Dissolving acids in aqueous media is a fundamental process in many areas of chemistry. One process of interest is the acid ionization of hydrogen bromide in water. In the past, scientists believed that the minimum number of water molecules needed to solvate the molecule and form the contact ion pair in isolated complexes was four. However, a team at Pennsylvania State University in University Park using femtosecond spectroscopy discovered that a fifth is needed to complete the process. Untrafast spectroscopy has probed the dissolution of hydrogen bromide in water, demonstrating that five, not four, water molecules are required to ionize one hydrogen bromide molecule. The first step in the mechanism is proton transfer from the acid to a water molecule, followed by solvent reorganization. Courtesy of Sean M. Hurley. The researchers investigated the ultrafast dynamics of mixed clusters of HBr-water using the pump-probe technique, combined with reflectron time-of-flight mass spectrometry, to form the contact ion pair H1*Br2 in the electronic ground state. They produced the mixed clusters by crossing a molecular beam composed of hydrogen bromide, which was formed by the supersonic expansion of a 10 percent hydrogen bromide mixture seeded in argon through a pulsed nozzle, with an effusive water vapor beam from a pickup source. It was observed that photochemistry coupled with solvation and clustering of hydrogen bromide with water molecules involves complex rearrangement and reaction pathways, resulting in the formation of the ion pair. Solvation influences the dynamics of the clusters, and a simple exponential decay of the excited species was not observed for any of the protonated water clusters.The discovery may play an important role in investigating the loss of ozone in the Arctic, where bromine contributes to ozone depletion near the Earth's surface.