Archive for the ‘Funded STSMs’ Category
H2+ in external XUV pulses. Calculations by using the exact prolate spheroidal coordinates
In recent years new and efficient numerical codes have been developed in order to solve the time-dependent Schrödinger equation (TDSE) for single or few-electron atomic and molecular systems. By applying the implemented algorithms on high performance computing platforms, exact solutions to the interaction of such systems with external and intense laser fields are obtained. For larger systems this laser field initiated dynamics is described in a simplified manner in the framework of the Born-Oppenheimer approach, where the nuclear dynamics is restricted to a few relevant electronic levels. A crucial part of these approaches is the calculation of high precision stationary electronic wave functions and the transition dipole moments between them.
The work of the STSM implied the development of an efficient algorithm for the calculation of the bound electronic eigenstates of H2+ for various internuclear distances. In the implemented approach we have discretized the electronic Hamiltonian in prolate spheroidal coordinates using finite difference (FD) discretization. The eigenstates were obtained via the direct diagonalization of the discretized Hamiltonian, and they were sorted according to their symmetry properties. The symmetry properties of each eigenstate was identified by counting the radial and angular nodal planes in prolate spheroidal coordinates (See figure below). After the convergence of the eigenstates (of the electronic wave functions) was carefully checked, the accurate transition dipole moments were also calculated.
In the next step the obtained ab initio electronic energy levels and transition moments will be included in the numerical code developed by the group of prof. Fernando Martin, and the combined electronic and nuclear dynamics of H2+ under the influence of ultrashort laser pulses will be studied.
In the existing approach of the host group the electronic wave functions are obtained by using one center partial wave expansion, which has the disadvantage of slow convergence, and low precision at large internuclear distances. During the present STSM this drawback was eliminated by the present approach.
STSM by Philipp Marquetand and Matthias Ruckenbauer, University of Vienna, Wien (AT) with Tamas Rozgonyi, Hungarian Academy of Sciences,Budapest (HU)
On July 21th, 2014 (6 days)
From AUSTRIA to HUNGARY
Adiabatic elimination for strong-field ionization with quantum dynamics
We develop a simplified theoretical description and the corresponding code for the quantum-dynamics simulation of strong-field multiphoton ionization. Any occurring intermediate off-resonant states are not treated explicitly in the propagation but removed by adiabatic elimination (ADEL) from the system of equations of motion. For the representation of the ionization continuum, a Legendre-polynomial based discretization ansatz (DIC) is used.
In this STSM, we evaluated various critical assumptions in the ADEL for the case of one-dimensional quantum-dynamics (QD). Especially, the validity of neglecting the kinetic energy on the states to be eliminated in the ADEL and the applicability of the multiphoton rotating wave approximation required in-depth consideration. The final equations of motion were integrated in a QD code and first tests showed an excellent agreement between the propagations of the full system including off-resonant states and when using ADEL.
In discussion with Prof. Dr. Thomas Weinacht (Stony Brook University) who was on a research visit in Budapest during the STSM we agreed on a concrete joint project for a mixed theoretical-experimental publication.
STSM by Dr. Giuseppe Sansone and Dr. Michele Devetta, Politecnico Milano and CNR-IFN Politecnico (IT) with Prof. Jens Biegert, ICFO (ES)
On July 7th, 2014 (5 days)
From ITALY to SPAIN
Temporal characterization of attosecond pulses generated by mid-IR pulses
The purpose of the STSM was to investigate the feasibility of the temporal characterization of ultrashort extreme ultraviolet radiation (XUV) generated by the process of high-order harmonic generation by mid-infrared driving pulses. By using pulsese centered around 2.0 microns, XUV photon energies up to 500 eV can be reached enabling the investigation of femtosecond and attosecond dynamics in systems characterized by resonances in the water window. In spite of its fundamental interest for biological applications, no temporal characterization of XUV radiation generated in this spectral energy range has been reported so far.
The experiment was performed at ICFO combining the expertise of the Milano group in characterization of isolated attosecond pulses and the carrier-envelop-phase stable driving pulses centered at 1.8 microns available at ICFO. The temporal characterization was based on the so called attosecond streak camera principle by measuring using a time-of-flight spectrometer the photoelectron spectra generated by the attosecond XUV radiation in the presence of an intense synchronized infrared field. A first test measurement based on the cross-correlation between the two driving pulses evidenced an excellent stability of the laser source and of the interferometric setup used for the measurement (see Figure). Preliminary indication of a sub-cycle modulation of the photoelectron spectra as a function of the relative delay between the attosecond and mid-IR pulses was obtained.
The visit will lead to future common projects between the two groups. A new application is undergoing in order to improve the quality of the streaking effect observed so far and to implement the FROG-CRAB retrieving algorithm to the acquired data. The experimental results will be included in a future publication describing, for the first time to the best of our knowledge, the characterization of XUV pulses generated by mid-IR pulses.
STSM by Nestor Aguirre, Universidad Autónoma de Madrid (ES) with Paul-Antoine Hervieux, Institut de Physique et Chimie des Materiaux de Strasbourg, IPCMS (FR)
On May 19th, 2014 (12 days)
From SPAIN to FRANCE
Fragmentation of (multi)charged carbon-based molecules by using statistical methods
The propose of this short term scientific mission hosted by Prof. Paul-Antoine Hervieux was to extend and improve the available options of our recently developed software code (M3C). This program allows to study the fragmentation of any kind of molecules by providing structural quantities from standard electronic-structure methods, e.g. electronic energies, vibrational frequencies and equilibrium geometries.
Angular momentum couplings play an important role in fragmentation processes of molecules, where the most important contribution comes from the coupling between the rotational and orbital angular momenta. A good description of this kind of effect is necessary for any fragmentation processes and it is crucial in the description of weakly bound systems, where its contribution reaches the maximum value. During this stay, we worked on the improvement of the fundamental aspects of the theory, paying particular attention to the combinatorial probability term and the rotational coupling scheme. By other hand, we also have taken advantage to discuss with Dr. Marin Chabot and Dr. Karine Béroff from Institut des Sciences moléculaires d’Orsay, consequently allowed us to adapt the program in order to compare with their most recent experimental results, e.g. calculation of breaking curves ( probability of the number of fragments as a function of excitation energy) and kinetic energy releases also as a function of excitation energy.
STSM by Juan Pablo Sanchez, Universidad Autónoma de Madrid (ES) with Marin Chabot,Institut de Physique Nucléaire d`Orsay (FR)
On May 19, 2014 (5 days)
From SPAIN to FRANCE
Structure, Geometry and Fragmentation of Neutral and Positively Charged Hydrogenated Carbon Clusters: CnHmq+ ( n=1-5; m=1-4; q=0-3 )
Partially hydrogenated carbon clusters are omnipresent in many interstellar bodies, they constitute one of the main ingredients of nebular clouds and in star forming regions. Hence their study is important to be able to understand the chemical and physical phenomena that are happening when those bodies form. Therefore the systematic study from small to big hydrocarbons can give insight on how some of the universe’s building blocks behave in the interstellar media.
The aim of this STSM was to bring theory and experiment together and find new goals. On the theory part, fragmentation was studied on the CnH systems using a statistical mechanical model: the Metropolis Microcanonical MonteCarlo(software in development by our group). This software needs the energies and geometries of all stable isomers of the species studied, all the input data was obtained using CCSD(T)/6-311++G(3df,2p) for energies and B3LYP/6-311++G(3df,2p) for geometry optimizations and frequencies. Only CnH with n from 1 to 4 were compared with experiment since it is what is available up to date thanks to the collision experiments in the Tandem Accelerator in Orsay. Other energetic and geometric theoretical studies for this family of compounds was done for the rest of the CnHm+q series ( n from 1 to 5, m from 1 to 4 and q from 0 to +3), thus enabling future collaborations. This extensive data on this compounds can be then introduced in astrochemical databases such as the KIDA ( http://kida.obs.u-bordeaux1.fr), whose data can be used in the astrochemistry kinetics models to study evolution of Interstellar clouds such as PDR regions ( like the Horsehead Nebula).
From this STSM some new considerations have been obtained in the theoretical study of the fragmentation processes, therefore improving the results of the software in development. Also this encouraged to perform more experiments on this partially hydrogenated carbon clusters which will reinforce the collaboration between this two groups as future papers are projected to converge our theory and their experiments and further the understanding of this interstellar building blocks.
Time-dependent atomic photoionisation with a Multi-Configuration-Hartree-Fock close-coupling approach
The most critical aspect in understanding electronic motion in matter is the phenomenon called correlation – the collective dynamics of the electrons by which they manage to lower the total energy of the system by avoiding each other. While being of utmost interest to understand, correlation is generally computationally demanding to describe. To account for it during a dynamic process such as ionization is particularly challenging and this problem is the focus of the program package currently being developed in Stockholm and Madrid in collaboration. The program will eventually be able to solve the time-dependent Schrödinger equation (TDSE) for arbitrary manyelectron atoms under the action of pulsed fields and builds on a multi-reference Hartree-Fock close-coupling ansatz.
The main objectives that were carried out during the present STSM were to test the recently added parallel solver as well as implementing an analyser for the output from it. For this analyser we wrote a program that calculates the partial photoelectron spectra dP
/dE, dependent on the symmetry and the ionization channel index, of the wave packets generated by the TDSE program, based on the projection on a set of scattering states. The scattering states are obtained in terms of a discretized solution to the multi-channel Lippmann-Schwinger equation.
The analyser was successfully tested on helium against a separate two-active-electron program that is known to work correctly.
With these additions a large step towards finalizing the program was taken. When ready, the program will be able to give a theoretical description of realistic attosecond pump-probe experiments that quantitatively accounts for the parent-ion rearrangement during a photoionization event.
STSM by Antonella Cartoni, Università di Roma “Sapienza” (IT) with Patrick Rousseau, Université de Caen Basse-Normandie (FR)
On June 15th, 2014 (6 days)
From ITALY to FRANCE
Study of the effect of the environment in the ion-induced fragmentation in uracil and halouracil
The halosubstituted analogues of DNA bases that can be efficiently incorporated in the DNA of fast replicating tumor cells to make them more sensitive to the lethal effect of radiation in radiotherapy are known as radiosensitisers. The 5-bromouracil (5BrU) as well as 5-fluorouracil (5FU, Figure 1), analogues of thymine, are considered good radiosensitisers even though the elementary mechanisms of their action is still not completely clear.
The main goal of the work performed at the ARIBE beamline in collaboration with Patrick Rousseau from CIMAP and Paola Bolognesi from CNR-ISM was to investigate the effect of the environment on the radiosensiting mechanisms of halouracil molecules, and in particular on the properties and behavior of the selected target molecules by studying and comparing the 12C4+ ion induced fragmentation of uracil, 5BrU and 5FU considered as isolated targets and embedded in pure and hydrated clusters. The aggregation source shown in Figure 2 has been used to efficiently produce the clusters (see for example Figure 3).
The study demonstrates that the presence of water molecules in the environment surrounding the 5BrU increases the formation of 5BrU in its enol tautomers with respect to Uracil molecule. Moreover the molecular modifications probably explain the extensive and the very fast fragmentation processes taking place on the halosubstituted rather than on the natural uracil base and hence its radiosensitising effects. These results are the first experimental evidence of the hypothesized mechanism  for the mutagenic activity of halouracil: the existence of keto-enol tautomerization induced by the water environment.
This work will produced more than one publication and future experiments on thymine and other bases have been planned with the CIMAP, CNR-ISM and Dipartimento di Chimica (Università Sapienza di Roma) groups. Bergen et al. Rev. Sci. Instrum. 70 (1999) 3244
 X. Hu, H. Li, J. Ding, S. Han, Biochemistry 43, 6361–6369 (2004).
Doubly charged metal ions Reactivity with peptides. An ab initio molecular dynamics perspective
A good knowledge of the interaction of formamide, as a suitable model of a peptide function, with alkaline-earth doubly charged metal ions is important to understand many aspects of the behavior of proteins at the molecular level. This motivated the study of such interactions from both the experimental and the theoretical viewpoints, the dynamical aspects being crucial to rationalize the experimental results (See Fig. 1)
Figure 1: In CID processes alternative trajectories (pink curve) to the minimum energy path may be accessible leading to non-thermodinamically favored products.
The first step focused on a thorough assessment of the model to be used in the ab initio molecular dynamics simulations of the [M(formamide)]2+, M=Ca and Sr, unimolecular reactivity. This assessment included 21 different functionals, and was carried out to account simultaneously for geometries, energetics and kinetics (by means of RRKM rate constants). The subsequent ab initio molecular dynamics simulations provide us with an atomic level description of the reaction mechanism (Fig. 2), which explains the preferential loss of formamide, for both Ca2+ and Sr2+containing systems, in spite of being the more endothermic processes. We could also explain the lower reactivity of the heavier metal and account for most of the other products of both reactions, either associated to Coulomb explosions or neutral loss.
Figure 2: Evolution with time of the natural charges and some representative distances for a trajectory yielding a Coulomb explosion product.
The aforementioned assessment will be published in Phys. Chem. Chem. Phys. The ab initio molecular dynamics simulation performed after the assessment will form the body of a second publication which was almost finished during the STSM. One of the most important conclusions of the analysis of the dynamical results was the necessity of doing post-TS dynamics to actually explain the formation of some products when the reactive processes are too slow. This will be the objective of our immediate work, which will show the importance of the bifurcations on the potential energy surface, that can only be accounted for through the aforementioned post-TS dynamics.
STSM by Danielle Dowek and Kevin Veyrinas, Institut des Sciences Moléculaires d Orsay, Université Paris-Sud (FR) with Fernando Martín and Alicia Palacios, Departamento de Quimica, Universidad Autónoma de Madrid (ES)
On March 17th, 2014 (5 days)
From FRANCE to SPAIN
Molecular frame photoemission in dissociative ionization of H2, D2 and HD
The main objective of this STSM was to discuss the interpretation of molecular frame circular dichroism induced in one-VUV-photon dissociative ionization (DPI) of H2 and D2 at resonance with the Q1 and Q2 doubly excited states. This observable is the most sensitive probe of the relative phases of the dipole matrix elements accounting for electronic and nuclear coupled dynamics at play on the femtosecond time scale during DPI. A paper is in preparation which compares the outcome of experimental observations based on electron-ion coincident momentum spectroscopy at the DESIRS beamline (SOLEIL) and time dependent Schrödinger equation (TDSE) calculations performed at UAM, for H2 and D2 at different photon energies.
During the short visit, new perspectives have been considered for the interpretation of the circular dichroism in the angular distributions (CDAD) supported by recent experimental results for dissociative photoionization of the HD isotope on the one hand, and by a refined level of calculations on the other hand. On this basis a new series of calculations has been decided with a focus on the role of the nuclear phase.
In parallel, the theoretical modelization of two-VUV-photon dissociative and non-dissociative ionization of the H2 and D2 molecules involving resonant excitation of an intermediate excited neutral state has been discussed aiming at the prediction of experiments planned at the FERMI FEL.
The next steps in the collaboration will include finalization of the manuscript dedicated to “Circular dichroism in resonant dissociative photoionization of H2 and D2” after analyzing the outcome of the on going TDSE calculations, preparing the paper focused on the isotope effects in HD, working on the two-VUV-photon project.
Figure: 3D molecular frame angular distribution induced by left handed circularly polarized light.
The results derived from this STSM have been published in:
J. F. Perez-Torres, J. L. Sanz-Vicario, K. Veyrinas, P. Billaud, Y. J. Picard, C. Elkharrat, S. M. Poullain, N. Saquet, M. Lebech, J. C. Houver, F. Martin, and D. Dowek, “Circular dichroism in molecular-frame photoelectron angular distributions in the dissociative photoionization of H-2 and D-2 molecules,” Physical Review A, vol. 90, iss. 4, 2014. https://journals.aps.org/pra/abstract/10.1103/PhysRevA.90.043417
Exterior complex scaling and momentum space approaches to matter in ultrafast intense pulsed lasers
To describe accurately observables such as the ATI spectrum of an atom or molecule in a low frequency pulsed laser field, one is required to represent a time dependent wavepacket for long times and very large radial distances. However the exterior complex scaling (ECS) approach allows one to restrict the radial description to moderate distances.
Scrinzi and co-workers have successfully used ECS with finite elements over a semi-infinite region and combined it with a surface flux integral approach (t-SURFF) to extract the ATI spectrum. We have shown that one can also use a complex B-spline basis defined over a finite region to efficiently calculate experimental observables. The combined ECS/t-SURFF approach allows one to restrict the spatial description of the wavepacket to relatively small finite regions of space at the expense of integrating the TDSE for times much longer than the duration of the pulse. We have extended the Arnoldi method to treat the complex symmetric matrices that arise in the time propagation with ECS.
Fig. Comparison of the ‘exact’ ATI spectrum for a 1-D Gaussian potential (crosses) with that obtained using the ECS/t-SURFF method for different integration times T, 2T and 4T, where T is pulse duration, for a 6 cycle cosine squared envelope pulse of frequency and intensity of 1013 W/cm2.