Max-Planck-Institut für Kernphysik Heidelberg

Atoms and molecules in ultra-short laser pulses

Priv.-Doz. Dr. Robert Moshammer

  Research topics:  |  Ions in Traps  |  Electrons in Collisions  |  Lasers in Time  |  
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Towards holographic imaging through electron wave packet interferences ?

Fig.1: 2D electron momentum distribution (linear scale) for a CEP with maximum asymmetry. The x-axis is the longitudinal momentum; the y-axis is the momentum transverse to the laser polarization axis. The red line indicates the position of p|| = 0. The projection onto the p|| -axis is indicated as a white line. Inset: TDSE calculations for a gaussian pulse (740 nm) with I0 = 0.3 PW/cm2, FWHM ~ 2.7 fs

Interferometry with coherent electron wavepackets (EWPs), modulated through ultrafast processes (10-18-10-15s) in atoms and molecules, opens new possibilities to enhance our understanding of these dynamics. EWP interferences, manifested in the photoelectron spectra of single ionization in atoms with Carrier Envelope Phase (CEP)-stabilized few-cycle (~5 fs) laser pulses, investigated by a ’reaction microscope’, present a hitherto unexplored path to map attosecond dynamics in atoms and molecules. In a recent experiment, linearly-polarized CEP stabilized 5 fs pulses at 740 nm (repetition rate: 3 kHz) obtained at the attosecond beamline at Max-Planck-Institut fr Quantenoptik, Garching with intensities up to 0.4 PW/cm2 at the focus was crossed with a supersonic, cold He jet (~1011 atoms/cm2) in the ultra-high vacuum chamber (~10-10 mbar) of the reaction microscope.

2D electron momentum spectra (Fig. 1) at certain fixed CEP reveal in addition to enhanced emission into one P-hemisphere, regular interference stripes, parallel to the transverse momentum axis in the corresponding hemisphere and radial structures in the opposite hemisphere, in good qualitative agreement with theoretical predictions and TDSE calculations. The spacing between the peaks, significantly smaller than the multi-photon peak structure previously observed for longer pulses, agrees with those calculated in a simple Strong Field Approximation (SFA)-based model. This model, invoking the interference of two quantum paths leading to the same final drift momentum, captures the basic mechanism of holographic imaging via the superposition of a re-scattered, modulated EWP on an unaffected, directly launched ’reference’ EWP (of the same electron!).

We believe that holographic imaging through EWPs has the potential to obtain unprecedented information on ultra-fast correlated electron dynamics, for example, the details of the (time dependent) scattering potential in atoms and molecules

Experiments at the Attosecond Beamline at MPI-K

With the dedicated Reaction Microscope installed at the attosecond beamline, the preliminary experiments and tests have begun. An XUV beam (harmonics centred on 46eV) was used to singly ionize Helium in the supersonic jet. The single ionization count rate confirms a photon flux of 1010 photons/s. Recoil ion time of flight reveal signatures of the harmonic structure of the XUV beam. Currently, optimization of the performance of the Reaction Microscope and the preparation of the beamline for the proposed pump probe experiments is underway.



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