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

An experiment is described to study temporal variations of the hydrogen bond length in diluted HDO/D₂O solutions. The principles of this laser spectroscopic experiment are explained first. The construction of a laser source generating 150 fs pulses in the 2.5-4.5 μm spectral region at a 10 μJ power level is detailed next. The OH stretching band is reproduced for different excitation frequencies and different pump-probe delay times. A theory, based on statistical mechanics of nonlinear optical processes, is proposed to calculate the lowest two spectral moments. A new effect is reported, the delay dependent vibrational solvatochromism. It is shown how this effect can be exploited to follow temporal variations of the OH..O bond length directly, in real time. Initially elongated hydrogen bonds were found to contract with time and to reach the equilibrium lengths. The corresponding times scales are of the order of 700 fs. No bond oscillations are observed.