Femtosecond mid-infrared spectroscopy of aqueous solvation shells

We use femtosecond two-color mid-infrared spectroscopy to study the dynamics of aqueous solutions of salts in HDO:D₂O. We find that the lifetime of the O–H stretch vibration of HDO molecules in the solvation shell of the halogenic anions Cl ^–, Br ^–, and I ^– is much longer than the lifetime of the O–H stretch vibration of the HDO molecule in bulk D₂O solution. This difference in lifetime allows for a clear separation of the response of the solvation shell from that of the bulk liquid. A detailed investigation of the spectral dynamics of the solvating HDO molecules reveals that the hydrogen-bond dynamics of these molecules are much slower than the hydrogen-bond dynamics of bulk liquid water.