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Physics of Atoms with Attosecond Light Pulses

Final Report Summary - PALP (Physics of Atoms with Attosecond Light Pulses)

The aim of the project was to advance the emerging new research field “Ultrafast Atomic Physics”, where electron wave packets are created by absorption of attosecond pulse(s) and analyzed or controlled by another short pulse. Our project was divided into three parts:
1. Interferometric measurements using tunable attosecond pulses
We studied photoionization dynamics with high temporal and spectral resolution, and in some cases even angular resolution. In the case of non-resonant photoionization, we measured photoionization time delays, representing the time of propagation in the atomic potential of an ionized electron for several systems: xenon, argon, neon and nitrogen. In the case of resonant photoionization, we studied the influence of an autoionizing quasi-bound state on the spectral and temporal properties of the electron wave packet. We could fully reconstruct the temporal dynamics of the ionizing electron wavepacket, close to the resonance and even studied its quantum coherence properties.
2. XUV pump/XUV probe experiments using intense attosecond pulses
Our aim was to study double ionization by absorption of two photons, at the attosecond time scale, to understand in depth the transition between non-sequential and sequential processes. An intense source of attosecond pulse trains, designed with help of scaling arguments, was developed. Two-photon sequential double ionization of neon atoms was observed. A split and delay unit was implemented for splitting the XUV pulse into two replicas and varying their relative delay. Tight and broadband focusing of high-order harmonics using toroidal mirrors in a Wolter configuration, in a vacuum chamber including a new double (ion and electron) velocity map imaging spectrometer was achieved. Finally, time-resolved measurements on the attosecond time scale were performed using XUV and IR fields. XUV pump/XUV probe experiments have revealed to be more difficult than originally thought. One possible reason for this, which was identified during the course of this project, might be the divergence properties of the high-order harmonics leading to strong spatio-temporal couplings of the attosecond pulses.
3.“Complete” attosecond experiments using high-repetition rate attosecond pulses
We developed a fully working high repetition rate attosecond laboratory with a reliable carrier-envelope-phase-stable optical parametric chirped pulse amplification laser system, a high vacuum chamber including a high pressure gas jet for attosecond pulse generation and an interferometric setup for pump-probe experiment, and a compact reaction microscope for the three-dimensional detection of the fragments emitted during the interaction. Interesting new results have been obtained regarding electron wave packets created in He after absorption of two or three attosecond pulses, in the presence of a weak dressing infrared field.

The intense attosecond beamline developed in subproject 2 has served as a prototype for a beamline being built for the European Facility ELI-ALPS in Szeged, Hungary. The stability requirement on our laser systems led to further development of the dispersion scan technique for the characterization of short laser pulse which is the subject of a proof-of-concept project.