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Stored and Cooled Ions Division
 
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Master and bachelor theses

Master thesis:
Ion Fragmentation at the Free Electron Laser FLASH

Scheme: Free Electron Laser (FEL) FLASH
FLASH, the Free-Electron LASer in Hamburg
Protons attached to neutral molecules
Soft x-rays close to stars
Proton bridged water clusters
Photon absorption cuts the molecule right at the proton bridge

Using the uniquely intense pulses of soft-x-ray radiation (~30 eV) at the Free Electron Laser (FEL) FLASH at DESY we study the photolysis of protonated molecules.

We use an ion beam (few keV) which is crossed with the FEL beam - the ion beam facility TIFF constructed by us and which is stationary at DESY. Nearby, in a range of 0.3 to 1 m, particle counters are installed which image the fragments of single reaction events. These reactions can be reconstructed completely.

The experiments are performed in a collaboration that consists of Aarhus University (Denmark), DESY, the Weizmann Institute (Rehovot, Israel) and the Max Planck Institute of Nuclear Physics, Heidelberg.

We are preparing new experiments on proton bridged clusters H2O-H+-H2O and N2-H+-N2.

Master project tasks:

  • Designing the reaction product imaging for breakup of N2-H+-N2.
  • Adapting and testing of a supersonic expansion jet ion source for N2-H+-N2 production.
  • Participation in data taking and analysis on reaction product imaging at the FLASH FEL.

Background information:

Protons attached to neutral molecules strongly increase their reactivity. Therefore these systems are important in nature (chemistry, biology).

Soft x-rays (such as, in nature, close to stars or in outer atmospheres) can create vacancies within the electron shell responsible for the chemical binding. The molecules are then broken up into fragments (photolysis). Since ions can be produced only at low density, their photolysis as been only little studies with conventional photon sources, and becomes accessible only now using FLASH or other FEL's.

Interesting protonated systems are proton bridged clusters, such as the water dimer structured as H2O-H+-H2O. At TIFF we observed that photon absorption "cuts" the molecule right at the proton bridge (as seen by the decay -> H3O+ + H2O+) - a process which recently was explained by "ultra-fast non-adiabatic coupling" on the potential surfaces of the molecules and should similarly occur quite generally in the vicinity of protons attached to molecules.

Publications:

L. Lammich et al., Fragmentation Pathways of H+(H2O)2 after Extreme Ultraviolet Photoionization, Phys. Rev. Lett. 105, 253003 (2010) external Link
Zheng Li et al., Correlated Dynamics of the Motion of Proton-Hole Wave Packets in a Photoionized Water Cluster, Phys. Rev. Lett. 110, 038302 (2013) external Link

Links: FLASH at DESY, Hamburg external Link

Contact: Apl. Prof. Dr. , MPI für Kernphysik, Phone: (06221)516-503