Dynamics of photoionization-induced processes in laser-prepared gas- and aqueous-phase samples


The group aims to investigate two different projects, one on gas-phase and one on liquid-phase experiments, having in common the preparation of the different samples by laser pulses. In detail, this will cover

  • studying X-ray-induced photoemission processes and their dynamics in laser pre-aligned gas-phase molecules in a reaction microscope (using cold target recoil ion spectroscopy, COLTRIMS),
  • observing and controlling bimolecular chemical reactivity and kinetics, involving aqueous-phase radicals (such as OH), hydrated electrons, and a range of co-reactants using the liquid-jet, picosecond-time-resolved photoemission spectroscopy technique.

To achieve this, a versatile and state-of-the-art high-average-power laser system of up to 1 MHz repetition rate, 0.2-200 picoseconds pulse-duration range, and 200-2500 nm wavelength range will be combined and synchronized with the soft-X-ray beamline P04 of synchrotron PETRA III (DESY, Hamburg). This beamline is a unique light source offering a broad soft-X-ray photon-energy range of 250-3000 eV, an exceptionally high photon flux of up to 1014 photons/s, a high repetition rate of 5.2 MHz and ~100 ps on target X-ray pulse durations for worldwide unique, high-data-collection-rate measurements.

For the experiments, existing and mobile COLTRIMS reaction microscope and liquid-jet photoelectron spectroscopy end-stations will be used.

Scientific goals:

For molecules in the gas phase, this will enable novel studies at the highest level of detail on molecular-frame photoelectron angular distributions of complex, pre-aligned molecules, via both inner-shell and valence orbital photoionization; the deduction of structure, handedness, and detailed geometrical features of complex molecules by photoelectron diffraction; and in the field of adiabatically aligned chiral molecules an extremely high contrast for photoelectron circular dichroism, allowing for chiral recognition of gaseous sample molecules towards useful applications with an unprecedented sensitivity.

For molecules in the liquid and especially aqueous phase, the new approach will enable time-domain tracking of chemical reactions (energetics and kinetics) of photoinduced species, including hydronium ions, radicals, and solvated electrons, and the associated structural reorganization of the solvation shell upon electronic excitation; the observation of time-resolved core-level chemical shifts as well as valence electronic-structure changes upon photoexcitation/photoionization of solution-vacuum interfaces (in conjunction with OH radicals); and the time-resolved exploration of photoelectron angular distributions in aqueous solutions containing surface-active surfactant molecules. The spectrum of applications stretches from molecular physics, chemical physics, the physics of liquids, physical chemistry, homogeneous catalysis, environmental science, to biochemistry.

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