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February, 27 2015 - h:15:00 - Aula Rossa, Dipartimento di Fisica Ed. 8, Politecnico di Milano

Stefan Pabst

Title: Atomic Many-Body Physics in the Attosecond World

Abstract: Ultrafast science is a young, vibrant, and highly interdisciplinary research field combining areas of biology, chemistry, and physics. Attosecond and strong-field physics focus on the electron motion in atoms, molecules, and solid state systems. In this talk, I discuss in more detail the importance of many-body processes in electronic motion as they appear in noble gas atoms when exposed to ultrashort and strong-field pulses. Furthermore, I explain the challenges and the perspectives of many-body theories in the strong-field regime. I show, in particular, how a time-dependent configuration interaction (TDCIS) approach can be used to uniquely identify collective many-body effects in high-harmonic generation, and to explain line deformations in the attosecond transient absorption spectrum as they occur when probing sub-cycle tunnel ionization dynamics.

host: Caterina Vozzi

May, 27 2014 - h:14:45 - Aula Rossa, Dipartimento di Fisica Ed. 8, Politecnico di Milano

Amelié Ferré
Université Bordeaux I

Title: Identification of multi-channel static High Harmonic Generation for applications on SF6 vibrational dynamics and PhotoElectron Circular Dichroism (PECD)

Nowadays, the  techniques and applications of High Harmonic Generation (HHG) are largely developped, either using the HHG process as a probe of the emitting medium (high-harmonic spectroscopy) or as a source of femtosecond XUV pulses. In both cases,  the coherence, brightness, and short duration of the HHG Process are essential. One of the challenges in high-harmonic spectrocopy is the identification of multiple channels associated to tunnel ionization from different orbitals, coherently summed in the harmonic emission.
For small molecules, many studies rely on aligning the molecular sample relative to the laser electric field, which is not possible for most polyatomic systems.
In this seminar, I will present results combining different techniques for channel identification in a non-alignable polyatomic system: SF6. We have access to a complete characterization of the emission Spectrum, polarization, ellipticity and harmonic emission time, which enables us to identify a transition between two dominant channels (HOMO-3 and HOMO-1) in the harmonic emission.
This study has important consequences for high-harmonic spectroscopy of excited molecules : I will show that the different channels respond differently to molecular vibrations, both amplitude and phase. On the other hand, this study also opens new perspectives for high-harmonic light sources. We have discovered that  harmonic 15 in SF6 could be almost circularly polarized (80% ellipticity), enabling  PhotoElectron Circular Dichroism (PECD) measurements in chiral molecules. We have ionized  Fenchone (C10H16O) by left or right elliptical XUV radiation and measured  an assymetry in the angular electron distribution,  encoding the signature of circular dichroism. This study opens the route to femtosecond and attosecond time-resolved PECD experiments.

host: Caterina Vozzi

December, 5 2013 - h:14:45 - Aula Rossa, Dipartimento di Fisica Ed. 8, Politecnico di Milano
Prof. Dr. Alejandro Saenz
Humboldt-Universität zu Berlin, Institut für Physik, AG Modern Optics

Molecules in Intense Laser Fields: From Molecular Hydrogen to Water Molecules

Presently, many efforts are spent in exploring the use of ultrashort intense laser pulses with long wavelengths for time-resolved imaging of molecular processes like chemical reactions. Both, the emitted high-harmonic radiation or the rescattered electrons may be used for obtaining structural information on a sub-femtosecond time scale. However, this requires a full understanding of the molecular response to intense laser fields. In this talk our efforts in understanding the behaviour of molecules in strong fields will be summarized. After an introduction of the theoretical challenges some results obtained with our two-electron code describing molecular hydrogen or our newly developed gauge-invariant single-active-electron approximation for arbitrary molecules will be shown and their consequences for imaging will be discussed.

host: Caterina Vozzi