26 Mars – Thesis defense - Arturo Sopena Moros

15 h Videoconference

Study of relativistic effect in non-linear interaction between molecules and XUV/soft X-ray short laser pulses.

The development of intense XUV sources through free electron lasers and high-order harmonic generation in the femtosecond (fs) and sub fs domains provides a unique tool to investigate non-linear laser matter interaction at ultra-short time durations in molecules. In this context, the resolution of Time Dependent Schrödinger Equation has proven to be powerful for interpretation of experimental results. The possibility of coherent and simultaneous control of both electronic and nuclear dynamics in the attosecond time scale urges for theoretical approaches beyond the Born-Oppenheimer (BO) approximation. On the other hand, it is well-known that, in the XUV/X-rays domains, the dipole approximation (DA) must be carefully used. The focus of this thesis is the study of two photons processes and their role in the coupled electron-nuclear dynamics they induce as well as their study beyond the DA.
We begin with a description of the formalism and mathematical methods used to describe the photoionization dynamics. Our approach is based on a spectral method, which demands that we first start by determining the quantum states of the field free molecule. To account for all electronic and nuclear degrees of freedom we resort to BO to obtain a set of vibronic eigenstates which are expanded in a basis of B-splines and spherical harmonics. Additionally, we resort to the Feshbach projectors and multi-channel scattering theory for a correct description of the electron continuum and autoionization. The interaction with radiation is included by introducing the first two terms of a Taylor expansion of the vector potential in the Hamiltonian. The first one corresponds to the well-known dipole approximation and the second accounts for retardation effects. Finally, we describe the propagation methods needed to obtain ionization amplitudes from which we can extract observables such as ionization cross-sections, photoelectron spectra and molecular frame angular distributions (MFPADs).
In the first part of the results, we present simulations of an experimental set-up in which a VUV attosecond train pulse triggers the electron-nuclear dynamics which is then probed by means of an intense IR pulse. We then discuss the potential of harmonic filtering techniques on the steering of 1-photon and 2-photon ionization routes and its consequences on the appearance of asymmetries in the MFPADs.
In the second part of the results of the thesis we report the first calculations of Stimulated Raman and Compton Scattering (SRS/SCS) in H2 with fixed nuclei. The high photon energy, XUV/X-ray, and intensity necessary for the observation of these phenomena requires the inclusion of non-dipole effects in the calculation. We validate the use of perturbation theory, by direct comparison with non-perturbative calculations, which enables us to present an interpretation of SRS and SCS in terms of two-photon processes.
We then report photoelectron spectra and MFPADs from calculations of SCS with photon energies up to 1.6 keV. We compare the relative contribution of dipole and non-dipole routes as a function of photon energy and analyze the manifestation of their coherent interference in the MFPADs. Special attention is given to the effect of molecular orientation.
Finally, we study SCS with two color, focusing on the effect of the angle between the pulses. As seen in atoms, non-dipole effects are enhanced for counterpropagating pulses. We also investigate the effect of molecular orientation and color separation.

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