Motion of hydrogen bonds in diluted HDO/D2O solutions: Direct probing with150 fs resolution

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.