Onset of charge separation in clusters: size-selective infrared spectroscopy of uncharged solute-solvent clusters

The dissolution of substances in solvents is familiar to everyone. When a solid dissolves in a solvent it breaks down at the molecular scale into simple building blocks, such as small ions or molecules, which mix intimately with the solvent molecules. While much is known about this process, it is not easy to gain detailed knowledge of what is happening at the molecular scale. One way of tackling this is to use the 'cluster approach', whereby a small cluster of solute and solvent atoms/molecules in the gas phase is subjected to detailed study, e.g. by spectroscopy. If the cluster sizes are not too large, a supporting theoretical study is feasible and the combination of theory and spectroscopic data can then be interpreted to extract detailed information, such as the arrangement of solvent molecules around the solute. If the spectra from different sized clusters can be distinguished and assigned, then one potentially has the means to explore how the cluster properties change with the number of solvent molecules. This makes it possible to follow the evolution from a small solute-solvent cluster through to cluster sizes where the solute-solvent behaviour may begin to resemble the bulk solution.The cluster philosophy is well-established and in recent years it has begun to be applied to the detailed spectroscopic study of ion-solvent clusters. Charged clusters are especially amenable to this approach because different sized clusters are easily separated by mass spectrometry. By combining this mass selectivity with new tuneable infrared laser technology, important and exciting new information on ion-solvent clusters has been extracted by several research groups in the past few years using mass-selective infrared spectroscopy.The work proposed here takes a different approach. Our aim is to apply size-selective infrared spectroscopy to uncharged solute-solvent clusters. Two systems are targeted: (1) metal atom solutes combined with common solvent molecules such as water and ammonia; (2) alkali halide molecules (MX) in water and ammonia. In both cases there is a tendency for charge separation to occur in bulk solutions. For highly electropositive metals, such as the alkalis, the metal atom can release an electron into the solvent to form a solvated electron. The extent to which this occurs in clusters will depend on the identity of the metal, the identity of the solvent, and the number of solvent molecules. It is well-known that the alkali halides, MX, dissolve in bulk aqueous solutions to form the solvent-separated ions, M+ and X-, but in small clusters this ionic dissociation may be incomplete. We want to answer the following basic question: how many solvent molecules are required before the MX salt molecule fully dissociates into M+ and X- ions? Our aim is to employ infrared spectroscopy to extract this information.A challenge for this work is to achieve mass-selectivity in the spectroscopy of uncharged clusters. Our chosen approach is to record action spectra by observing the impact of the IR laser excitation on the mass-selected ion signals originating from UV laser photoionization. Three related action techniques will be available to tackle this problem, all requiring essentially the same equipment.The techniques we propose here are not limited to metal-solvent and alkali halide-solvent clusters. Other solute-solvent systems, such as base-solvent, acid-solvent and indeed other salt-solvent systems are potentially observable by this means. Consequently, this unique project will also serve as a stepping-stone for a longer term programme investigating solute-solvent interactions in neutral clusters and their link to solvation mechanisms.