Ultrafast aqueous bimolecular proton transfer

Femtosecond infrared spectroscopy is used to monitor the different proton transfer stages in the neutralization reaction between an optically triggered proton donor (the photoacid pyranine) and carboxylic bases in aqueous solution. When acid and base are directly connected, the transfer occurs faster than the time resolution (150 fs). In contrast, for the case of the photoacid and base being separated by one water molecule, the transfer from photoacid to the water bridge is faster than 150 fs, whereas the next transfer from water to the base proceeds on picosecond time scales. (author)