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Dr. Christian Schröder
Photodissociation of I2

Energy transfer of CH2I2

Energy transfer of azulene compounds

Protein - water interface

Ionic liquids

Photodissociation of iodine

The photodissociation and recombination of iodine is one of the most extensively studied chemical reactions in liquids and, therefore, served as a model system for understanding various aspects of condensed phase reaction dynamics.
The interest goes back to Franck and Rabinowitch proposing the possibility that after photoexcitation separating iodine atoms could be trapped in a solvent cage reducing the dissociation quantum yield with respect to the gas phase.

After light absorption in the range 450 - 600 nm the dissociative B"1u or the predissociative B0u+ electronic state is populated causing the iodine atoms to seperate on repulsive potentials. A fraction of atoms collide with surrounding solvent particles and recombine geminately in the cage. Partially they get trapped in weakly bound A1u and A'2u electronic states from which they can relax to the ground state. Partially they immediately recombine to the ground state where the iodine molecules are stabilised by vibrational energy relaxation. A fraction of atoms succeeds to escape from the solvent cage. These atoms call still recombine geminately within several tens of picoseconds or separate by diffusion and finally recombine nongeminately on a microsecond time scale.
The dynamics is characterised by a variety of competing processes summarised in Fig.1.

Fig.1. Potential energy curves of iodine

We investigated the geminate recombination dynamics of iodine by determing photodissociation quantum yields after laser excitation at 532 nm in compressed liquid n-alkanes and supercritical CO2 and xenon. For all solvent the quantum yield was found to decay nearly linearly with increasing density. The quantum yield is independent to the nature of the solvent but determined by the free space available to the separating iodine atoms during the dissociation process (Fig.2.). Our results are supported by molecular dynamics simulations and clearly show that the geminate recombination of iodine can be understood in terms of a purely kinematic effect.
Fig.2. Quantum yields of iodine in various solvents.

Literature :
Zeitschrift für Physikalische Chemie, 215, 2, 183-195 (2001)

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