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MIDAS Report Summary

Project ID: 336468
Funded under: FP7-IDEAS-ERC
Country: Israel

Mid-Term Report Summary - MIDAS (Multidimensional Spectroscopy at the Attosecond frontier)

Attosecond science has revolutionized our ability to resolve electron dynamics on its natural time scale. Currently, one of the main challenges in this field is imposed by the multiple degrees of freedom inherent to strong light-matter interaction. The main goal of the MIDAS project is to establish novel approaches in attosecond time-resolved measurement. Specifically, we aim at integrating multidimensional approaches into attosecond science in order to decouple its primary degrees of freedom.
Performing time-resolved measurements with attosecond precision is a significant challenge. Currently, the main approaches to attosecond measurements include Attosecond Pump-Probe Spectroscopy and Attosecond Self-Imaging. We have integrated the two branches, introducing XUV-initiated HHG spectroscopy. In this scheme the initial excitation is induced by an attosecond XUV pulse. Within one optical period, an electron is ionized by the XUV pulse via photoionization, accelerated by the strong laser field, and it re-collides with the parent ion, consequently emitting XUV radiation.
We have constructed a new apparatus for the generation and measurement of XUV-initiated HHG. In the first stage an attosecond pulse train (APT) is generated by focusing an intense IR laser pulse into a source gas cell. The IR and APT beams co-propagate and are refocused into a second gas cell in order to produce XUV-initiated HHG. An in-depth study of the underlying mechanism requires applying a multidimensional scheme. We increase the dimensionality of the measurement by adding a weak second harmonic (SH) field to the strong fundamental field. When the SH field is synchronized at the first stage it modifies the spectrum of the XUV ionizing field and therefore allows us to study the ionization mechanism. When the SH is synchronized in the second stage it modifies the recollision mechanism and allows us to study its dynamics. For both measurement schemes we performed a 3D experiment where we map, for each harmonic number, its dependence on the XUV-IR delay and the IR-SH delay.
An additional study aims at revealing an electron’s rearrangement during the HHG mechanism. The strong laser field that triggers the high harmonic response opens up multiple quantum pathways for multielectron dynamics in molecules, resulting in a complex process of multielectron rearrangement during ionization. So far, the sub-cycle nature of this process has been hidden in most experimental observations. We have introduced an advanced HHG spectroscopy and have demonstrated its ability to follow multielectron dynamics of a core rearrangement on a sub-laser cycle time scale. Collaborating with the group of O. Smirnova from the MBI, Berlin, we reconstructed the relative phases for all relevant ionization channels in CO2 molecules, with sub-cycle resolution. These phases encode the dynamics of multielectron rearrangement upon strong-field ionization. Our study brings us one step closer to fulfilling this initial promise in developing robust schemes for sub-femtosecond imaging of multielectron rearrangement in complex molecular systems.

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