Archive for the ‘Funded STSMs’ Category

STSM by Cristina Sanz-Sanz, Department of Physical Chemistry, Universidad Autónoma de Madrid with Graham Worth, School of Chemistry, University of Birmingham
On September 23rd, 2013 (10 days)stsm_uam-birm-3
From SPAIN to UNITED KINGDOM

 

Fitting of field dependent potential energy curves, spin-orbit and elements of the dipole moment matrix of the first 36 states of IBr

The control of the photodissociation of IBr through a curve crossing was studied in the group of Prof. Stolow using the dynamic Stark effect. Until now, theoretical simulations have used a reduced model including just 3 electronic states. However, the system consists on 36 electronic states dissociating to the ground states of atoms. We have computed and fitted the 36 potential energy curves, spin-orbit and elements of the dipole matrix for several electric field strengths and orientations. Those curves will be used in the dynamical calculations to reproduce the experiment.

The electronic structure calculation programs do not maintain the phase of the wavefunction and it translates into jumps in the spin-orbit and transition dipole moment curves. In order to use these curves in dynamical calculation programs the curves have to be fitted. Because of the sudden changes in the curves normal fitting methods do not work. We have used an optimization method to smooth out the spin-orbit and transition dipoles.

During the STSM visit we finished with the fittings of the potential energy curves, spin-orbit couplings and the elements of the dipole moment matrix for several electric field strengths and orientations. The dynamical calculations will be done using MCTDH package and we created the input files and fittings required for the wavepacket calculations. A test calculation was done for a free field example using the 36 spin-orbit states. In addition, we wrote the outline of the first publication of a series of works that will include the global fittings of potential energy curves, spin-orbit couplins and dipole moment matrix.

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STSM by Patrick O`Mahony, Royal Holloway, University of London with Bernard Piraux, Universitè Catholique de Louvain, Louvain-la-Neuve
On August 22th, 2013 (9 days)
From UNITED KINGDOM to BELGIUMstsm-MOTA-img

Time Scaling and Momentum Space Models in Intense Fields

The Time Dependent Schrödinger Equation (TDSE) in momentum space provides a very useful alternative to the coordinate representation to describe atomic and molecular processes in an intense laser field.  We wish to solve this equation in the very low frequency limit.

Taking a model in which the kernel of the nonlocal Coulomb potential in momentum space is replaced by a finite sum of separable potentials each supporting one bound state of atomic hydrogen, we have cast the kernel for the resultant integral equation in a form which is a sum of a pole term and an integral with has a rapidly decaying integrand which is smooth and doesn’t oscillate making it easy to evaluate numerically. We showed that in the limit of the frequency going to zero the pole term doesn’t contribute and we expect the integral to be strongly peaked making it possible to easily solve the integral equation.

We are now in a position to study semi-analytically the limit as the frequency of the laser goes to zero which would be impossible to do by solving the TDSE as the pulse length becomes extremely long. In this way we should be able to obtain new insights into the low energy structure (LES) found in both atoms and molecules in intense fields.

STSM by  Francisca Mota-Furtado,Royal Holloway, University of London with Bernard Piraux, Universitè Catholique de Louvain, Louvain-la-Neuve
On August 22th, 2013 (9 days)
From UNITED KINGDOM to BELGIUMstsm-MOTA-img

Time-dependent methods for strong laser fields

Robust time propagators are required to solve the Time Dependent Schrödinger Equation (TDSE) for atomic, molecular and solid state systems as for example when matter interacts with an intense low frequency laser field.

The direct numerical ab initio solution of the TDSE using spectral methods leads to large systems of first order equations with a high degree of stiffness. We focussed on two explicit methods, Fatunla’s method and the Arnoldi algorithm.  Both of these methods have optimum stability properties but they differ in the degree of accuracy they are able to reproduce. We identified strategies to use them both successfully.

During the short visit we finished a joint paper with our hosts on this topic which has been submitted to Physical Review A.

STSM by  Patrick O’’Keeffe, National Research Council of Italy (Rome) with Michael Meyer, European XFEL Hamburg
On August 19th, 2013 (6 days)
From ITALY to GERMANYstsm-POK-thumbnail

Multiphoton ionization combining radiation from synchrotrons and free electron and optical lasers

The principal scientific goal of the mission was to allow the applicant (Patrick O’Keeffe, National Research Council of Italy, Rome) and the host (Michael Meyer, European XFEL Hamburg, Germany) to collaborate with Alexei GrumGrzhimailo and Elena Gryzlova of the Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University in order to prepare a manuscript based on experimental results obtained during a beam time at the French synchrotron source, SOLEIL, on the two-photon resonant ionization of Xe combining synchrotron and optical laser light.

The novelty of the results lies in the fact that the angular distributions of the emitted electrons were measured in coincidence with the isotope resolved Xe ion formed in the same event. This allowed the photoelectron angular distributions of the nuclear spin (I) zero atoms to be separated from the non-zero I atoms. This has the important consequence that the effects of nuclear spin depolarization of the intermediate state due to precession of the electronic angular momentum about the total angular momentum vector can be eliminated. This allows the pure electronic dynamics of the photoionization to be isolated.

The manuscript prepared during the STSM was recently published in Physical Review Letters:
Isotopically Resolved Photoelectron Imaging Unravels Complex Atomic Autoionization Dynamics by Two-Color Resonant Ionization, P. O’Keeffe, E.V. Gryzlova, D. Cubaynes, G.A. Garcia, L. Nahon, A.N. Grum-Grzhimailo, and M. Meyer, Physical Review Letters, 111,  243002 (2013).

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Figure: The raw photoelectron images for parallel linear polarizations of both SR and optical laser light of the photoelectrons taken in coincidence with the non-zero and zero nuclear spin isotopes of Xe. The images are shown as 3D representations in order to visualize clearly the effect of the nuclear spin on the photoelectron angular distribution. Copyright: American Physical Society.