Max-Planck-Institut für Kernphysik Heidelberg

Atoms and molecules in ultra-short laser pulses

Priv.-Doz. Dr. Robert Moshammer and Dr. Claus Dieter Schröter

  Research topics:  |  Ions in Traps  |  Electrons in Collisions  |  Lasers in Time  |  
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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 a billion!

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.

Ionization of lithium using a magneto-optical trap


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.


Kinetic energy of heavy fragments after dissociation of N2 molecules as function of the emission angle with respect to the laser polarization axis. (top): single photon induced ionization and dissociation. (bottom): Two-photon (sequential) induced fragmentation.


Schematic drawing of the Reaction Microscope with split-mirror for pump-probe experiments.

Robert Moshammer

Priv.-Doz. Dr. Robert Moshammer

Atoms and molecules in strong fields

Room: Bo. 328b
Tel.: +49 (0) 6221 516 - 461
Email: robert.moshammer@please delete

Claus Dieter Schroeter

Dr. Claus Dieter Schroeter

Room: Bo. 335
Tel.: +49 (0) 6221 516 - 244
Email: claus.schroeter@please delete

Hannes Lindenblatt

MSc Hannes Lindenblatt

Room: Bo. 323
Tel.: +49 (0) 6221 516 - 523
Email: hannes.lindenblatt@please delete

Severin Meister

MSc Severin Meister

Room: Bo. 321
Tel.: +49 (0) 6221 516 - 438
Email: severin.meister@please delete

Florian Trost

MSc Florian Trost

Room: Bo. 322b
Tel.: +49 (0) 6221 516 - 296
Email: florian.trost@please delete

Dr. Kirsten Schnorr

Room: PSI
Tel.: +49 (0) 6221 516 - xxx
Email: kirsten.schnorr@please delete

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