Motion of hydrogen bonds in liquid water: Direct observation in real time

An experiment is described to study temporal variations of the hydrogen bond length in liquid water. The principles of this laser spectroscopic experiment are explained first. The construction of a laser source generating 150 fs pulses in the 2.5-4.4 μm spectral region is detailed next. The theoretical analysis is proposed in terms of statistical mechanics of nonlinear optical processes. A new spectral effect is reported, the pump-probe delay dependent solvatochromic shift of the OH band. It is shown how this effect can be exploited to "photograph" the dynamics of the OH...O bonds in real time.