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 mm spectral region, at a 10 mJ 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 OH · · · O motions are "photographed" in real time. [S0031-9007(98)08298-2] PACS numbers: 82.20.Wt, 78.47. + pElementary chemical processes in gases and liquids take place on time scales ranging from 10 fs to 10 ps. It took ultrafast laser technology over 20 years to reach these scales. An important breakthrough was accomplished when Zewail and his colleagues, applying laser spectroscopic methods, succeeded in following the photochemical dissociation of gaseous iodine cyanide (ICN) in real time [1], producing in this way a "photographic" picture of the process. A number of related studies were published later; see, e.g., . Although essential, this was only the first step; thermally activated chemical reactions in liquids necessitate further studies. They require the availability of femtosecond infrared sources, the knowledge of statistical mechanics of nonlinear optical processes, and that of large scale quantum molecular dynamics simulations.Published work on femtosecond dynamics of thermally activated processes mainly refers to an isotopic variety of liquid water, the diluted solution HDO͞D 2 O. The following directions of research were explored: (i) Pumpprobe experiments were realized by Graener, Seifert, and Laubereau at 10 ps [5] and by Laenen, Rauscher, and Laubereau at 1 ps [6] time scales. Hole burning of the OH-stretching band was observed for the first time and a population relaxation time of 1.5 ps was inferred. (ii) Another midinfrared pump-probe study of HDO dissolved in D 2 O, realized with polarization resolution at 250 fs time scales, was published by Woutersen, Emmerichs, and Bakker [7]. The orientational relaxation of HDO was found to be biexponential, with time constants of 0.7 and 13 ps. (iii) Theoretical work on this subject, mainly due to Bratos and Leicknam [8,9], employed the correlation function approach of statistical mechanics of nonlinear optical processes. A method was proposed to study OH · · · O motions of water in real time.The purpose of this paper is to realize the experiment suggested in Ref.[9], "photographing" H-bond motions in this way. The principle of the experiment is as follows.Its key ingredient is the well-known relation between the OH-stretching frequency and the length of an H bond; it is described in Refs. [10,11] and is illustrated in Fig. 1. Pumping the HDO͞D 2 O solution at a given frequency V 1 thus results in selecting the OH · · · O bonds of a specified length; an alternative statement is that the pump pulse generates a coherently excited wave ...

An experiment is described to study temporal variations of the hydrogen bond length in diluted HDO/D2O solutions. The principles of this laser spectroscopic experiment are explained first. The construction of a laser source generating 150 fs pulses in the 2.5-4.5 microns spectral region at a 10 microJ 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 OH...O bond length directly, in real time. The corresponding time scales are of the order of 700 fs. No bond oscillations are observed.

A new theory is proposed to describe spectral effects of the coupling between molecular rotations and OH¯O motions in liquid water. The correlation function approach is employed together with a special type of development in which the coupling energy of these two motions is the expansion parameter. The isotropy of the liquid medium plays an essential role in this study. Based on this theory, a new infrared pump-probe experiment is described permitting a visualization of molecular rotations at subpicosecond time scales. Full curves relating the mean squared rotational angle and time, and not only the rotational relaxation time, are measured by this experiment. However, very short times where the incident pulses overlap must be avoided in this analysis. The lifetime of OH¯O bonds in water is rotation-limited.