Photoinduced Charge-Transfer Processes in Gas-Phase Clusters

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. This term has been coined a charge-transfer-to-solvent transition. Charge-transfer transitions are also observed in ionic systems such as metal-sulphate and metal-halide clusters. To understand these charge-transfer processes at a molecular level, laser spectroscopy in the ultraviolet and visible region will be carried out. Hydrated iodide will be generated using laser vaporization, while electrospray ionization will be used to produce salt clusters. Clusters will be stored in the cell of a Fourier transform ion cyclotron resonance mass spectrometer. Interesting clusters will be mass-selected, which allows for study of individual cluster sizes. Laser systems using optical parametric oscillators will be used, providing intense tuneable laser light in the 225 2600 nm region. For hydrated iodide, upon absorption of light evaporation of water, or a neutral iodine atom, is proposed, revealing vital information on the charge- transfer-to-solvent transitions. Formation of the solvated electron, along with its reactivity, will also be investigated. For salt clusters, evaporation of varying salt masses will be recorded, elucidating photochemical pathways connected to charge-transfer transitions. These evaporation channels will be revealed by mass spectrometry, whereby an electronic absorption spectrum can be generated in each case, in addition to wavelength-specific photochemistry. These experiments will be complemented with simulated spectra generated using quantum chemical calculations. The photochemistry of a series of metal sulphate and metal halide salts, with varying positive and negative charges, will be studied as a function of cluster size and wavelength exploring charge-transfer behaviour. 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.