Archive for the ‘Funded STSMs’ Category
Interaction of short laser pulses with atoms
During the present STSM we developed and tested a semi- classical simulation code for the study of laser-atom interactions. In our recent case we focus on the multielectron atomic systems. The interaction between the active target electron and the target nucleus mimic with the Garvey-type model potential. In the semiclassical model, it is assumed that wavepacket propagation in the post-tunneling process can be well described within the framework of the classical mechanics. The method is similar to the Classical Trajectory Monte Carlo (CTMC) method based on the inclusion of the classical phase information of the motion. Using this code we study the ionization of H, He, Ne an Ar. We also analyzed the classical trajectories individually. We sorted the events according the number of recollisions as a function of the final energy of the ejected electrons. We clearly identify and separate the regions in momentum distributions of the ejected electrons according to the number of recollisions (see Fig. 1). We analyzed the similarities and differences between the pure classical description and the semi-classical treatment of the collision problem.
Fig.1. Ionization probability densities for He atom as a function of the electron parallel and perpendicular momentum measured from the polarization vector which coincides with the Oz axis. The electric field is defined
STSM by Ignacio Sola Reija, Universidad Complutense de Madrid (ES) with Agnes Vibok, University of Debrecen (HU)
On March 28, 2016 (6 days)
From SPAIN to HUNGARY
LICCIs: Laser Induced and Controlled Conical Intersections
Whether they are chemical reactions or photophysical processes, chemists conceive most chemical processes as events occurring in the potential energy surface (PES) of a molecule. The excited PES are the main roads for large transformations, but they are quagmired with ridges and bottlenecks that quickly lead to other PES and more often than not, to the ground potential. These topological features are usually called conical intersections. A strong field completely transforms this landscape and offers the opportunity to place and design laser induced conical intersections (LICIs) to control the flow of energy and population in the excited state of molecules.
In this work our main goal was to analyze the set of quantum control techniques that could be used to control the dynamics of wave packets around LICIs. We studied the simplest set-ups that involve LICIs either as deactivation mechanisms or as drivers to the desired target, and conceived different scenarios where the control was possible or very unlikely.
As the outcome of the STSM we elaborated a short memorandum for LICCIs (laser-induced and –controlled conical intersections) and planned the first steps for a future project involving the UCM and the Debrecen groups.
Photo-induced dissociation of hydrogenated polycyclic aromatic hydrocarbons at ELISA
Pyrene (C16H10) is a Polycyclic Aromatic Hydrocarbon (PAH) consisting of four fused aromatic rings. On one hand hydrogenation of PAHs allows for enhanced deexcitation through dissociation of the additional hydrogen atoms, a fact that has been found to potentially protect the carbon backbone against fragmentation . On the other hand conversion of unsaturated carbon-carbon bonds into single bonds weakens the carbon backbone, and CID experiments have shown that this weakening effect might prevail [2,3].
During this STSM hydrogenated pyrene (C16H10+m, m = 0, 6, or 16) molecules were injected into the ELISA storage ring and overlapped with high-intensity laser pulses in the optical range (420-650 nm). We have measured action spectra and fragmentation mass distributions. We have further measured the power dependencies of the total fragmentation yield as well as the power dependencies for individual fragmentation channels.
Figure 1 shows preliminary results of the power dependencies of the total fragmentation as a function of laser pulse energy. The exponent in the power law gives the number of photons involved in the fragmentation process. This number decreases from three 2.72 eV photons for m = 0, to two 2.88 eV photons for m = 6, to one 2.95 eV photon for m = 16. This shows that photo induced carbon backbone fragmentation is more likely for hydrogenated pyrene. The results will be published in an article in a peer-reviewed journal later this year. During this STSM we have also started additional measurements on hydrogenated coronene (C24H12+m), which will set the stage for continued collaboration.
 M. Gatchell et al., Phys. Rev. A 92, 050702(R) (2015).
 M. Wolf et al., Eur. Phys. J. D, in press.
Producing cold ions in He nanodroplets for DESIREE
The focus of this STSM was to establish a partnership between groups at Stockholm University and Universität Innsbruck with the goal of developing a He droplet cluster source for the DESIREE facility. Superfluid droplets of He consisting of up to 105 atoms can be doped with other molecules or atoms. Through evaporative cooling the dopants will reach the temperature of the surrounding droplets (0.37 K). The dopants will collect at the center of the droplets where bound complexes may be formed. The droplets can be ionized by electron impact so that they can be manipulated with electric fields. This offers an efficient yet gentle way of producing ions and charged complexes for a wide range of experimental applications.
During my time with the group in Innsbruck we performed two different sets of experiments. Figure 1 shows results from a measurement of C60 anions enclosed in H2 molecules that formed in He droplets. We can see that a wide range of cluster sizes are formed and that clusters with 32 or less H2 molecules are more stable than larger clusters. This is due to shell closing where the H2 molecules align with the 32 faces of a C60 molecule (20 hexagons and 12 pentagons, see inset).
This STSM will result in two publications and the future joint development of a He droplet source for the DESIREE facility at Stockholm University.
Figure: Cluster size distribution of H2 molecules attached to C60 anions. These are produced in a He droplet and
ionized by electron impact. A shell closure is observed at 32 H2 molecules, which form the first complete layer around
the fullerene molecule.
Stereoselective fragmentation of nitroimidazoles following UV-photoabsorption
The primary ionizing beams and the secondary particles (electrons, ions, radicals, excited fragments) may provoke significant alterations to biological systems, particularly within living cells and the DNA/RNA molecules. At the molecular level these modifications are mostly related to the bond cleavages of the DNA building components, which before decomposition may be a subject of excitation, ionization and/or isomerization. To determine the most sensitive part of the DNA molecular chains to the photon-induced bond rupture, it is therefore important to explore the possible fragmentation mechanisms of their constituents. In this perspective, investigations of the excitation, relaxation and fragmentation processes of the cyclic hydrocarbons containing oxygen and nitrogen heteroatoms (furan, tetrahydrofuran, isoxazole, and pyrimidine) are of particular relevance, because they are often considered to be simple archetypes of the structure units of the DNA. The results of the photon-induced dissociation of these heterocyclic molecules in the inner-valence photon energy range showed that the general fragmentation mechanism involves initial excitation of these molecules into the superexcited states, which are inner-valence or high-Rydberg excited states, lying at higher energies above the first ionization threshold.
During the STSM we investigated a new class of mechanisms of the photon-induced fragmentation of the heterocyclic molecules, namely ultrafast recapture processes to Rydberg states by detection of the neutral high-Rydberg (HR) fragments after inner-shell C1s, N1s and O1s core excitation and ionization. First, we measured the NEXAFS spectra by recording total ion yields at each inner-shell edge (C1s, N1s and O1s) without field ionization. Then, we switched on field ionization and measured the sum (or total yield) of HR fragments and energetic photons. The differences between both curves (see figure below) demonstrate that core ionization of the molecule with a photon energy just above the 1s ionization potential leads to ultrafast photoelectron recapture processes where the photoelectron is pushed back to HR orbital of the molecular ion. Then, neutral HR fragments can be created together with ions after subsequent dissociation processes. In order to identify particular HR fragments, the TOF spectra were measured at the selected energies using field ionization. For better understanding of the fragmentation processes we also measured the PEPICO spectra (i.e. coincidence spectra without field ionization) at numerous photon energies at the C1s, N1s and O1s edges.
All the results measured during this STSM are being currently analyzed. They will be presented in imminent international conferences and published in high-profile journals as soon as possible. The main goals of this STSM were successfully accomplished and I am grateful to the COST Action CM1204 XLIC for the opportunity to collaborate with the Elettra GasPhase beamline team. This cooperation will be continued during the upcoming years.
Figure shows the example patterns of total ion yield and HR fragments+VUV photons yield recorded in isoxazole at the N1s edge in the photon energy range of 398-416 eV. Both yields are normalized to the photon flux and scaled to have the same intensity at the lowest energy resonance.
During this Short Term Scientific Mission the UV photoionization of two imidazolic derivatives, 2-nitroimidazole and 4(5)-nitroimidazole (see Fig. 1), were studied. These compounds have been proposed as radiosensitizers and have shown very interesting effects in their fragmentation patterns upon collision induced dissociation and low energy electron attachment,. These studies suggest that even rather small structural changes can alter the fragmentation chemistry significantly thus also affecting the radiosensitizing properties.
We found out that the fragmentation processes induced by valence photoionization depend greatly on the site of the nitro (NO2) group, especially when the six outmost orbitals are ionized. Basically, 2-nitroimidazole seems to be more unstable upon valence ionization compared to 4(5)-nitroimidazole. Furthermore, the imidazole ring always breaks via the same bond cleavages; in 2-nitroimidazole the NO2 at the C2 site blocks the HNCH+ production. As for in 4(5)-nitroimidazole, the NO2 at the C4(5) site blocks the C2H2N+ formation.
Figure 1 Mass spectra of 2-nitroimidazole (top) and 4(5)-nitroimidazole (bottom) following the ionization of the six outmost valence orbitals.
Future collaborations between the Institute for Ion Physics and Applied Physics have been planned during the STSM. Especially experiments concerning clustered (molecular and hydrated) nitroimidazoles were envisioned. A publication concerning this STSM, the photofragmentation of 2-nitroimidazole and 4(5)-nitroimidazole, is currently under preparation and will be finished within a month.
 M.R. Horsman & A.J. van der Kogel. In: Joiner & van der Kogel, Basic Clinical Radiobiology, Hodder Arnold, 2009
 K. Tanzer et al. Angew. Chem. Int. Ed., 2014, 53, 12240-12243
 L. Feketeová et al. Phys. Chem. Chem. Phys., 17, 12598 (2015)
STSM by Eva Lindroth, Stockholm University (SE) with Fernando Martín, Universidad Autonoma de Madrid (ES)
On December 13th, 2015 (6 days)
From SWEDEN to SPAIN
Fully-correlated transition matrix elements for the description of atomic attosecond experiments
During the last decades laser source technology has seen substantial progress. In the spectral range from extreme ultraviolet to soft X-rays
highly coherent, short pulsed, radiation, can now give access to time resolved information of elementary charge-transfer processes in atomic and molecular systems. An accurate theoretical description, using either ab-initio methods or models, is seminal for the understanding of the features appearing on the attosecond time scale, and for the interpretation of the sometimes cumbersome results obtained in experiments.
Angularly-resolved experiments on rare gases with the RABBITT -technique (reconstruction of attosecond beating by interference of two-photon transitions) have recently been performed by the group of Ursula Keller at ETH, in Zürich, demonstrating an angular anisotropy
of the photoionization delay. This anisotropy is clearly seen even in helium where both the initial atomic state and the final ionic state are isotropic, and is there solely due to the second photon being exchanged in the RABBITT process, allowing for final quantum states with two
different symmetries. The groups in Madrid and Stockholm have used different approaches, both ab initio and models, to account for the anisotropy.
In the next step also the results in neon and argon have to be explained and here we want to combine the approaches and use transition matrix elements calculated in Stockholm in the model that has been developed in Madrid
The helium results are submitted as a joint experimental and theory paper, and the calculations on argon and neon started during the STSM. The results will be published later this year.
Calculated angular dependence of the photoemission delay in neon for different photoelectron energies. The angle is measured relative the linear polarization axis of the laser field, and the delay is given relative that in the forward direction. The calculation is done using lowest order perturbation theory for the light-matter interaction and with many-electron effects included at the level of RPAE (random-phase approximation with exchange).
Recurrent fluorescence of organic molecules
Highly excited molecules tend to fall apart, usually by losing an atom or a small molecule. At sufficiently long time scales they will also emit photons. Recently, an extremely efficient radiation channel has been found in a few carbon based molecules. The purpose of the experiments was to search for a similar effect in a list of other molecules, using the electrostatic storage ring ELISA.
The molecules studied were of the PAH type, i.e. containing an aromatic ring, and were positively charged. The quenching radiation, known as recurrent fluorescence (RF), was measured by the exponential decay of a one photon laser induced signal.
The measurements will be published in an article in a peer-reviewed journal. I expect that the visit is the start of a long-term program that will take place at different storage rings and give important input for the modeling of circum- and interstellar clouds.
Interfacing DFT-like methods with classical motion to treat electron dynamics in aminoacids
Several theoretical works predicted the possibility of inducing ultrafast charge migration along the skeleton of large biological molecules upon their photoionization [1,2]. Recent experimental data together with the theoretical results obtained using a DFT-like methodology, captured for the first time the ultrasfast charge migration in the amino-acid phenylalanine after the interaction with a single attosecond pulse . This first experimental evidence was obtained using a XUV-IR pump-probe scheme with an unprecedented attosecond resolution, measuring a doubly charged species to track the dynamics in the singly charged molecule. The theoretical method was able to reproduce the measured electron dynamics, accurately describing the pumped electronic wave packet that involved up to 34 ionic states leading to a highly delocalized hole and giving rise to charge fluctuations occurring in less than 4-5 fs. Despite the breakthrough of this joint experimental and theoretical work, a number of questions remains open, such that the role of nuclear motion and the possible fragmentation paths responsible for the lost of the double charged signal after a few tens of femtoseconds. In the present approach, we aim to include the nuclear degrees of freedom within a classical picture, combining our ab initio DFT-methods with existing CPMD (Carr Parrinello Molecular Dynamic) packages.
The main goal of this STSM was to learn the basic use of CPMD code in order to merge the current DFT-like methodology in the field ab-initio calculations of light-molecule interactions with the time-dependent picture of Ehrenfest dynamics for the physical problem of charge migration upon photoionization by XUV laser pulses. The first tests have already been carried out, although first definitive calculations and convergence checks will be perform within the next weeks. The computational cost of TDDFT dynamics is high in comparison with the previous approach, yet we expect to obtain clarifying results in the next weeks.
Multi-scattering effects in slow and fast ion-atom collisions
The main focus of this proposal is to broad our understanding regarding the multi-scattering phenomena in electron emission and electron transfer processes from atomic-target interacting with slow and fast ions. In order to reach this objective, the work is based on classical and nonperturbative approaches to deal with the electronic dynamics in time-dependent processes.
Our preliminary results for single electron capture in slow energies (2 keV/u) He2+ projectile colliding with a helium target, are presented in Fig. 1. Here the process is treated using a semiclassical close-coupling approach in combination with an Eikonal approximation to calculate the differential cross sections (DCSs) of the scattered projectile . Our theoretical investigations are reported in two-electron processes to actively take part in the collision dynamics. The DCSs reveal oscillatory structure in the scattering angles θ, in agreement with the experimental data of Goa et al. . The observed structure at large scattering angle has not been discussed neither theoretically nor experimentally, and therefore, its origin has not been explored yet.