Optical manipulation of electron photodetachment in aqueous halide solutions: verification of atom-electron contact pairs via pump-repump probe spectroscopy

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Electron transfer (ET) reactions are ubiquitous in chemistry, biology and physics, but the large number of degree of freedom in most ET systems makes it difficult to obtain a fundamental understanding of the charge transfer process. Therefore, it makes sense to study model systems that consist of atomic reactants as the aqueous halides. Quantum simulations predicted two channels for the electron release out of charge-transfer-to-solvent (CTTS) states of aqueous halide: adiabatic separation out of the lowest CTTS state via a weakly bound halogen-electron pair, the so called contact pair, and a less effective channel for direct electron injection into the solvent. Our main goal in this project is a more detailed understanding of the electron photodetachment in aqueous halides, and in particular the verification and characterization of the predicted atom-electron contact pair. It is recalled that it is difficult to distinguish the two species, the atom-electron pair and the final hydrated electron, because of the very similar ground state absorption band around 720nm. For this purpose, we plan to apply punip-repump-probe (PReP) spectroscopy in combination with the well-known pump-probe technique. The secondary excitation in the PReP spectroscopy allows an optical manipulation of the relaxation pathway and can provide novel information on the involved intermediates. The electron photodetachment will be accomplished by a short UV pump pulse and the following separation and solvation dynamics will be monitored in a broad spectral region from UV to middle-infrared with femtosecond probing pulse. The measurements will be conducted at various temperatures in order to verify the binding energy of the predicted contact pair.