Vibrational and Structural Relaxation of Hydrated Protons in Nafion Membranes

We study the vibrational dynamics of the bending mode at 1730 cm^-1 of proton hydration structures in Nafion membranes with polarization-resolved infrared (IR) pump-probe spectroscopy. The bending mode relaxes to an intermediate state with a time constant T₁ of 170±30 fs. Subsequently, the dissipated energy equilibrates withT_eq of 1.5±0.2 ps. The transient absorption signals show a long-living anisotropy, which indicates that for part of the excited proton hydration clusters the vibrational energy dissipation results in a local structural change, e.g. the breaking of a local hydrogen bond. This structural relaxation relaxes with a time constant of 38±4 ps.