Archive for the ‘Funded STSMs’ Category

STSM by Philipp Marquetand, University of Vienna (AT)  with Tamas Rozgonyi, Hungarian Academy of Sciences (HU)
On July 2nd, 2015 (8 days)

Strong-Field Ionization Simulations Using Adiabatic Elimination

Strong-field ionization including multiphoton transitions is hardly understood yet due to the complex interplay of intermediate resonant states, their dynamic Stark shift, the ponderomotive potential and many more fundamental processes. In order to obtain nonetheless a simple and more intuitive picture, we have developed a theoretical description and a corresponding code, which makes use of two main approximations. All off-resonant intermediate states are not treated explicitly but removed by adiabatic elimination (ADEL) from the system of equations of motion. Furthermore, the ionization continuum is represented by a Legendre-polynomial based discretization approach.

In this STSM, we extended our code to include non-adiabatic coupling between multiple intermediate resonant states. In this way, we are able to test a hypothesis developed by our experimental partner, Thomas Weinacht (Stony Brook University), that such vibrational couplings are at the origin of some of the peaks in the photo-electron spectrum of Iodobromomethane. We have tested our new implementations, comparing a 0-dimensional, a 1-dimensional and our new 1-dimensional code with couplings. First simulations on a model with arbitrarily chosen parameters show that indeed non-adiabatic couplings can lead to additional peaks in the photo-electron spectrum, see Fig. 1. The model consists of a ground state |0>, two intermediate resonant states |i1>, |i2> and two electronic continua |c1>, |c2>. The states |0> and |i1> are coupled via a 5-photon process, while the states |i1> and |c1> as well as |i2> and |c2> are coupled via a 2-photon process, respectively. A non-adiabatic coupling between |i1> and |i2> can be switched on and off. If the coupling is present, an additional peak appears in the photo-electron spectrum.


STSM by Sylvain Maclot, Lund University (SE)  with Alicia Palacios, Universidad Autónoma de Madrid (ES)
On June 28th, 2015 (7 days)

Setting up collaboration towards ultrafast molecular dynamics using XUV-XUV pump-probe techniques

The relaxation dynamics of an ionized/excited molecular system leads to different fundamental processes occurring at sub-femtosecond timescales as electron dynamics, charge transfer/migration and nuclear motion. Time-resolved XUV-XUV pump-probe experiments should allow deeper insight to the key roles of these processes. Such studies should be feasible soon at the high-intensity high-order harmonic generation beamline of Lund Laser Centre. Main part of the time, the results of these kinds of experiments are quite complicated to interpret and require theoretical support. The team of Fernando Martin, managed by Alicia Palacios, has already started to develop theoretical tools based on time-dependent ab initio methods to account for the full dimensionality of the problem. The main purpose of this application was to start up collaboration between theoreticians from Madrid and experimentalists from Lund in order to prepare the future experiments/calculations and results whose can be done together.

We evaluated the suitability of our existing tools, both experimental and theoretical sides, PulseTrainto apply XUV-XUV pump-probe schemes in different contexts (atoms, small and large molecules; single or double ionization processes; excitation processes). Then we discussed the type of calculations and models that can be perform in a short-term period for different problems, and the details about the beamline and experimental set-up. For instance, investigated the effect of attosecond pulse train (APT) compared to single attosecond pulse (SAP) (figure).

During this mission, we have limited the targets and processes that we can tackle both experimentally and theoretically and we have carried out a scientific exchange specifying the physical parameters and the observables that can be measured/computed considering existing tools and their current limitations, as well as methodologies than could be developed in a near future. Now, we have common working bases to plan our future and specific tasks.


STSM by Lucas SCHWOB, CIMAP, Caen (FR)  with Thomas Schlathölter, University of Groningen (NL)
On June 2nd, 2015 (7 days)

Photostability of superhydrogenated nitrogen-subsituted polycyclic aromatic hydrocarbons

Polycyclic Aromatic Hydrocarbons (PAHs) are of great interest in astrophysics and astrochemistry fields because of their presence in interstellar gas clouds. In this particular environment, PAHs undergo irradiation of photons with energies ranging from UV to X-ray. Moreover, the existence of highly hydrogenated PAH in the interstellar medium could potentially lead to the formation of H2, the most abundant molecule in the universe. During this STSM we studied the photoionization and fragmentation of coronene and hydrogenated coronene under VUV photon irradiation.

Experiments were performed at the BESSY II synchrotron in Berlin, using a home-made tandem mass spectrometer. A Slevin-type hydrogen discharge source has been used to attach single H atoms to coronene radical cations produced by electrospray ionization. Photon irradiation occurred in a 3D RF-Paul trap, and cationic fragments were analyzed by time-of-flight mass spectrometry. Photofragmentation spectra were recorded for VUV photon energies ranging from 10 to 25eV. For coronene and coronene+H, we observed, as shown in figure 2, a strong yield of 2H or H2 loss from both dication by photoionization, whereas losses of 2nH or n(H2) with n < 5 were observed after absorption of one photon of 284-300eV (C 1s-edge electron removal). Unlike soft-Xray, no deexcitation through H+ loss from the dication was observed after absorption of a single VUV photon.

HydrogenenSourceFigure 1 : Hydrogenation discharge source in function.

The spectra obtained are still under analysis and will lead to the publication of a new manuscript with original results. Moreover, this STSM has initiated a new collaboration between our two groups. A new research proposal for beamtime at the BESSY II synchrotron, has already been submitted and accepted. This will allow us to study human type II collagen peptides upon VUV and soft X-ray irradiation.


Figure 2 : Mass spectrum of protonated coronene after the absorption of one 17.25eV photon.

STSM by Katrin Tanzer, University of Innsbruck (AT)  with Edwin Kukk, University of Turku (FI)
On April 27th, 2015 (26 days)

Photo-Ionization of Nitroimidazolic Compounds

During this Short Term Scientific Mission two nitroimidazolic derivatives, 4-nitroimidazole (4NI) and 1-methyl-5-nitroimidazole (Me5NI), were studied. These compounds have been proposed as radiosensitizers[1] and have shown very interesting effects in their fragmentation patterns upon low energy electron attachment[2][3]. To study the response of these molecules towards ionizing radiation, experiments with UV radiation and soft X-rays were undertaken.

Of special interest for us are the results of our PEPIPICO (Photoelectron-Photoion-Photoion Coincidence) study following C 1s core ionization of 4NI and Me5NI since this process is highly relevant for the study of high energy (keV range, cf. medical X-rays) radiation absorption. We obtained PIPICO maps by recording the flight times of two correlated ions from which the most dominant fragmentation pathways upon photoionization can be determined. The PEPIPICO maps of 4NI and Me5NI exhibit significant differences, albeit their similar structures (Figure 1). While in the case of 4NI most fragmentation patterns show a coincidence with the ionized nitro group (NO2+; 46 amu), NO+ (30 amu), or O+ (16 amu), these cations can hardly be observed for Me5NI, where the fragmentation is thought to be proceeded via loss of the neutral nitro group and sequential loss of the ionized methyl group (CH3+; 15 amu), which leads to subsequent cleavage of the ring.

Future collaborations between the Institute for Ion Physics and Applied Physics have been discussed during the STSM. It is planned to compile two publications from this STSM – one concerning the C 1s core ionization of 4NI and Me5NI that is presented in this report and one discussing the fragmentation pattern obtained by PEPIPICO measurements upon valence ionization. Furthermore, the possibility of more scientific exchanges in the future has been discussed.

Figure1_KatrinTanzerFigure 1: PEPIPICO maps of 4-nitroimidazole (left) and 1-methyl-5-nitroimidazole (right). The axes show the flight times of the slower and the faster ion, respectively and the numbers signify the mass of the ionic fragments.

[1] M.R. Horsman & A.J. van der Kogel. In: Joiner & van der Kogel, Basic Clinical Radiobiology, Hodder Arnold, 2009

[2] K. Tanzer et al. Angew. Chem. Int. Ed., 2014, 53, 12240-12243

[3] K. Tanzer et al. J. Phys. Chem. A. 2015, Just Accepted Manuscript. DOI: 10.1021/acs.jpca.5b02721

STSM by Sandra Gomez,  Complutense University of Madrid (ES)  with Volker Engel, Institut für Physikalische und Theoretische Chemie der Universität Würzburg (DE)
On March 15th, 2015 (75 days)

Two-dimensional spectroscopy of coupled nuclear and electronic motion.

The analysis of the dynamics of molecular systems is normally done by solving the Schrödinger equation for the electronic degrees of freedom for multiple fixed geometries of the nuclei obtaining potential energy surfaces, molecular orbitals,densities and more interesting properties. This description, within the Born-Oppenheimer approximation, fails in many cases. However, the computational effort of treating both kinds of particles at the same level of theory is huge, and this problem can only be solved if drastic simplifications are perfomed.

Here we use a model introduced by Shin and Metiu which consists in a four particles system, three nuclei and one electron, where only the electron and one of the nuclei are allowed to move in one dimension. The interaction between charges is parametrized such that we can consider two cases: one in which the coupling between the electron and the nucleous in movement is small enough to consider the Born-Oppenheimer approximation still valid, and the other one, where the BO approximation breaks down.
For obtaining the 2D-Spectra we focused on electric-dipole transitions caused by the interaction with three laser pulses, where the second and the third come at the same time and the response function is recovered in direction -k1+k2+k3 (as in a photon-echo experiment).
The spectra are calculated for the case of soft and large coupling and compared with those obtained applying the Born-Oppenheimer approximation, i.e. using electronic eigenfunctions only dependant on nuclear coordinates.

SAGOM2-imgAs we can observe in the picture, the results show that the Born-Oppenheimer approximation is valid for the case of small coupling, where the potential energy surfaces are far away enough, but it breaks completely down for the case of large coupling.

The results obtained on this stay are pending publication.

STSM by Fernando Aguilar-Galindo, Universidad Autónoma de Madrid (ES) with Michele Pavone, University of Naples (IT)
On February 8, 2015 (90 days)

AGUILAR-LOGOSShifting in the electronic levels of the acrylamide due its interaction with a Cu(100) surface

When the light interacts with a molecule it suffers internal changes due to the energy of the photon. With the appropriate light, it is possible even to break the molecule. This have been used in the recent years to treat cancer (phototherapy). In this work, we want to study the effect of a metallic surface (as a nanoparticle) on a molecule, in order to modify its electronic structure and reach the breaking with a less energetic light, which can be helpful to use less invasive radiation in the treatment of cancer.
We have performed a methodological assessment, in which we have compared several combinations of DFT functionals and basis with the results obtained with by CASPT2(8,6)/ANO-S (active space: the lone pairs, the π orbitals and the empty π* orbitals). We have found that for the acrylamide the transition S0->S2 (mainly a single excitation from the lone pair of the nitrogen to a π* orbital) can be correctly represented by TD-DFT with a deviation lower than 0.05 eV from the CASPT2 result by employing these combinations: LC-BLYP/6-31G, LC-wPBE/6-31G and M06HF/6-31G.
With those levels of theory, we have calculated the excitation of the molecule adsorbed on a cluster of Cu atoms emulating the Cu(100) surface (shown in the figure 1). In the results obtained with the three
functionals we have observed a red shift in this electronic level respect to the gas phase (see figure 2)

Now, and thanks to this STSM, a new collaboration between two groups of the COST Action have been consolidated. Since the first results obtained with TD-DFT are in good agreement with those at the CASPT2 level in the gas phase, we are currently working in the development of a more complex methodology to compute the electronic excited states of a molecule adsorbed on a metallic surface with a greater precision, using ab initio techniques.

FERAG_IMG1Figure 1. The cluster model used to simulate the Cu(100) surface with the molecule adsorbed.

FERAG_IMG2Figure 2. The excitation energies of the acrylamide in the gas-phase and on the cluster model.

STSM by Marcos del Cueto, Universidad Autónoma de Madrid (ES) with Geert-Jan Kroes, Leiden University (NL)
On March 02, 2015 (91 days)

Theoretical study of molecular scattering and dissociative adsorption on surfaces

Diffractive scattering is a technique extensively used to study the topology of surfaces and gain insight in the atom/molecule – surface dynamics. This process may also shed light on molecular dissociative chemisorption on surfaces, which is of special interest in the field of heterogeneous catalysis.
We have used a quantum treatment of the H2/LiF(001) system using the SPO-DVR implementation proposed by Prof. Kroes’ group [1], and we intend to compare our results with previous experimental surface analysis results at grazing incidence using fast atom/molecule diffraction, that show a strong dependence with respect to the crystallographic incidence direction studied [2-3]. In addition, we have studied the dissociative adsorption of HCl molecules on a Au(111) surface (see Fig. 1) by performing AIMD simulations, which produce results that can be compared to experimental data obtained by the group of Prof. Alec Wodtke [4].
During this collaboration we have been able to set up both systems and obtain promising preliminary results. We expect to publish complete results shortly.

MARDEC_imgFigure 1: Scheme of a HCl molecule motion on a Au(111) surface. The molecule can follow one of the two pathways depicted in the figure, leading to a) scattering or b) adsorption.

[1] – G. J. Kroes, Prog. Surf. Sci. 60, 1 (1999)
[2] – P. Rousseau et al, J. Phys. Conf. Series 133, 012013 (2008)
[3] – H. Winter. Private communication
[4] – A. Wodtke. Private communication

STSM by Manuel Lara Astiaso, Universidad Autónoma de Madrid (ES) with Christoph Meier, IRSAMC – Université Paul Sabatier III, (Toulouse (FR)
On February 13, 2015 (64 days)


Controlling the harmonic spectrum of diatomic molecules

The process of generating high-order harmonics is a result of the non-linear response of the electrons in a strong laser field, and leads to the emission of light of much higher frequency than that of the driving laser. One of the most appealing applications of HHG is the generation of attosecond pulses of light, which can in be used in time-resolved pump-probe experiments, to resolve physical processes at such timescales.

We analyzed to which extend the harmonic spectrum can be modified by using shaped laser pulses, starting with chirped pulses in the H2+ molecule. As major new finding we show that the sign of the chirp plays a crucial role, with identical spectrum.

In this context, two distinct situations have been considered: the first one considers pulses of constant pulse duration, the second one of constant spectrum.

In the case of a constant pulse duration, chirping the pulses inevitably means broadening the spectrum, which is at the origin of the previously documented chirp effects. However, in our case, we find significant differences for time-symmetric pulses having exactly the same spectrum, i.e. it is the appearance of the different colors within the laser pulse which -in combination with the electron dynamics- leads to a different HHG process.

This simple model of collinear laser-molecule interaction is being used to get deeper insights of the chirp effects in the coupled electron-nuclear dynamics. The numerical much more time consuming simulations shall elucidate if the observed chirp sign effect is still visible, or even enhanced within this more realistic description. The upcoming results of this STSM are to be presented at the XXIX International Conference on Photonic, Electronic, and Atomic Collisions (ICPEAC 2015). A publication intended to a refereed journal is also in preparation.

MANLA-imgFigure: Chirp effects on harmonic generation. The harmonic yield is greatly enhanced near the cutoff frequency, especially for the down-chirped pulse (green) when normal intensities are applied. In the high-intensity regime, up chirps (red) become more efficient to extend the cutoff.

STSM by Zdenek Masin, The Open University (UK) with Olga Smirnova, Max Born Institute, Berlin (DE)
On February 17, 2015 (12 days)

Tracking coupled electronic-nuclear dynamics at conical intersections in NO2

The purpose of the STSM was to generate the data required to model high harmonic spectra for NO2 which include the effects of the coupled electronic-nuclear dynamics. Specifically, the data obtained will be used to probe the mixing amplitudes of the electronic states at the conical intersection and to track the footprint of the mixing in the high harmonic spectra.

The work carried out concerned optimization of the UKRmol suite of codes that was used to generate the recombination dipoles between the bound and the continuum states of NO2.

For the present application the recombination dipoles are required for relatively high photon energies (up to approximately 80eV), i.e. for electron energies up to approximately 70eV. For electron energies much larger than 10-15eV the unbound electron’s wavefunction oscillates rapidly making it very hard to represent accurately using the methodology implemented in UKRmol (i.e. using Gaussian-type orbitals). We have overcome this problem using the new parallel integral core of the UKRmol suite which has the capability to generate the required molecular integrals in quadruple precision. The original code (running on 5 nodes on the MBI cluster) took approximately 5 hours to calculate integrals for one geometry (in quad precision). After optimization the compute time has been reduced to just under 2 hours per geometry.

The success in overcoming difficulties with representation of the continuum by employing the quad precision code is demonstrated in Fig. 1 where the total photoionization cross section for NO2 for one (non-equilibrium) geometry is shown as calculated using the double precision and the quad precision code. The calculations using the double precision code employed a smaller continuum basis set (in order to guarantee numerical stability) while the calculations in quad precision used a larger continuum basis. The calculations carried out using the quad precision code show no signs of numerical instabilities and an excellent quality of the continuum.

The main outcomes of the STSM included optimization of the parallel molecular integral transformation routines and a utility program to compute relative phases between Dyson orbitals computed for different geometries of NO2. The relative phases are obtained computing the overlap integrals between a pair of Dyson orbitals: the sign of the overlap integral corresponds to the relative phase between the orbitals. The details of the scattering calculations and the results obtained will be published in a publication that is in preparation.


Figure 1: Total photoionization cross section for NO2 (non-equilibrium geometry) obtained using a Close-Coupling scattering model and with a small and a large continuum basis. The small continuum basis was obtained using double precision and deleting those orbitals from the basis which caused the most severe numerical problems. The large continuum basis kept all available continuum orbitals in the basis and used quad precision to compute the molecular integrals.

STSM by Simone De Camillis and Grace Alexander, Queens University Belfast (UK) with Jean-Christophe Poully, GANIL, Caen (FR)
On March 9, 2015 (13 days)

Damage to DNA Nucleosides by Bragg Peak Energy Ar10+ Ions

The study of the interaction of ionising particles with biological molecular systems and the resulting events is of great importance, particularly for understanding and improving cancer treatment. The main purpose of our work was to investigate the mechanisms by which the biological building blocks of DNA are ionised and dissociated by energetic heavy ions. Using our home-built time-of-flight (ToF) mass spectrometer (KEIRA) the nucleoside fragmentation processes driven by Ar10+ ion collisions at an energy of 35 MeV were studied.

The results for the nucleoside thymidine (the DNA base thymine attached to a deoxyribose sugar) are shown in figure 1 and compared to that obtained from 5 keV proton impact (from our previous study1). Despite the higher energy and charge of the Ar ions, fragmentation patterns for the two measurements are similar even though the ionisation processes being different in each case. However, the fragment peaks in the present results have an asymmetric tail at longer ToFs, which indicates that there is substantial delayed fragmentation of metastable ions in the acceleration region. This is not observed for lower-mass fragments suggesting that they are instead being generated by prompt dissociation of ions with higher internal energies.

gralex-stsm-figureFigure 1: Mass spectra of thymidine after interaction with proton (red) and Ar10+ (black) beams.

This STSM has provided the first experimental study of MeV ion irradiation of nucleosides in the gas phase which is directly relevant to DNA damage induced by ion beams in heavy therapy as they approach the Bragg peak.

J.-C. Poully, J. Miles, S. De Camillis, A. Cassimi and J. B. Greenwood, Physical Chemistry Chemical Physics, 2015, 17, 7172.