass-selected FAD anions with a high-intensity pulsed OPO laser at a fixed wavelength and separating the daughter anions with an electrostatic energy analyzer. For the most abundant daughter ions, excitation energy (wavelength) dependence measurements, so-called action spectra, and pulse energy dependence measurements were performed. A surprising result was the observation of dissociation channels activated by the absorption of a single photon, and which were much less prominent in collision induced dissociation (CID) experiments. This suggests that ultrafast excited state processes may lead to non-statistical fragmentation of these ions.

This highly productive STSM resulted in a wealth of new data on the photo-physics FAD anions. After further analyzing these results, including computational modeling of the dissociation processes, we will prepare a manuscript for publication. We expect follow-up measurements and experiments on similar systems to lead to a long a fruitful collaboration between Stockholm and Aarhus Universities.

STSM by Alicja Domaracka, CIMAP-CNRS (FR) with Paola Bolognesi ISM-CNR, (IT)
On January 27, 2017 (5 days)
From FRANCE to ITALY

A new method to determine of energy-transfer distributions in ionizing ion-molecule collisions via photoelectron-photoion coincidence experiments in furan molecule

A detailed knowledge of the response of complex molecular systems to ionization or excitation and its influence on chemical reactivity is required to fully understand processes in astrophysical environments, planetary atmospheres as well as mechanisms associated with radiation damage. The study of such systems requires the support of theoretical investigations. However, a meaningful comparison between experimental and theoretical results requires the knowledge of the energy transferred in the radiation interaction. This is straightforward in photon absorption processes, but in case of ion interaction very difficult as collisions occur at different impact parameters, thus associated with a wide distribution of energy transfer. Recently, the joint effort of the CIMAP group (France), the CNR-ISM group (Italy) and the UAM group (Spain) has proposed a new method to determine of energy-transfer distributions in ionizing ion-molecule collisions via photoelectron-photoion coincidence experiments in the case of thymidine molecule (S. Maclot et al, Phys. Rev. Lett. 117, 07321 (2016)).

The objective of the present mission to the Elettra Synchrotron (Trieste, Italy) was to investigate the state-selected fragmentation of furan (a prototype of planar five membered hetero-aromatic compounds) and glycine (the simplest amino acid) molecules in the valence and inner valence ionization region. Figure 1a) shows obtained photoelectron spectra (PES) at 60 eV for glycine molecule. Based on obtained PES we selected binding energy values in vide range from the ionization threshold up to 30-40 eV to perform Photoelectron-Photoion Coincidence spectra (PEPICO), see Figure 1b).

The obtained results will be implemented in the proposed method to determine energy transfer ion –molecule collisions. Our aim is to fully validate the proposed method and to estimate the energy transfer in ion collisions with different impact parameters. The CIMAP group, from the French ion beam facility (GANIL), has already measured mass spectra for different ions (He²⁺, O³⁺, O⁶⁺, Ar¹¹⁺ and Xe²⁵⁺) at for glycine molecule and the experiments with furan molecule are planned in this year.

  Figure 1  a) Photoelectron spectra obtained at 60 eV for glycine (83°C) molecule. The lines indicate selected binding energy values for PEPICO investigations. b) PEPICO spectra for glycine molecules for six selected binding energy between 8.5 eV and 32 eV.

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 Marta Tarkanovskaja, Tartu University (EE) with Edwin Kukk, University of Turku (FI)
On March 27, 2017 (20 days)
From ESTONIA to FINLAND

Photochemistry of small acetamide and acetic acid clusters

Experimental studies of biological bulk systems (proteins, DNA) tend to obscure the details of photoinduced reaction mechanisms due to the influence of the environment. Therefore, experiments with clusters of small molecules of biological interest offer an elegant solution to the problem mentioned above.

We combined the synchrotron radiation with the ion mass spectroscopy to study gas-phase clusters of acetamide, (CH₃CONH₂)_n, and acetic acid molecules, (CH₃COOH)_n, produced by the supersonic expansion source. Clusters of studied compounds are capable of contributing to C–H^…O=C, O–H^…O=C and N–H^…O=C types of intermolecular hydrogen bond interactions that play an important role in biology, being responsible, for example, for the structural organization of proteins. We explored photodissociation pathways of the clusters using mass spectroscopy as well as their electronic properties as a function of their size using partial ion yield (PIY) technique, where an array of time-of-flight mass spectra was measured at increasing photon energies (see PIY map of acetamide clusters and extracted PIY curves in Fig.1).

Our study showed that almost identical structures of the studied compounds that differ only by one functional group result in different photodissociation behavior under vacuum ultraviolet ionization. They both readily undergo dissociation by proton transfer; in acetamide clusters, proton transfer mainly occurs from the amino group, while in acetic acid clusters from the hydroxyl group. The second fragmentation channel for acetamide clusters is multistep ammonia ion transfer that was detected for clusters up to tetramers, while in acetic acid clusters, only dimer fragments by the methyl group loss (not a transfer). The structures of the ionized dimers were optimized, and the proton transfer and ammonia transfer processes probed using ab initio calculations.

As an outcome of the visit, the initial draft for the publication with reported experimental ionization and appearance energies of the clusters and their fragments was prepared.

Fig. 1. Partial ion yield (PIY) map of acetamide clusters plotted as ion flight time versus photon energy. Below that are extracted PIY curves of acetamide monomer and protonated monomer. Blue lines represent the fit to the curves. The intersection point of the lines (marked with vertical black line) is assigned to the ion appearance energy (AE).

STSM by Ewa Erdmann, Gdansk University of Technology (PL) with Manuel Alcamí, Universidad Autónoma de Madrid (ES)
On March 26, 2017 (14 days)
From POLAND to SPAIN

Dynamical, energetic and entropic aspects of the fragmentation of excited neutral and cationic furan molecules in the gas phase

The aim of this Short Term Scientific Mission within the COST XLIC Action was to continue our collaborative study on the fragmentation of small ring molecule: furan (C₄H₄O). Furan belongs to the family of ring structures that are analogous to the deoxyribose building block of the DNA helix; hence it can serve as model system for track simulations in biological medium. The fragmentation mechanism of going through various intermediates is still unclear, so in this work our goal is to extend and complement previous studies.

Firstly, we investigated possible isomerization and dehydrogenation of furan by exploration of the appropriate regions in the potential energy surface with Density Functional Theory. In order to improve the description of the process, we applied a statistical approach that allows to describe the unimolecular decomposition of excited systems: M₃C (Microcanonical Metropolis Monte Carlo). With the new 2.0 version of the M₃C program we performed multiple simulations. In Figure 1 we present the probability of the most populated species as a function of the internal energy. In accordance with the results of previous pyrolysis experiments, the major observed products were the following species: CO, H₃C₄, H₂C₂ and H₂C₂O. Additionally, we note the constantly high probability of CO production for higher energies (purple line in Figure 1). This result is consistent with our previous calculations using ADMP molecular dynamics method. In conclusion, the improved M₃C code has the capacity to become a convenient tool for the description of fragmentation processes.

We are currently working on a manuscript describing the obtained results that we plan to submit to the special issue of the PCCP journal devoted to the XLIC-COST Action. We also intend to continue the collaboration between the groups in Gdańsk and Madrid by extending the applied methodology to charged furan. These results will be useful to help in the interpretation of recent experimental measurements carried out by other groups in the XLIC network.

Figure 1 M3C results: Species probabilities as a function of the internal energy

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 Daniela Ascenzi, University of Trento (IT) with Christian Alcaraz, Laboratoire de Chimie Physique, Paris (FR)
On February 13, 2017 (8 days)
From ITALY to FRANCE

Ion-molecule reactivity monitoring with synchrotron radiation: reactions of CH2CN+ isomers with hydrocarbons

The atmosphere of Titan, Saturn’s largest satellite, hosts one of the most complex organic chemistry in the Solar system, initiated by N₂ and CH₄ and leading to the synthesis of complex hydrocarbons, nitriles and prebiotic molecules. Titan’s atmosphere is very similar to the Earth’s primordial atmosphere, thus understanding Titan atmospheric chemistry is extremely relevant for the chemical evolution of our planet.

Titan has a significant ionosphere and results from the Cassini-Huygens mission have demonstrated a strong implication of ionospheric chemistry in the synthesis of complex N-containing molecules, that maybe the precursors of stratospheric tholins.

Among N-containing ions, C₂H₂N⁺ have been detected on Titan, and during the STSM we have studied the reactivity of C₂H₂N⁺ isomers with CH₄, C₂H₂ and C₂H₆. Using dissociative photoionization of appropriate neutral precursors, we have successfully demonstrated the possibility to generate different C₂H₂N⁺ isomers, namely the cyclic one (from the CH₃CN precursor) and the cyanomethyl CH₂CN⁺ cation (from the ICH₂CN precursor). The experiments have been performed using the CERISES set-up, a guided ion beam mass spectrometer that permits the measurement of absolute reactive cross sections and branching ratios as a function of photon and collision energies.

Among the most relevant results we mention:
a) in the case of CH₄, the CH₂CN⁺ cation is the only reactive isomer, and one of the three most abundant product is C₃H₄N⁺ (plus H₂) in which a new C-C bond has been formed
b) in the case of C₂H₂, both c-C₂H₂N⁺ and CH₂CN⁺ are responsible for the synthesis of the most abundant C₃H₃⁺ product, while minor channels CH₃⁺ (plus HC₃N) and C₄H₂N⁺ (plus H₂) derive exclusively from the linear isomer, as shown in the Figure.

The experiments have been carried out in a joint collaboration among the Host Institution in Orsay/SOLEIL Synchrotron (C. Alcaraz, C. Romanzin, R. Thissen), the Trento group (D. Ascenzi), the Stockholm group (W. Geppert) and the Prague team (M. Polasek) and we are confident that they will results in at least one scientific publication.