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 T 1 of 170 ± 30 fs. Subsequently, the dissipated energy equilibrates with T 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.