Dynamics of Hydrogen Bromide Dissolution in the Ground and Excited States

The dissolution of acids is one of the most fundamental solvation processes, and an important issue is the nature of the hydration complex resulting in ion pair formation. We used femtosecond pump-probe spectroscopy to show that five water molecules are necessary for complete dissolution of a hydrogen bromide molecule to form the contact ion pair $H^{+} \cdot Br^-(H_{2}O)_n$ in the electronic ground state. In smaller mixed clusters (n < 5), the ion pair formation can be photoinduced by electronic excitation.