Probing the Stepwise Solvation of the Sulfate Dianion: Gas Phase Infrared Spectroscopy of SO$_{4}^{2-}$$\cdot$(H$_{2}$O)$_{n}$ Clusters (n=3-24)

Hydrated sulfate dianions are of paramount importance in diverse branches of science. However, a microscopic, molecular level understanding of the nature of the solute-solvent interaction as well as the composition of the solvation shell has not yet been achieved. Here we report the first gas phase infrared spectra of multiple charged anions in general, namely of mass-selected SO$_{4}^{2-}\cdot $(H$_{2}$O)$_{n}$ dianions ($n$=3-24) employing the infrared multiple photon dissociation technique in combination with radiation from the free electron laser FELIX. The infrared spectra (550-1800 cm$^{-1})$ were measured in the region of the stretching and bending modes of the sulfate core as well as characteristic intra- and intermolecular water modes, allowing an unprecedented, atomic level insight into structure of the complex as well as the nature and strength of the underlying hydrogen bonding interaction.