Archive for the ‘Funded STSMs’ Category
STSM by Jorge Alejandro Budagosky Marcilla, Institute for Biocomputation and Physics of Complex Systems (Zaragoza), with Esa Räsänen, Tampere University of Technology (Tampere)
On February 16th, 2014 (7 days)
From SPAIN to FINLAND
Optimal control of high harmonic generation
At sufficiently high intensities, matter reacts non-linearly to light, and may re-emit at integer multiples (harmonics) of the frequency of the incoming source. The spectrum of atoms and molecules exposed to very intense laser pulses was found to present unexpectedly high harmonics, and its shape was observed to have a plateau extending over many orders of magnitude – a process known as high harmonic generation (HHG). The light emitted in this manner is coherent and may reach the extreme ultraviolet and soft X-ray frequency regime. These properties can be of paramount importance for many technological and scientific purposes.
We examine computationally the possibility of optimizing the HHG spectrum of Hydrogen atoms by shaping a laser pulse in the THz range. The spectra are computed with a fully quantum mechanical description, by explicitly computing the time-dependent dipole moment of the systems, which are modeled in one dimension. Specifically, by the optimal control theory (OCT), we studied the possibility of arbitrarily adjusting the plateau extension in harmonic spectra.
Preliminary results obtained so far show that it is possible to optimize the HHG spectrum in order to arbitrarily extend the plateau length. The length of the plateau can be controlled not only by using a frequency window (target) or by increasing the pulse intensity, but increasing the length of this. In general, we have observed the presence of characteristic structures in the pulses that can be directly associated with particular processes (ionization, recombination, etc.). The latter is still under discussion.
High Harmonic Generation from biological molecules
High Harmonic molecular spectra provide insights into attosecond electronic dynamics in the target molecule. Accurate values of the dipole matrix elements between bound and continuum molecular states are required to generate the spectra. This STSM has allowed Zdeněk Mašín to learn to use the codes developed at Max Born Institute to generate the dipole matrix elements from the results of the R-matrix calculations and to apply them to calculations of photoionization cross sections.
The calculations were performed using the UKRmol suite of codes implementing the molecular R-matrix method. In order to benchmark the quality of our calculated dipole matrix elements for pyrazine we calculated the photoionization cross sections and photoelectron angular distributions (shown in the Figure). The results, performed using a simple Close-Coupling model including the lowest-lying 40 electronic states, show an encouraging agreement with previous experiments and theory. However, the description of the correlation/polarization interaction was limited in this model and therefore the shape resonances, such as the broad resonance in the 6ag cross section, appear too high in energy. This deficiency will be removed in more sophisticated models which we are developing.
This STMS has allowed us to obtain the first photoelectron angular distributions for pyrazine and to identify directions for further improvement of our calculations so that accurate High Harmonic spectra can be obtained. The next steps in the collaboration will include:
- Improving on the target CI description in the scattering calculations by including dynamical correlation.
- Calculating the population transfer in the target cation induced by the laser field.
- Analyzing the Dyson orbitals generated using the R-matrix codes.
- Generating the High Harmonic spectra.
STSM by Jan Franz, Faculty of Applied Physics and Mathematics, Gdansk University of Technology, with Graham Worth, University of Birmingham.
On February 2nd, 2014 (14 days)
From POLAND to UNITED KINGDOM
Simulations of the photodissociation of amino acids
The goal of this project is to set up a computational framework which will enable us to simulate the photodissociation of amino acids and other molecules of biological interest. Ultimately these simulations should contribute to the ongoing discussions about the origin of homochirality in the building blocks of living organisms and about the origin of life.
The first aim of this mission was to familiarise with the Multi Configuration Time Dependent Hartree (MCTDH) method to solve the time-dependent Schrödinger equation for multi-dimensional multi-state problems. The second aim was to set up quantum chemistry calculations to compute the parameters of the effective vibronic Hamiltonian used in the MCTDH simulations.
Some initial test calculations of the photodynamics of alanine have been performed. The figure shows the population of the three lowest-lying electronically excited states as a function of time after excitation into the third excited state. We will continue to collaborate on this project.
STSM by Tamas Rozgonyi, Institute of Materials and Environmental Chemistry Research Centre for Natural Sciences of the Hungarian Academy of Sciences (Budapest), with Philipp Marquetand, Institute of Theoretical Chemistry, University of Viena.
On January 30th, 2014 (10 days)
From HUNGARY to AUSTRIA
Simulating quantum dynamics in the presence of strong field ionization
Simulating strong field (multiphoton) molecular ionization by ultrashort pulses in the presence of nuclear dynamics by sophisticated methods is extremely time consuming. The goal of our collaboration is to develop a numerically efficient approximate description balanced between completeness and efficiency.
As a solution we combine the methods of continuum discretization and adiabatic elimination of off-resonant electronic states. During the short visit we thoroughly discussed the theory behind these two methods involving also an undergratuate student from the host institute into the project, derived the equations for the case when nuclear dynamics is taking place on resonant electronic states (these are treated explicitly) and developed a new routine for the quantum dynamics (QD) program of the host institute.
We plan to apply the new program to describe the dissociative photoionization of halomethanes and to rationalize pulse-shape dependent photoelectron spectra and photoelectron-photofragment coincidence measurements on both halomethane and CS2 molecules.
Time-dependent Schrödinger equation for coherent electron dynamics in poly-electronic atoms
The energetics of chemically relevant processes is largely affected by the way electrons manage to avoid each other, thus reducing their mutual repulsion, a phenomenon called correlation. The need to go beyond independent-particle models is also the reason why an accurate theoretical description of many-electron systems is so hard to achieve. As a consequence, correlation is firmly at the center of any investigation of atomic and molecular reactivity.
Photoionization with controlled sequences of sub-femtosecond pulses is a good way to monitor the dynamical manifestation of correlated motion since electrons respond to the sudden ejection of one of them on a characteristic timescale of few tens of attosecond to a few femtoseconds. To reproduce theoretically the outcome of similar processes, it is necessary to integrate numerically the time-dependent Schroedinger equation (TDSE), the equation of motion of isolated quantum systems, in a very big configuration space. We achieve this in practice by running large simulations on parallel supercomputers.
The objective of this short-term scientific mission is to adapt a parallel solver for the TDSE to the output of the multi-reference Hartree-Fock close-coupling program developed in Stockholm, which provides bound and single ionization states for arbitrary atoms, as well as the radiative transition amplitudes between them.
The parallel solver is based on a second-order split-exponential propagator on a spectral basis. The action of the exponential of the dipole non-diagonal interaction on the propagating state vector is achieved with an iterative Krylov space method implemented using the PETSc parallel library. The close-coupling multi-reference Hartree-Fock representation of the single-ionization space for arbitrary atoms and ions, provided by the Stock package developed in Stockholm, was interfaced with the parallel solver and a first simulation for the ionization of the Ne+ ion was successfully carried out.
This result is a step towards a theoretical reproduction of realistic attosecond pump-probe experiments that takes quantitatively into account the parent-ion rearrangement following a photoionization event. During this visit, we extended to the time domain the Stockholm atomic-structure program with a parallel solver for the TDSE. We also planned the upgrade of the program that computes the scattering states used to extract asymptotic photoelectron distributions from time-dependent wave packets.
STSM by Károly Tökési, from Institute of Nuclear Research of the Hungarian Academy of Sciences, Debrecen with Joachim Burgdörfer, Institute for Theoretical Physics Vienna University of Technology, Vienna
On January 10th, 2014 (16 days)
From HUNGARY to AUSTRIA
Classical simulation of laser-matter interaction
Since the first observation of different (apparent) emission times of photoelectrons in laser-surface interactions we have started to simulate the transport of photoexcited electrons through the target material. We determine the apparent delay in the emission of low-energy electrons depending on their initial state (conduction band or bound state). We therefore plan to simulate low-energy electron transport in the combined fields of the probe pulse and the localized hole left behind.
During the present STSM we address our expertise on simulating electron transport and calculating the (linear) response of electronic structures to external perturbations. The most work was invested into the improvement of our classical transport code. The special appeal of the transport simulation was its conceptual simplicity and the resulting numerical efficiency which allows for Monte-Carlo simulations with excellent statistics within a reasonable amount of computing time.
A computer code was improved, which allows us the theoretical study of electron transport in solid. With our electron transport code we are able to perform benchmark simulations of streaking experiments. The results will be published in international journals.
STSM by Bernard Piraux, from Université catholique de Louvain (Belgium) with Patrick O’Mahony, Royal Holloway, University of London.
On January 20th, 2014 (5 days)
From BELGIUM to UNITED KINGDOM
Interaction of one-active electron systems with two perpendicular pulsed oscillating fields
The main objective of this STSM was the development of a computer code to solve numerically the Schrödinger equation that describes the interaction of a one – active electron system with two perpendicular oscillating pulsed electric fields. This code should allow one to treat various field configurations as for instance, elliptically polarized fields, fields with time-dependent polarization and pump-probe schemes with linearly polarized fields along perpendicular directions. Emphasis is put on low frequencies making this problem very challenging and thereby requiring state-of-the art time-propagation methods.
During the short visit, we performed the first tests of the code in the case of the interaction of pulsed circularly polarized high frequency fields with atomic hydrogen initially in various excited states where few benchmark results exist for the total ionisation yield. Moving to the low frequency regime requires efficient explicit time propagators. We took the opportunity of this short visit to finalize a paper which has just been accepted for publication in Phys. Rev. A:
Explicit schemes for time propagating many-body wave functions, A.L. Frapiccini, A. Hamido, S. Schröter, D. Pyke, F. Mota-Furtado, P. O’Mahony, J. Madroñero, J. Eiglsperger and B. Piraux.
Post-transition state chemical dynamics for gas phase reactivity of dications
Our aim is to understand reaction mechanisms that characterize reactivity of dications in the gas phase, in particular complexes formed by a divalent metal (Ca, Sr for example) and an organic molecule. Two general pathways are possible, Coulomb explosion and neutral loss and chemical dynamics simulations are performed to understand weather and when reactivity proceeds through a statistical or to a non-statistical mechanism, and how the metal affects the reactivity.
Post-transition state dynamics were able to explain the difference in reactivity found between the two systems, in particular the Sr2+ + formamide reaction channel was found to be much more probable than the analogous Ca2+ + formamide one when starting from a particular transition state. Another particular result was obtained by analyzing the reaction pathways dealing to [Ca(H2O)]2+ that proceeds to a different and fast pathway with respect to what obtained from PES study. This pathway was not observed for related Sr2+ system, confirming experimental results.
We are now planning to perform detailed analysis on wave-function character and bond character across the two reaction pathways to couple the dynamical differences with chemical differences. Furthermore, we decided to run a fourth (and probably last) set of chemical dynamics simulations from another transition state. Finally, we are setting up publications related to the project, and in particular thanks to the ad hoc analysis tools provided during the visit, we are rationalizing all post-transition state dynamics results to show how this approach can be complementary to the usual PES approach.
STSM by Ana Martín Sómer, Universidad Autonoma de Madrid with Marie-Pierre Gaigeot, Laboratoire Analyse et Modelisation pour la Biologie et l`environnement, LAMBE, Evry
On November 17th, 2013 (14 days)
From SPAIN to FRANCE
Statistical approaches and chemical dynamics simulations to study CID experiments
This STSM allowed the Early Stage Researcher (Ana Martín Sómer) to learn how to perform the kinetic analysis of the unimolecular fragmentation dynamics of formamide-M^(2+) (M=Ca, Sr). Now, the researcher will be able to acomplish the kinetic analysis of the whole surface following the learned procedure.
The ESR also learn to use VENUS-MOPAC program, design the set-up for the AlaGlnAla protonated tripeptide system CID modelization and do some preliminary trajectories on the system. Some preliminary trajectories firsts to set-up the right parameters for running the dynamics simulations were submitted to have an idea of the computational time required to compute one trajectory.
Time-dependent atomic photoionisation with a Multi-Configuration-Hartree-Fock close-coupling approach
This STSM concerned the collaboration between the COST nodes in Stockholm and Madrid to build a new program to solve the timedependent Schrodinger equation (TDSE) for arbitrary poly-electronic atoms under the action of pulsed fields.
The program under construction extends a Multi-Conguration-Hartree-Fock (MCHF) atomic-structure package to include the coupling to an ionized electron. It is based on close-coupling ionization states built from MCHF parent-ion states coupled to radial B-splines. Prior to the STSM, the package could already reproduce photoionization cross sections for arbitrary atoms with both the initial bound state and the final continuum states described
at the MCHF level as detailed in a recent publication [Carette, Dahlstrom, Argenti
and Lindroth 2013 Phys. Rev. A 87 023420].
As a result of the STSM:
- The inclusion of the K-matrix package for the calculation of multichannelsingle-ionisation scattering states to resolve energetically and angularly the partial photoelectron spectra encoded in the electronic wave packetsobtained from simulations of pump-probe experiments on atoms was started. A similar integration technique was already successfully demonstrated in 2010 [Argenti et al. 2013, Phys. Rev. A 87 053405].
- The user friendly setup of the package was tested
- The benchmarking of neon- and argon photoionization, and especially the issue of how to obtain a correct energy position of the so-called Cooper minima in argon was discussed.