Motion of hydrogen bonds in dilutedHDO/D2Osolutions: Direct probing with 150 fs resolution

An experiment is described to study temporal variations of the hydrogen bond length in diluted ${\mathrm{H}\mathrm{D}\mathrm{O}/\mathrm{D}}_2\mathrm{O}$ solutions. The principles of this laser spectroscopic experiment are explained first. The construction of a laser source generating $150\hspace{0.5em}\mathrm{fs}$ pulses in the $2.5–4.5\hspace{0.5em}\mu \mathrm{m}$ spectral region at a $10\hspace{0.5em}\mu \mathrm{J}$ power level is detailed next. The OH stretching band is reproduced for different 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 delay dependent vibrational solvatochromism. It is shown how this effect can be exploited to follow temporal variations of the $\mathrm{OH}\dots \mathrm{O}$ bond length directly, in real time. The corresponding time scales are of the order of $700\hspace{0.5em}\mathrm{fs}.$ No bond oscillations are observed.