Hydrogen Bond in Liquid Water as a Brownian Oscillator

We present the first experimental observation of a vibrational dynamic Stokes shift. This dynamic Stokes shift is observed in a femtosecond pump-probe study on the OH-stretch vibration of HDO dissolved in ${\mathrm{D}}_2\mathrm{O}$. We find that the Stokes shift has a value of approximately $70{\mathrm{cm}}^{-1\mathrm{}}$ and occurs with a time constant of approximately 500 femtoseconds. The measurements can be accurately described by modeling the hydrogen bond in liquid water as a Brownian oscillator.