Energy Relaxation and Structural Dynamics of Protons in Water/DMSO Mixtures.

We investigate the structure and dynamics of proton solvation structures in mixed water/dimethyl sulfoxide (DMSO) solvents using two-color mid-infrared femtosecond pump-probe spectroscopy. At a water fraction below 20%, protons are mainly solvated as (DMSO-H)+ and (DMSO-H)+-H2O structures. We find that excitation of the OH-stretch vibration of the proton in (DMSO-H)+-H2O structures leads to an ultrafast contraction of the hydrogen bond between (DMSO-H)+ and H2O. This excited state relaxes rapidly with T1 = 95 ± 10 fs and leads in part to a strong local heating effect and in part to predissociation of the protonated cluster into (DMSO-H)+ and water monomers.