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.