The charge transfer to solvent spectrum of iodide in supercooled water and glass-forming aqueous solutions

Measurements of the peak position and the line shape of the charge transfer to solvent (CTTS) band of aqueous I^- were carried out using thin samples squeezed between quartz disks. Results obtained in glass-forming aqueous magnesium acetate (Mg(OAc)₂) solutions show a temperature dependence similar to that observed in pure water above T_g, but below T_g both peak position and bandwidth suddenly become much less temperature dependent. This is a direct indication of the importance of configurational, as opposed to vibrational, degrees of freedom in accounting for temperature effects on CTTS transitions. In pure water supercooling to -31°C was achieved. Below 0°C, the temperature dependence of the energy of the transition deviates from linearity and becomes gradually smaller, and changes in bandwidth are arrested. These effects are discussed in terms of water structure in the supercooled region. Correlations of structural effects on CTTS with data concerning the absorption of the hydrated electron are discussed. Results are also given concerning the effects of solutes (e.g., I^-, OAc^-, Br^-) on the CTTS band. In particular, systematic observations were made on the line shape as a function of solute concentrations. The results are discussed using the physical picture of the diffuse model, attributing most structural effects to changes in the ground state energy.