Vibrational dynamics of the bending mode of water interacting with ions

We studied the vibrational relaxation dynamics of the bending mode (ν₂) of the H₂O water molecules in the presence of different salts (LiCl, LiBr, LiI, NaI, CsI, NaClO₄, and NaBF₄). The linear and nonlinear spectra of the bending mode show distinct responses of water molecules hydrating the anions. We observe that the bending mode of water molecules that are hydrogen-bonded to an anion exhibits much slower relaxation rates (T₁ ∼ 1ps) than water molecules that are hydrogen-bonded to other water molecules (T₁ = 400 fs). We find that the effect of the anion on the absorption spectrum and relaxation time constant of the water bending mode is not only determined by the strength of the hydrogen-bond interaction but also by the shape of the anion.