Photoinduced Charge Transfer Processes

Charge-transfer processes, particularly in hydrated iodide and salt clusters, depend sensitively on the chemical environment and number of water molecules solvated around the iodide ion. Studying such charge-transfer behaviour is ideally suited to gas-phase clusters, whereby the size and chemical composition, along with number of water molecules, can be controlled. For example, when hydrated iodide interacts with ultraviolet light, the electron fully separates from the iodine ion, forming a solvated electron. Charge-transfer transitions are also observed in ionic systems such as metal-halide clusters. To understand these charge-transfer processes at a molecular level, laser spectroscopic measurements in the ultraviolet and visible region are utilised and, initially, focussed on the ionic salt systems. Electrospray ionization is employed producing salt clusters, which can then be stored in the cell of a Fourier transform ion cyclotron resonance mass spectrometer. Laser systems using optical parametric oscillators are implemented, providing intense tuneable laser light in the 225 – 2600 nm region. For each size-selected salt cluster, evaporation of stoichiometric and non-stoichiometric fragments are recorded, elucidating photochemical pathways connected to charge-transfer transitions. These evaporation channels are revealed by mass spectrometry, whereby an electronic absorption spectrum can be generated in each case, in addition to wavelength-specific photochemistry. These experiments are complemented with simulated spectra generated using quantum chemical calculations. Unanswered questions such as where the charge is located, and where it moves to within the cluster, along with whether the charge is localised to one atom, are yet to be fully understood. Isolating size-selected hydrated iodide alongside a systematic series of salt clusters and exploring their photochemistry offers a targeted approach to tackle such questions. Studying these systems not only provides fundamental insight into charge-transfer mechanisms in cluster physics, but also offers a laboratory model system for a molecular level understanding of reactions occurring during marine aerosol ageing or radiation-induced cell damage. \includegraphics[scale=0.3]{Figure1.eps