Here we aim to use the powerful isolated attosecond XUV and X-ray pulses realised in the framework of the project A.3.1 for establishing nonlinear attosecond science by demonstrating the feasibility of using attosecond pulses for both triggering and probing electronic phenomena simultaneously. Such measurements are necessary for studying dynamics that are obscured or even destroyed by an intense laser pulse, which is used in all contemporary attosecond spectroscopies.
High harmonic generation in gases is expected to deliver sub-100-as XUV pulses in the 100-eV photon energy range with at least two orders of magnitude higher pulse energy than any previous source before. Such a source created ideal prerequisites for attosecond XUV-pump / XUV-probe measurements with a resolution currently inaccessible to sources based on free-electron lasers. As a first step, we plan to apply two-photon ionisation of a noble gas for temporal characterisation of the attosecond pulse via second-order nonlinear autocorrelation. This can be extended to a more sophisticated frequency-resolved temporal gating scheme to perform a complete temporal characterisation of the XUV pulses.
Once fully characterised, the pulses will be used for proof-of-concept attosecond pump / probe spectroscopy. By promoting an electron from a core state to a manifold of unoccupied states with an attosecond XUV pump pulse and tracing the unfolding wave packet dynamics and core hole decay with an attosecond XUV probe pulse, we can study correlated electron motion in atoms and molecules. The investigations can be extended to higher, soft-X-ray photon energies (--> 1 keV), once energetic attosecond pulses in this range become available from surface harmonics.