Femtosecond dynamics of hydrogen bonds in liquid water: A real time study

experiment is described to study temporal variations of the hydrogen bond l ength in diluted HDO/D2O solutions. The principles of this laser spectrosco pic experiment are explained first. The construction of a laser source gene rating 150 fs pulses in the 2.5-4.5 mu m spectral region at a 10 mu J power level is detailed next. The OH stretching band is reproduced for different excitation frequencies and different pump-probe delay times. A theory, bas ed on statistical mechanics of nonlinear optical processes, is proposed to calculate the lowest two spectral moments. A new effect is reported, the de lay dependent vibrational solvatochromism. It is shown how this effect can be exploited to follow temporal variations of the OH..O bond length directl y, in real time. initially elongated hydrogen bonds were found to contract with time and to reach the equilibrium lengths. The corresponding times sca les are of the order of 700 fs. No bond oscillations are observed.