Vibrational Relaxation Dynamics of the Core and Outer Part of Proton-Hydration Clusters

We study the ultrafast relaxation dynamics of hydrated proton clusters in acetonitrile using femtosecond mid-infrared pump-probe spectroscopy. We observe a strong dependence of transient absorption dynamics on the frequency of excitation. When we excite the OH vibrations with frequencies ≤3100 cm^-1, we observe an ultrafast energy relaxation that leads to the heating of the local environment of the proton. This response is assigned to the OH vibrations of the water molecules in the core of the hydrated proton cluster. When we excite with frequencies ≥3200 cm^-1, we observe a relatively slow vibrational relaxation with a T₁ time constant ranging from 0.22 ± 0.04 ps at ν_ex = 3200 cm^-1 to 0.37 ± 0.02 ps at ν_ex = 3520 cm^-1. We assign this response to water molecules in the outer part of the hydrated proton cluster.