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

An experiment is described to study temporal variations of the hydrogen bon d length in liquid water. The principles of this laser spectroscopic experi ment are explained first. The construction of a laser source generating 150 fs pulses in the 2.5-4.4 mu m spectral region is detailed next. The theore tical analysis is proposed in terms of statistical mechanics of nonlinear o ptical processes. A new spectral effect is reported, the pump-probe delay d ependent solvatochromic shift of the OR band. It is shown how this effect c an be exploited to "photograph" the dynamics of the OH...O bonds in real ti me.