Martin-Luther-Universität Halle-Wittenberg

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Current Research Interests ...

  • Photoionizations via Radical Ions or Charge-Transfer States
  • Detection and Characterization of Excited States and Reactive Intermediates
  • Mechanism and Kinetics of Complex Radical Reactions
  • Dynamic Exchange Processes
  • Magnetic Field Effects on Chemical Reactions
  • Diffusional Dynamics

... and Methods

  • Two-Pulse Two-Colour Nanosecond Laser-Flash Photolysis with Optical Detection
  • Fluorescence, Phosphorescence and UV/vis Spectroscopy
  • Time-Resolved Photo-CIDNP Spectroscopy
  • Fourier-Transform NMR
  • Computer Sumulations with Monte-Carlo Methods

Time-Resolved Photo-CIDNP Spectroscopy

This method is essentially laser flash photolysis with detection of the products by a pulsed and Fourier transform NMR spectrometer. Although NMR is generally regarded as a "slow" method, this is feasible on a submicrosecond time scale because the CIDNP (chemically induced dynamic nuclear polarization) effects are generated during the life of intermediate radical pairs (1 ... 10 nanoseconds) but persist in the diamagnetic products for the spin-lattice relaxation time (seconds for protons). The time resolution is limited by the width of the NMR acquisition pulse, typically a few microseconds. However, by deconvolution techniques it can be reduced to about 100 nanoseconds.

We use Varian Unity and Gemini NMR spectrometers at 200, 400, and 500 MHz proton resonance frequencies. The probes are the only components of these spectrometers that have to be modified for CIDNP experiments: optical systems were inserted to allow side-on illumination of the sample within the active region. Besides, a trigger generator was built to synchronize the laser and the rf pulses. With an excimer laser that can also be used to pump a two-stage dye laser, we cover a wide range of photochemically relevant wavelengths (ca. 300 ... 600 nm).

Two-Pulse Two-Colour Laser-Flash Photolysis with Optical Detection

The essence of this method is to generate a reactive intermediate with the first laser flash and to investigate its photochemistry with the second. By varying the delay between the pulses, the decay reactions of the intermediate can also be investigated.

With our setup, the two flashes pass the cell in a collinear arrangement and the observed volume is excited uniformly. The optical path has been optimized such that very high excitation fluxes are possible. Detection (of optical absorbance or luminescence) occurs at right angles to the excitation light. Because of the well-defined path length and excitation conditions, absolute concentrations and quantum yields can be measured. A flow system is used to avoid a depletion of the reactants. The whole experiment is controlled by a PC that allows signal averaging.

With an excimer laser and a Nd:YAG laser, which can also be used to pump two dye lasers, and a home-made digital delay generator complete flexibility of the experiments is available.

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