An experiment is described to study temporal variations of the hydrogen bon
d length in diluted HDO/D2O solutions. The principles of this laser spectro
scopic experiment are explained first. The construction of a laser source g
enerating 150 fs pulses in the 2.5-4.5 mu m spectral region at a 10 mu J po
wer level is detailed next. The OH stretching band is reproduced for differ
ent 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. An 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 dire
ctly, in real time. The corresponding time scales are of the order of 700 f
s. No bond oscillations are observed.