Investigation of the continuum-continuum phase in attosecond time delays

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The process of photoionisation has been assumed for a long time to be an instantaneous process: a photon of sufficient energy impinges on a quantum system (atom, molecule or surface) and an electron is instantaneously released in the continnum. The development of attosecond technologies has allowed, for the first time, to resolve in time this fundamental process and to measure time delays between the photoelectrons emitted from different energy levels in the same system or from different quantum systems. The measurement of these delay is relevant, as it gives access to the characteristics of the potential experienced by the electrons on its way out. All experiment focused on the characterization of time delays are based on the combination of an attosecond waveform in the extreme ultraviolet and (typically) an infrared laser pulse. The interaction with the latter introduces an additional delay (indicated as continuum-continuum (CC) delay), which must be corrected for, in order to isolate the response of the quantum system to the attosecond field. The estimation of the CC delay has been based so far on theoretical models and no experimental measurement has been reported so far. The goal of this project is to provide the first experimental characterization of the CC delay, by taking advantage of the results recently demonstrated at the Free-Electron Laser FERMI by the proponent. The project will be based on the development on an optical parametric amplifier that, combined with an attosecond train source, will give the possibility to generate photoelectron spectra encoding directly the information on the CC delay. The results will be compared with numerical solution of the TDSE and predictions of perturbation theory that will be delivered by two theoretical groups supporting the project.