A pump-probe experiment is described to study femtosecond dynamics of hydrogen bonds in liquid water. The key element of the experimental setup is a laser source emitting 150 fs pulses in the 2.5–4.4 μm spectral region, at a 10 μJ power level. The OH-stretching band is recorded for different excitation frequencies and different pump-probe delay times. Time-dependent solvatochromic shifts are observed and are interpreted with the help of statistical mechanics of nonlinear optical processes. Using these spectral data, the $\mathrm{OH}\cdots \mathrm{O}$ motions are “photographed” in real time.

• Received 22 September 1998

DOI:https://doi.org/10.1103/PhysRevLett.82.1068