Berlin 2014 – scientific programme

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A: Fachverband Atomphysik

A 10: Atomic systems in external fields

A 10.2: Talk

Monday, March 17, 2014, 14:30–14:45, BEBEL E44/46

Time-resolved Fano spectroscopy and control of laser-coupled doubly-excited states — •Andreas Kaldun, Christian Ott, Alexander Blättermann, Martin Laux, Kristina Meyer, Thomas Ding, Andreas Fischer, and Thomas Pfeifer — Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany

The precise characterization of quantum states is at the heart of atomic and molecular physics. In absorption spectroscopy, different spectral line shapes arise by the interference of the system's dipole response with the incoming light. We here present and experimentally confirm a method to extract dynamical phase- and amplitude modifications of quantum states in laser coupled few-level systems on the example of the lowest doubly-excited states in helium atoms. These states are coherently excited by an attosecond-pulsed extreme-ultraviolet (XUV) field and coupled by a near-visible (VIS) laser pulse. A controlled time delay between the pulses results in small changes in the absorption line shapes of the doubly-excited states, resolved with a home-built high-resolution (20 meV) XUV spectrometer. The observed changes can be understood by considering a coupled four-level system in second order perturbation theory, giving experimental access to the created two-electron wave packet. From the experimental analysis of dipole amplitudes and phases as a function of time delay and intensity, we are able to separate different few-photon transition pathways. We find evidence for a coherent contribution of the N=2 continuum to the two-electron wave packet.