Free Electron Laser
Starting with an ultra-short electron pulse (blue illustrated cloud in the lower left corner of the figure) of a few hundred femtoseconds and allowing it to travel further and further through the undulator, the initially
emitted light begins to interact with the electron packet in
such a way that the electrons (center bottom) come increasingly
in phase with the light: They move in common mode and the emitted radiation becomes coherent, i. e., the wave maxima and minima of the light trains emitted by the individual electrons lie on top of each other. This has dramatic consequences: The intensity of the emitted light pulse is no more, as in a normal, about 2 m long synchrotron undulator, proportional to the number of electrons in the bunch, but to the number squared, such that the emitted light intensity in
the up to 200 m long FEL undulators increases by a factor of
Fragmentation of Atoms and Molecules
In our experiments at the free-electron laser in Hamburg (FLASH), the SCSS in Japan and the LCLS in Stanford we investigate the response of atoms and molecules exposed to extremely intense und very short EUV light pulses. Under these conditions atoms and molecules undergo ionization and fragmentation reactions with high probability by instantaneous or sequential absorption of two or three photons. Of particular interest are transitions with two or more active electrons like e.g. double or multiple ionization. We use a specially designed Reaction Microscope to detect all created charged fragments (i.e. ions and electrons) in coincidence. The reconstructed 3D momentum vectors of all particles reveal insights into the underlying fragmentation dynamics and enable us to separate different ionization mechanisms. Reactions that we study this way are:
- Direct (instantaneous) two-photon two-electron transitions in atoms
- Sequential (step-wise) multiple ionization of atoms and molecules
- Coulomb explosion of molecules
Very recently we succeeded in performing the very first EUV-Pump - EUV-Probe experiments with molecules (in collaboration with MPQ Garching). Here the FEL-beam is focused onto our target using a multi-layer split mirror (half-moon geometry). By moving one half-mirror with respect to the other the arrival time of both light pulses is adjustable with fs resolution. The goal is to follow the fragmentation or dissociation of molecules as function of time and to produce a "molecular movie".
See some pictures of our group's activities.
Schematical drawing of the self-amplifying light emission from the electron packet (blue) in the undulator. Traversing the alternating magnetic structure, the electrons emit light. By interaction with the light field, a density modulation is imposed on the electron packet (bottom), leading to an amplified coherent light emission.
Momentum distributions of recoiling target ions for two-photon double ionization of Neon
at different photon energies. The light intensity is in the order of 1013 W/cm2 (pulse-length
25 fs). In one case (left) the two photons were absorbed instantaneously (direct transition),
at higher photon energies the sequential (step-wise) ionization dominates.