Vibrational relaxation of liquid water in ionic solvation shells

Femtosecond two-color pump–probe spectroscopy is used to measure the vibrational lifetime of the O–H stretch vibration in solutions of KF, NaCl, NaBr, and NaI in HDO:D₂O. We observe a slow component (roughly 2–4 times slower than in HDO:D₂O) in the decay of the absorption change, which is due to O–H groups that are hydrogen bonded to the dissolved anions. The time constant of this slow component depends on the nature of the anions and is observed to decrease with temperature, in contrast with the temperature dependence of the relaxation of the OH stretch vibration in pure HDO:D₂O.